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Attentional requirements of postural control in people with spinal cord injury : the effect of dual task Tse, Cynthia Munting 2015

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 ATTENTIONAL REQUIREMENTS OF POSTURAL CONTROL IN PEOPLE WITH SPINAL CORD INJURY: THE EFFECT OF DUAL TASK   by  Cynthia Munting Tse  BScPT, University of Alberta, 1992    A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF   MASTER OF SCIENCE   in  The Faculty of Graduate and Postdoctoral Studies  (Kinesiology)  THE UNIVERSITY OF BRITISH COLUMBIA  (Vancouver)  July 2015   © Cynthia Munting Tse, 2015 ! ii!ABSTRACT Background: The simultaneous performance of a postural and suprapostural task has been shown to result in the deterioration of the performance of one or both tasks. For people with spinal cord injury (SCI), whose standing balance is challenged, it is unknown the extent to which they rely on attentional resources to maintain quiet stance. The overall aim of this study was to use a dual task paradigm to investigate the attentional requirements for maintaining standing balance in people with SCI.   Methods: We recruited 9 adults with incomplete SCI and 8 matched able-bodied controls. Subjects were asked to perform two suprapostural tasks: a mathematical task (counting backwards by 3s) and an auditory reaction time (RT) task with eyes open/closed. Three single task (ST) trials were recorded: i) standing on force plates; ii) math task while seated; iii) RT task while seated. Two dual-task (DT) trials were recorded: i) standing + math task; ii) standing + RT task. The primary outcome measures were the change in performance between ST and DT between SCI and controls for: i) RT, ii) maximum standing time, iii) error ratio and total number of words uttered, and iv) movement reinvestment. Secondary outcomes such as center of pressure (CoP) measures from force plates as well as perceptual measures such as fear, confidence and perceived mental workload were also recorded.   ! iii!Results: SCI subjects stood for shorter duration during DT (stand and count) than ST (stand) compared to controls during eyes closed. Main effects between groups were observed for movement reinvestment, CoP performance, perceived mental effort, fear and confidence. No significant effects were observed for RT task or math task performance.  Conclusion: Total standing time during eyes closed is adversely affected with the addition of a math task for SCI subjects. Perceptual measures such as increased fear and perceived mental workload and decreased confidence correspond to increases in postural sway and conscious control of standing in subjects with SCI. Individuals who can stand for >60 seconds eyes closed do not appear to be significantly affected by the addition of a concurrent secondary task of minimal mental workload.   ! iv!PREFACE My study was inspired by my observation of SCI participants in the human locomotion laboratory at ICORD. In particular I noted how much attention SCI patients seemed to devote to maintaining postural control, for example, losing concentration and balance by simply engaging in conversation. As nearly all activities of daily living involve some form of postural control along with a suprapostural task, improved understanding of the impact of a dual task on attention and balance seemed important. In particular, it could have potential implications for SCI rehabilitation. A literature reviewed suggested that the influence of a suprapostural task on postural control had not been previously studied in people with SCI.    The specific research hypothesis grew out of discussion with my thesis supervisor, Dr. Tania Lam. The experimental design and choice of math task was researched by myself in consultation with Dr. Lam. The secondary aim of associating dual task performance with clinical measures was suggested by Dr. Lam.   We observed a great deal of heterogeneity in postural control in people with SCI due to factors such as differing injury levels and whether the injury is complete or not. Dr. Mark Carpenter suggested a second suprapostural task – reaction time – in order to capture those SCI subjects who could stand for only a short period of time. Dr. Carpenter also suggested adding perceptual measures such as movement reinvestment, fear and confidence to see how they might relate to postural control. Dr. Teresa Liu-Ambrose highlighted the importance of using cognitive testing such as the Montreal Cognitive ! v!Assessment and the executive role of the brain. Consequently, the study addresses postural control from multiple aspects: physiologically, psychologically and clinically.   I was responsible for data collection and analysis under the guidance of Dr. Lam. For example, I collected force plate center of pressure measures, math task and reaction time performance as well as all clinical measures in the course of the study. Staff and students at ICORD were extremely helpful and beneficial in identifying possible trial subjects and in generally assisting to administer the data collection. The data was analyzed by me with input from Dr. Lam.   This project was approved by the Clinical Research Ethics Board of UBC (“Postural control in persons with SCI: the effects of dual task”, H14-02177).    ! vi!TABLE OF CONTENTS Abstract .............................................................................................................................. ii Preface ............................................................................................................................... iv Table of Contents ............................................................................................................. vi List of Tables .................................................................................................................. viii List of Figures ................................................................................................................... ix Acknowledgements ........................................................................................................... x 1 Introduction .................................................................................................................... 1 1.1 Impact of SCI on postural control ...................................................................................... 2 1.2 Neural control of posture ..................................................................................................... 3 1.3 Cognitive control of upright stance .................................................................................... 6 1.4 Allocation of attention ........................................................................................................ 10 1.5 Current dual task frameworks ......................................................................................... 13 1.6 Task considerations ............................................................................................................ 17 1.6.1 Balance ability .............................................................................................................. 18 1.6.2 Difficulty of the suprapostural task .............................................................................. 20 1.6.3 Difficulty of the postural task ....................................................................................... 22 1.6.4 Types of task ................................................................................................................. 23 1.6.5 Focus of attention and instruction ................................................................................. 24 1.6.6. Perceptual factors and postural control ........................................................................ 27 1.7 Rationale ............................................................................................................................. 28 2 Methods ......................................................................................................................... 32 2.1 Participants ......................................................................................................................... 32 2.2 Postural conditions ............................................................................................................. 32 2.2.1 Sitting ............................................................................................................................ 32 2.2.2 Standing ........................................................................................................................ 33 2.3 Experimental tasks ............................................................................................................. 33 2.3.1 Reaction time task (Experiment 1) ............................................................................... 33 2.3.2 Math task (Experiment 2) ............................................................................................. 34 2.4 Supplementary experiment ............................................................................................... 34 2.5 Measures ............................................................................................................................. 35 2.5.1 Reaction time ................................................................................................................ 35 2.5.2 Standing time ................................................................................................................ 36 2.5.3 Math task error ratio and total words uttered ................................................................ 37 2.5.4 Movement Specific Reinvestment Scale ...................................................................... 37 2.5.5 Center of Pressure (CoP) .............................................................................................. 38 2.5.6 Perceptual measures ...................................................................................................... 39 2.5.7 Clinical and functional measures (SCI subjects only) .................................................. 40 2.5.8 Dual-task cost ............................................................................................................... 41 2.6 Statistical analysis .............................................................................................................. 42 2.6.1 Subject characteristics .................................................................................................. 42 2.6.2 Dual task effects on primary and secondary performance measures ............................ 42 2.6.3 Dual task effects on perceptual measures ..................................................................... 43 2.6.4 Associations between performance and functional measures (SCI only) ..................... 44 ! vii!2.6.5 Associations between chronicity, cognitive impairment and performance measures (SCI only) .............................................................................................................................. 44 3 Results ........................................................................................................................... 45 3.1 Subjects characteristics ..................................................................................................... 45 3.2 Reaction time task performance (Experiment 1) ............................................................ 45 3.3 Math task performance (Experiment 2) .......................................................................... 46 3.4 Standing time performance (math task) .......................................................................... 48 3.5 Movement reinvestment .................................................................................................... 49 3.6 Postural performance ........................................................................................................ 51 3.7 Berg Balance Scale (SCI subjects only) ............................................................................ 53 3.8 Subjective mental workload (NASA-TLX) ...................................................................... 53 3.9 Fear ...................................................................................................................................... 54 3.10 Confidence ........................................................................................................................ 55 3.11 Correlations ...................................................................................................................... 56 3.11.1 Associations between performance and functional measures (SCI subjects only) ..... 57 3.11.2 Associations between chronicity, cognitive impairment and performance measures (SCI only) .............................................................................................................................. 57 4 Discussion ...................................................................................................................... 58 4.1 SCI subjects characteristics .............................................................................................. 59 4.2 Reaction time tasks during standing do not elicit dual-task effects .............................. 60 4.3 Math task performance is not affected by standing ........................................................ 60 4.4 People with SCI have greater postural sway and movement reinvestment during quiet standing ..................................................................................................................................... 62 4.5 Postural sway and movement reinvestment was not affected by dual-tasking ............ 63 4.5.1 Perceptual measures affect postural sway and movement reinvestment in SCI ........... 65 4.5.2 Lack of findings due to task parameters and high functioning SCI group ................... 66 4.6 Influence of dual task postural task on clinical measures .............................................. 69 4.7 Cognitive function and dual tasking in SCI ..................................................................... 70 5 Conclusions and Future Directions ............................................................................ 72 6 Tables ............................................................................................................................ 74 References ........................................................................................................................ 85 Appendices ....................................................................................................................... 95 Appendix A: The Movement Specific Reinvestment Scale-Modified .................................. 95 Appendix B: NASA-TLX ......................................................................................................... 96 Appendix C: The Berg Balance Scale ..................................................................................... 97 Appendix D: Spinal Cord Independence Measure ............................................................. 103 Appendix E: Canadian Occupational Performance Measure ........................................... 109 Appendix F: Montreal Cognitive Assessment ..................................................................... 110 Appendix G: Correlations between performance and clinical measures ......................... 111    ! viii!LIST OF TABLES !Table 1 Descriptive statistics of subject characteristics and clinical evaluations ............. 74 Table 2 Subject characteristics and clinical outcomes of individual SCI subjects ........... 75 Table 3 Experimental conditions ...................................................................................... 76 Table 4 Summary of sequence of data collection ............................................................. 77 Table 5A  Reaction time task performance ....................................................................... 78 Table 5B  Subjective mental workload for reaction time task … ……………………….77 Table 6 Math task performance for eyes open and closed ................................................ 79 Table 7 Standing time performance eyes closed ............................................................... 80 Table 8 Movement reinvestment performance for math task, eyes open and closed ....... 80 Table 9 Center of pressure postural performance eyes open and closed .......................... 81 Table 10 Subjective mental workload for math task, eyes open and closed ..................... 82 Table 11 Fear ratings for math task, eyes open and closed .............................................. 83 Table 12 Confidence ratings for math task eyes open and closed .................................... 84    ! ix!LIST OF FIGURES  Figure 1 Reaction time processing .................................................................................... 36 Figure 2 Reaction time and perceived mental workload between SCI and AB groups .... 46 Figure 3 Math task performance between SCI and AB groups ........................................ 47 Figure 4 Standing time eyes closed between SCI and AB groups .................................... 48 Figure 5 Movement reinvestment between SCI and AB groups ...................................... 50 Figure 6 CoP displacement for an SCI and AB subject .................................................... 51 Figure 7 Subjective mental workload for the math task between SCI and AB groups .... 54 Figure 8 Fear and confidence for the math task between SCI and AB groups ................. 56   ! x!ACKNOWLEDGEMENTS I offer my deepest gratitude to Dr. Tania Lam for her enduring support and guidance throughout my journey towards pursuing a Masters degree. She has inspired me to continue to work in the field of SCI research. I owe particular thanks to my committee members Dr. Mark Carpenter and Dr. Teresa Liu-Ambrose for challenging and enlarging my views. Together with Dr. Lam, they have taught me to question more deeply.  Special thanks to the Lam Lab ICORD family; Dr. Amanda Chisholm, for answering my unending questions, Raza Malik for opening my eyes to Matlab and greeting me with a smile no matter what the day was like. Thanks to Franco Chan who went above and beyond everything I asked him for including writing a special program for reaction time.  I could not have contemplated a Masters degree without the support of my husband Scott. He gave up being with his family for 10 months while I pursued my dream. To my son Alex who patiently (for a 4 year old) waited for his mother to finish her “homework” and who filled me with joy when he squealed with excitement when we went to the lab instead of Science World.!! 1!1 INTRODUCTION One of the most important goals for people with spinal cord injury (SCI) is the recovery of balance or postural control. Postural control in sitting or standing is essential for performing the functional activities that underpin all activities of daily living. Standing is traditionally considered to be an unconscious, automatic task, suggesting that postural control systems use minimal attentional resources. However, evidence from dual task paradigms, where postural and suprapostural (secondary) activities are performed concurrently, suggest that there are significant attentional requirements in postural control and that these activities may share common resource requirements in the brain (Maylor et al., 2001; Maki and McIlroy, 2007). In people with SCI, pathways mediating the sensorimotor integration required for postural control may be interrupted. As a result, individuals with SCI may need to devote more attentional resources to maintain posture, which may limit their ability to undertake concurrent tasks (e.g. engaging in conversation while standing). While there is increasing research in understanding the recovery of postural control after spinal cord injury (Lajoie et al., 1999; Datta et al., 2009; Lemay et al., 2014), none so far have considered the potential impact of concurrent suprapostural tasks on the ability to maintain balance. Thus, the overall objective of this study is to understand how the addition of a suprapostural task may affect standing postural control in people with SCI.  !! 2!1.1 Impact of SCI on postural control Spinal cord injury (SCI) encompasses a broad constellation of altered physiological functions, secondary medical complications and changed psychosocial roles. In Canada, there are 85 000 people living with SCI with 3600 new cases each year (Noonan et al., 2012). Causes include traumatic events such as motor vehicle accidents, violence and falls. Non-traumatic SCI comprises a host of pathophysiological conditions such as congenital and developmental disorders, degenerative and metabolic conditions, toxicity and tumors to the spinal cord. The resultant complete or partial paralysis and sensorimotor impairments compromises postural control and mobility, which in turn may increase the risk of secondary complications such as pneumonia, heart disease, pressure sores, or urinary tract infections.  The extent of recovery after SCI depends on the level and severity of the injury. Injury to the spinal cord in the cervical region, with associated loss of use in all four limbs and trunk, is classified as tetraplegia. Paraplegia encompasses injury to the thoracic, lumbar and sacral segments, including the cauda equina and conus medullaris. Incomplete SCI, which accounts for 45% of SCI (Jackson et al., 2004; Rahimi-Movaghar et al., 2013), have variable neurologic findings with partial loss of sensory and motor function below the level of the lesion. A complete injury to the spinal cord is characterized clinically as a complete loss of motor and sensory function below the level of the lesion.   Research has shown that people with motor incomplete SCI may have the capacity to recover some motor function that appears to be dependent on the integrity of spared supraspinal pathways. For instance, improvements in walking using body weight !! 3!supported treadmill training have been associated with increased activation of cortical areas involved in motor control (Dobkin et al., 2004). Also, studies tracking muscle responses to transcranial magnetic stimulation or analysis of inter-muscular coherence all suggest that improvements in walking are associated with the strengthening of spared descending inputs from the brain (Yang, 2006). In people with stroke, the recovery of standing balance was found to be associated with increased supplementary motor cortex activation after rehabilitation (Fujimoto et al., 2014). Given these previous findings, we may similarly expect that the recovery of the control of postural stability in people with SCI, which will be challenged by spasticity, clonus, weakness and sensory loss, will rely on the integrity and facilitation of supraspinal pathways.   1.2 Neural control of posture Postural control is the regulation of the body’s position in space for two important behavioral goals, postural orientation and postural equilibrium. Horak and Macpherson define postural orientation as the active control of trunk and head alignment and tone with respect to gravity, support surfaces, visual environment and internal reference frames (Horak and Macpherson, 1996). Sensory information from the visual, vestibular, and somatosensory systems are integrated and weighted depending on the task goals and environmental context. Postural equilibrium or balance is the state in which the sum of all the forces and moments acting on the body are balanced such that the body tends to stay in the desired position and orientation (static equilibrium), or is able to progress through an intended movement without losing balance (dynamic equilibrium) (Horak and Macpherson, 1996; Horak, 2006). !! 4! The most important biomechanical constraint to maintaining balance is the size and quality of the base of support; for standing, this means that the placement and orientation of the feet is key. The ability to maintain balance during quiet standing can be achieved as long as the center of mass (CoM) or the vertical projection of it to the ground, the center of gravity (CoG), remains within the base of support. The CoM, which is the point at which the entire mass of the body is balanced, is controlled by the vector sum of the ground reaction forces at the feet, the center of pressure (CoP). The position of the CoP fluctuates around the CoG position. The CoP signal is higher in frequency and amplitude than the CoG, and this interplay between the CoG and CoP has been likened to a cat and mouse game where the CoG is regulated by the CoP signal (Winter, 1995). This difference between the CoG and CoP displacement is thought to reflect a signal error as the CNS attempts to adapt the body to the new position (Winter, 1995).  In quiet standing, humans sway at a mean frequency of 0.27-0.45Hz (Carpenter et al., 2001) and any limitations to the base of support such as that due to size, strength, pain, and control will adversely affect balance (Tinetti, Speechlev & Ginter, 1998). In people with SCI, weakness, spasticity, sensory deficits, and pain could all affect the ability of the body to control the CoM within the base of support (Lee et al., 2012). For example, in people with SCI, weak dorsiflexion and plantarflexion ankle control was hypothesized to account for lower antero-posterior excursion of the CoP compared to able-bodied controls in a limits of stability test (Lemay et al., 2014). Moreover, the tests of the limits of stability revealed that SCI subjects demonstrate increased sway path (reduced movement precision), which could be attributed to deficits in sensorimotor integration !! 5!(Lemay et al., 2014). Individuals with SCI also show greater postural sway that is exacerbated with increasing somatosensory challenge (e.g. standing on a pillow) compared to controls (Lee et al. 2012).   Sensory information from the vestibular, visual and somatosensory systems must be integrated to interpret complex environments. As individuals change from one environment to another, such as walking into a dark room, they must re-weight their relative dependence on each of the senses (Horak, 2006). Individuals with SCI rely more on visual information to maintain their balance due to deficits in their somatosensory systems (Lee et al., 2012; Lemay et al., 2013). Uneven weight distribution during quiet stance was observed under circumstances when insufficient visual information such as tilting the head backwards was provided (Lee et al., 2012). In a study examining the use of visual inputs to maintain standing postural steadiness in people with SCI, subjects were asked to stand quietly on force plates with eyes open and eyes closed. Higher Rhomberg ratios (eyes open:eyes closed) for CoP mean velocity and sway area indicated that SCI subjects were more unstable in both conditions than able bodied controls and that the eyes closed condition challenged postural stability the most (Lemay et al., 2013). These results indicate that individuals with SCI have increased reliance on visual input to compensate for presumed deficits in their somatosensory system resulting from the injury.   !! 6!1.3 Cognitive control of upright stance Cognitive processing of standing involves the higher order integration of neural systems for sensory orientation, multi-joint coordination, environmental adaptation, and other functions that interact with the biomechanical constraints of the musculoskeletal system to accomplish postural orientation and equilibrium (Horak and Macpherson, 1996). Attention encompasses dynamic processes that involve the enhancement or selection of particular information and the inhibition of other information. Attention may be thought of as a mechanism that controls cognitive processing and can depend on our own goals or a novel stimulus that captures our attention away from the task at hand. Quiet standing requires attentional processes as can be seen by increased reaction times in individuals who are standing compared to when sitting (Lajoie et al., 1993; 1999; Maki et al., 2001). Dual task studies have revealed that increased dependency on attentional processes for postural control seems more apparent with changes in the central nervous system, such as with aging (Maki et al., 2001; Maylor et al., 2001), or becomes impaired after injury or disease, such as in vestibular disorders (Andersson et al., 2003), multiple sclerosis (Boes et al., 2012), stroke (Bourlon et al., 2014), or Parkinson’s Disease (Dromey et al., 2010). Such situations requiring a shift from an automatic, unconscious level of control to one relying more on conscious control may have detrimental effects on function (Beilock et al., 2002). This process of switching from an automatic to conscious motor control is termed “reinvestment” (Masters and Maxwell, 2008). Understanding the propensity for movement reinvestment may help researchers probe the attentional demands associated with maintaining upright stance in people with neurological injury.  !! 7!Dual task paradigms provide an opportunity to probe the automaticity of the control of quiet stance. Kerr et al demonstrated that the performance of a cognitive visuospatial task was modified if the subjects were asked to perform a difficult postural balance task at the same time. Subjects performed tandem standing concurrently with either the Brooks spatial task1 or a non-spatial task (memorizing number-word pairs). Results showed that maintaining a difficult stance affected memory recall in the spatial processing task but not the verbal control task. Spatial processing involves organizing visual information into meaningful patterns and understanding how they might change as they move or rotate through space. The findings indicate that visual information is an important factor in postural regulation, as revealed by the interference caused by the concurrent Brooks spatial task, and that cognitive spatial processing might rely on the same neural mechanisms required for postural control (Kerr et al., 1985).   Further evidence of cognitive involvement in postural control can be found in gait studies and research in the elderly. In one study of older adults, subjects took longer to complete the Timed Up and Go test (TUG) while carrying a full cup of water than during the single task condition (Shumway-Cook et al., 2000). In another study examining the effect of a reaction time task on postural control, individuals with incomplete SCI were asked to walk and respond verbally as rapidly as possible to an auditory stimulus. Researchers found that reaction times were significantly longer when walking than standing, particularly during the single support phase of gait (the phase with greatest postural challenge), indicating that subjects with SCI needed to allocate significantly more !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1!The!Brooks!Spatial!Task!tests!visual!short:term!memory,!requiring!subjects!to!imagine!and!remember!the!location!of!number:word!pairs!on!a!4x4!grid.!!! 8!attentional resources to walking (Lajoie et al., 1999). Other studies supporting this finding demonstrate that gait speed was most affected during mental tracking tasks, which index sustained attention and information processing speed, (eg. naming objects, walks while talks test) in people with neurological disorders (Al-Yahya et al., 2011) and healthy young subjects (Szturm et al., 2013). Together, these studies indicate that during complex tasks such as walking, individuals appear to require greater attentional resources in order to navigate their environment when engaged in suprapostural tasks that challenge their postural control.    There is also evidence that cognitive processing can affect reflex pathways during postural tasks. The H-reflex primarily involves the automatic monosynaptic pathway consisting of the Ia afferents and alpha motor neurons and is thought to reflect the neural excitability at the spinal level. In a study examining changes in H-reflex excitability during dual task performance, subjects were asked to respond to an auditory reaction time task while either lying prone or standing, with or without stimulation to the soleus muscle to elicit the H-reflex. The H-reflex was six times smaller when dual tasking compared to single tasking (Weaver et al., 2012). More unstable tasks such as standing or walking are also associated with a depressed H-reflex, presumably to minimize unintended, reflex-mediated contractions of the plantarflexors in order to maintain postural stability (Llewellyn et al., 1990; Solopova et al., 2003). Similarly, dividing attentional resources between two tasks leads to a reduction in these reflexive contractions consequently promoting a neural strategy that prioritizes posture (Weaver et al., 2012).  !! 9!Other forms of standing postural control, such as balance recovery from an external perturbation, also require cognitive processing (Maki and McIlroy, 2007). Feedback mechanisms where the central nervous system responds to information received during and after a movement are used to attempt to restore stability. Many reactive or feedback postural responses occur 100-200ms after an external perturbation which is later than the stretch reflex (>30-40ms) but earlier than can be generated voluntarily (>200ms). This is typically referred to as the automatic postural response and is observed at ~100ms. Early studies involving visuomotor tracking reported that attention was substantially diverted away from the tracking task when balance was disturbed. Subjects visually tracked a moving target on a computer screen while holding a rotary potentiometer. They had to maintain sitting balance during an unpredictable perturbation while tracking the target (McIlroy et al., 1999; Maki et al., 2001). Visuomotor tracking performance was measured using the root mean square (RMS) tracking error (i.e. the difference between target and the measured hand held potentiometer voltages). They proposed three distinct stages of compensatory reactions. The initial balance response to a perturbation was thought to be automatic as the tracking response was preserved. At approximately 200-300ms after the onset of the balance reaction, they observed a pause in tracking behavior (no measurable movement of the potentiometer), which was thought to represent a shift from automatic to attentional control of balance. Finally, they reported a period of divided attention between balance and tracking, reflected by inaccurate performance of the tracking task, until equilibrium was restored several seconds after the perturbation (McIlroy et al., 1999; Maki et al., 2001). These results indicate that competing demands for attentional resources appear to affect the later stages of balance reactions while the earliest postural !! 10!responses appear to be primarily automatic, unaffected by attention. Combined with the attenuation of the H-reflex, the body appears to choose a neural control strategy prioritizing posture under dual task conditions (Weaver et al., 2012).   1.4 Allocation of attention Woollacott & Shumway-Cook define attention as “the information processing capacity of an individual” and that this capacity is limited (Woollacott and Shumway-Cook, 2002). It is thought that dual task interference is proof of limited cognitive capacity (Watanabe and Funahashi, 2014). Postural control is thought to become more attentionally demanding as the postural environment is made more challenging and the secondary task more complex (Remaud et al., 2013). Under these dual task conditions, there may be more interference or competition between processes involved in cognition and postural control leading to a disruption of the automaticity of movements (as in quiet standing) to reinvestment in a more conscious form of movement control.   Higher order cognitive processes are known as executive functions (EF).  Executive function is a construct that encompasses many cognitive processes such as initiating and inhibiting actions, planning, working memory, and aspects of cognitive flexibility and judgment (Salthouse et al., 2009). These are divided into three distinct processes (Miyake et al., 2000). Set shifting is the ability to switch attention back and forth between tasks. Response inhibition is the ability to control or prevent automatic tendencies not relevant to the current task or environmental context. Working memory involves monitoring and coordinating incoming information by directing attention and priority to some tasks and !! 11!not to others. Since the EF is made up of several constructs, there is no single measure that can assess its entirety. For example a trial using three different types of tasks, each assessing a different component of EF, found that only set shifting was independently associated with a complex dual task version of walks while talks in elderly women (Ambrose-Liu et al., 2009). More recently, dual task gait trials have begun to look at the ability to flexibly allocate attention between tasks in elderly subjects. One study used a visuospatial task switch test to isolate the shifting and inhibition functions of the EF. They found that age did not account for the longer reaction times between balance impaired older adults and healthy older adults. Instead, they reported that age and balance impairments do contribute to differences on Berg Balance Scores between the two groups (Hawkes et al., 2012). The role of the EF and not just the type of task should be considered when choosing tasks for dual task paradigms.  Nearly all the suprapostural tasks in postural dual task paradigms involve one or more of the processes related to working memory. Baddeley’s model of working memory (WM) serves as a framework for understanding the allocation of attention in dual task paradigms. It consists of 3 components: the central executive which is involved in controlling attention; the phonological loop, which processes verbal and auditory information; and the visuospatial sketch pad (VSSP), which receives visual and visuospatial information (Baddeley, 2012). Examples of tasks testing the phonological system include auditory reaction times and mathematical tasks. Stroop and Brooks spatial tests are examples of tests of the VSSP while tasks such as random digit generation test the central executive. The central executive receives information from the VSSP and !! 12!phonological systems and then allocates attention to incoming visual or auditory stimuli so that the information may be processed or stored and acted on at a later time. Using this framework, we can suppose that if postural control is attentionally demanding, a secondary task occupying either the phonologic or visuospatial mode (or both) may affect the performance of one or both tasks.  There is a distinction between tasks that are perceptual in nature vs. suprapostural tasks that are cognitive tasks. Performance can be affected by some kind of perceptual contact (i.e. visual, auditory or kinesthetic) with the environment. Conversely, mathematical rehearsal or random digit generation tasks are examples of suprapostural tasks that do not maintain perceptual contact with the environment because such tasks do not require the postural system to facilitate gaze, orient to sound or maintain touch (Stoffregen et al., 2007). There are no claims about the automaticity of stance or capacity of mentation, but rather that two tasks are functionally linked if they are integrated based on common behavioral goals. Support for this comes from a study where healthy young subjects were asked to perform either quiet stance or a visuopostural alignment task (Mitra et al. 2013). The visuopostural alignment task consisted of keeping two targets, which were placed at different distances along the subject's line of sight, visually aligned. Subjects could achieve this by generating their own mediolateral sway. They were then asked to perform a spatial task consisting of making judgments about the relative locations of buildings from particular vantage points, and a non-spatial task consisting of answering questions about academic and operational aspects about the university. During performance of the spatial task, decreases in sway were observed only when performing the visuopostural !! 13!alignment task but not during quiet stance (Mitra et al., 2013). This seems to indicate that when two tasks are functionally linked, postural sway decreases in order to facilitate the performance of the suprapostural task (i.e. the body stabilizes so that the subject was able to make accurate location judgments). In another study (Stoffregen et al. 2007), researchers used a visually perceptive task where subjects had to detect subtle, critical signals in a visual display (line height and space differences) and a mathematical task consisting of sequential subtraction by 3s to study attentional processes on postural control. They found that postural sway was altered with the perceptive task, whereas no changes in sway were reported during the mathematical task, which tested the phonological system (Stoffregen et al., 2007). The difference between the two tasks was not that they measured different aspects of working memory, but that the visuospatial task was functionally linked to achieving standing postural control.   1.5 Current dual task frameworks Situating dual task study results within a framework helps researchers understand how concurrent tasks interact with one another. Here we briefly present three proposed frameworks currently cited in dual task paradigms.  The resource competition theory proposes that attentional resources are limited and that performing a given task uses up a portion of this limited information processing capacity (Woollacott and Shumway-Cook, 2002). If two tasks are performed concurrently, and they exceed the information processing capability, the performance on either or both deteriorates. Some authors argue for a single pool of attentional resources (Lajoie et al., !! 14!1993; Dault et al., 2001b) while others suggest multiple pools of attentional resources as in the case of structural interference (Kerr et al., 1985). For example, if both tasks require visual processing, such as a visual search task and upright standing, structural interference may occur in the visual system and performance of one task or both may suffer. Conversely, a task from the phonological loop may draw on a different processing area of the brain. Even if the two tasks are rated the same in subjective mental workload, this does not imply they drew on the same processing resources (Stoffregen et al., 2007). Difficulty of task is independent of the area of the brain being probed using a suprapostural task.   This framework assumes that the control of stance does require higher level processing despite its highly practiced nature. It suggests that cognitive demand is inherent in postural control, with healthy able-bodied people on one end of a continuum, where postural control requires very little cognitive demand. On the other end are people where postural control requires more attention, such as the elderly, people with Parkinson’s, stroke and SCI. With this framework, cognitive demand for postural control conflicts with cognitive demand associated with other tasks.  The bottleneck theory (Broadbent, 1958) proposes that attentional resources are also limited, similar to the resource competition model, and only a certain amount of information can be processed at a time, leading to a filtering of information. The idea is that parallel processing may be impossible for certain operations and that a single mechanism may be dedicated to their operation (Pashler, 1994). When two or more tasks !! 15!require attention, they pass through a sensory buffer where one task is selected, filtered and processed while the others are filtered out except for basic physical features (e.g. identifying the sex of the speaker or the type of tone) (Broadbent, 1958). The tasks that were rejected are stored briefly and either decay or are processed later. This may result in a delay of one of both tasks. Tasks may compete for access to one or more bottleneck mechanisms or resource pools associated with different stages of processing (Pashler, 1994). This is best illustrated by the dichotic listening task where a different three-digit number is presented to each ear. People tend to only repeat what they heard in one ear compared to the other ear. In fact, in the unattended ear, they were not able to report even a change in language (Cherry, 1953). From this information, Broadbent concluded that only one channel (one ear) could be involved in selective attention.   While these two theoretical frameworks can account for the broad range of results in dual task studies, they cannot explain why some researchers have found an improvement in postural control while performing a suprapostural task (Dault et al., 2001a; Stoffregen et al., 2007). A facilitatory view proposes that by minimizing postural sway, performance on the suprapostural task may be facilitated, particularly if it is a visual or visuospatial task. This decrease in postural sway is interpreted as a strategy to facilitate the execution of the suprapostural task rather than to improve postural control (Riccio and Stoffregen, 1988; Mitra and Fraizer, 2004).   This brings to question the problem of interpreting sway patterns in postural-suprapostural dual task paradigms. An increase in sway during a concurrent suprapostural !! 16!task should not be necessarily interpreted as postural destabilization. Likewise, a decrease in sway in response to a dual task cannot be automatically interpreted as improved postural control. In a recent study where postural sway was experimentally minimized, researchers observed an increase in CoP variability regardless if the subject was previously aware of how and when stabilization would occur (Murnaghan et al., 2013). These findings suggest that postural sway may be used by the central nervous system (CNS) as more of an exploratory mechanism continuously allowing inputs from multiple sensory systems for online information processing (Carpenter et al., 2010). Instead of interpreting sway only as an indicator of postural performance, it may be viewed as feedback used by the CNS to scale the amount of sway needed for postural stability in order to facilitate a suprapostural task goal during quiet stance.  More recently an adaptive resource-sharing framework has been proposed that recognizes that observed increases or decreases in sway can have their origins in either facilitation or stabilization actions or partly in both (Mitra and Fraizer, 2004). This framework considers that postural control is not considered autonomous but is constrained by the requirements of the dual task. The exact pattern of postural sway that is observed depends on the specific characteristics of the postural and suprapostural components. It predicts three patterns of results. The first is a facilitatory pattern where performance on the suprapostural task may be aided by postural adjustments such as standing and reading a sign at a distance. In this situation, sway reduction would facilitate reading the sign. The second is an autonomous sway minimization pattern where a threat to balance leads to complete cessation of the suprapostural task and prioritization of the !! 17!postural task known as the posture first principle (eg. stop reading when bumped on a bus to avoid falling) or when the suprapostural task is non-demanding while the balance task is challenging (eg. general inspection of a scene while standing at the edge of a curb). The third pattern is a hybrid where attentional demands for both suprapostural and postural tasks are high and the system cannot spare the capacity for facilitatory actions. The system is forced to share or prioritize between the two tasks, thus leading to the performance tradeoffs seen in resource competition. This framework proposes that dual tasks are not independent actions but rather are integrated into a single higher order skill. Therefore postural sway can either increase or decrease depending on suprapostural task goals and whether or not they are functionally linked to the postural task.  1.6 Task considerations Increasing evidence supports that postural control and cognition have common resource requirements, however, there is a great deal of variability among the results. Studies using dual task paradigms examining the reciprocal modulating effects of concurrent postural and suprapostural activities have shown that suprapostural task performance is decreased but only when postural control is excessively challenged in healthy young adults (Remaud et al., 2013; Woollacott and Shumway-Cook, 2002). Effects on the postural task by manipulating a concurrent suprapostural task has produced mixed results, variously reporting increased (Roerdink et al., 2006; Boes et al., 2012), or decreased sway (Cavanaugh et al., 2007; Swan et al., 2007), or leaving sway unchanged (Stoffregen et al., 2007). Some studies reported that increasing suprapostural task complexity led to increased postural sway (Pellecchia, 2003; Mitra et al., 2013; Remaud et al., 2013). !! 18!Similarly, increasing the difficulty of the postural task adversely affected postural sway (Kerr et al., 1985; Andersson et al., 2002). Postural sway has been observed to increase in older adults and populations with neurological pathology compared to young adults and controls. However, several studies have shown that postural sway decreased in older but not younger adults in conditions that especially taxed postural stability (Melzer et al., 2001; Swan et al., 2004). Taken together, the variability in the data across studies may be the result of intrinsic factors such as the baseline balance and cognitive abilities of the different populations, in addition to extrinsic methodological factors such as task novelty and difficulty, the environmental context, behavioral goals such as maintaining trunk and head alignment and postural equilibrium in space, (Horak and Mcpherson 1996), arousal and instructions. In people with SCI, we presume that cognitive abilities are unaffected, but argue that baseline balance ability as well as methodological factors, such as task difficulty, type of task, and the effect of arousal and instruction needs to be considered in understanding the attentional requirements of postural control in this population.  1.6.1 Balance ability The attentional demands on postural control can be affected by balance ability. For example, balance impaired older adults take longer to regain their stability after a perturbation than healthy older adults during a reaction time dual task (Brauer et al., 2001). Populations with decreased balance ability due to deficits in either their somatosensory, visual, or vestibular systems compensate by allocating greater cognitive resources to remaining systems to maintain postural stability. This was observed in a study where an increase in attentional demands for postural control in older adults was seen when visual and sensory information was withdrawn under six different sensory !! 19!conditions that changed the availability of visual and somatosensory cues for postural control (Shumway-Cook and Woollacott, 2000). Subjects were asked to perform a choice auditory reaction time task while standing on either a firm or sway referenced surface. The three visual conditions were eyes open, eyes closed and simulated visual motion (moving vertical lines). For healthy elderly subjects, postural sway increased by 29% in the sway-referenced eyes closed condition (the most difficult) during the dual task. For balance-impaired older adults, the addition of the auditory task resulted in a significant increase in postural sway under all sensory conditions (Shumway-Cook and Woollacott, 2000).  These results indicate that the sensory re-weighting of information may not always be successful in maintaining postural stability in individuals whose balance is challenged.  Manipulating the balance ability of otherwise healthy young adults can also be achieved by changing the sensory environment. Donker et al increased the difficulty of the postural task in healthy young subjects by having them stand with eyes closed. They found that decreasing the balance ability of subjects in this way resulted in a decreased ability to maintain postural control as reflected in the regularity of the CoP time series sample entropy (increased regularity corresponded to decreased postural control). During the dual task condition of reciting names spelled backwards (Simon becomes “nomis”), they found a significant increase in sway path length. They suggested that this represented an increase in attention to postural control and that reduced balance ability might imply an increase in the active monitoring of postural control thereby decreasing the “automaticity” of movement (Donker et al., 2007).  !! 20! Greater attentional resources may be required for postural stability in populations in whose balance is challenged. Andersson et al reported that there was an increase in the allocation of attention to the postural task of standing quietly (with or without calf stimulation) when challenged with counting backwards by 7s in vestibular patients (Andersson et al., 1998; 2003). It was reported that during the condition of calf stimulation and backwards counting, postural sway decreased without concurrent change in suprapostural task performance indicating prioritization of posture (Andersson et al., 2003). In another study, a dual task paradigm was used to examine the attentional requirements in individuals with multiple sclerosis during quiet standing. With the addition of a suprapostural task of word list generation (e.g. list animals; list words that start with “H”), sway area and RMS of CoP displacements were significantly higher than in the single task condition even in individuals with mild disability (Boes et al., 2012). People with SCI will also have sensorimotor deficits affecting balance. We have also already seen that individuals with SCI have greater reliance on visual input to maintain quiet stance compared to controls (Lee et al., 2012; Lemay et al., 2013), and so may expect that dual-task paradigms should further reveal their attentional requirements on postural control.   1.6.2 Difficulty of the suprapostural task The characteristics of the suprapostural task are very important in analyzing the results of dual task paradigms. With a sufficiently difficult suprapostural task such as a combined motor and suprapostural task (counting backward in 7s out loud) vs. a suprapostural task alone (counting backward in 7s silently), measures of postural sway were significantly !! 21!increased compared with performance of the motor and suprapostural tasks on their own (Swanenburg et al., 2009). However, there is also evidence that greater diversion of attention away from the postural task (by increasing the difficulty of the suprapostural task) could actually improve postural control. Swan et al manipulated the level of difficulty of the Brooks spatial task under increasingly challenging postural conditions. They reported that increasing difficulty of the suprapostural task was the critical factor that led to decreased postural sway, not the level of difficulty of the balance task (Swan et al., 2007). They postulated that the more difficult suprapostural task directs attention away from balance related cues and prevents overcorrection, leading to improved postural control (Swan et al., 2004). These results are consistent with the notion that for well-learned, automated motor tasks, attending to the task can actually be a decrement to performance (Beilock et al., 2002), and intersects with issues raised in the previous section around the baseline balance ability of the population under study.  People with SCI whose injury has disrupted the intrinsic processes of postural control must relearn to stand again, and so we presume that the control of quiet stance is not as automated as that in able-bodied controls. Whereas in the healthy nervous system, the addition of a difficult suprapostural task may result in an improvement in postural control due to a distracting effect, for people with neurological injury, a difficult suprapostural task might direct attention away from the balance task and consequently degrade postural performance. Indeed, in people with multiple sclerosis, an increase in sway area and anteroposterior displacement was observed with the addition of a word list generation task, and this effect was associated with the level of disability (Boes et al., 2012). !! 22!1.6.3 Difficulty of the postural task  The characteristics of the postural task will also affect postural control and suprapostural task performance. Postural task alterations usually involve altering the base of support (Dault et al., 2003; Mitra and Fraizer, 2004; Van Daele et al., 2010), restricting vision (Donker et al., 2007; Sherafat et al., 2014), or stimulating the calf muscles (Andersson et al., 2003; Hwang et al., 2013) or a combination of these (Dault et al., 2001b; Cavanaugh et al., 2007; Swan et al., 2007; Remaud et al., 2013). Generally, the more novel and difficult the postural task, the more postural sway is increased (Dault et al., 2001b; Remaud et al., 2013). Dual-task paradigms further reveal greater number of errors in supra-postural tasks as the postural task is made more challenging (e.g. standing on one foot to tandem Rhomberg) (Kerr et al., 1985). In people with SCI, there is increased postural sway in standing with eyes either open or closed, compared to able-bodied controls (Lemay et al., 2013). Thus, depending on the level and severity of injury, simple unassisted quiet standing in people with SCI may be sufficiently challenging to affect postural control.  It is suggested that the difficulty of both the postural and suprapostural task load be quantifiable, scaled by experimental manipulation and confirmed by subjects’ performance (Fraizer and Mitra, 2008). Information reduction tasks such as combining two single digits and then classifying them as high (>50) or low (<50) and odd or even, yield a 4.5 bit reduction (Pellecchia, 2003) and visual search tasks by altering the number of items in the search display (Mitra and Fraizer, 2004) are examples where suprapostural task load may be quantified. Altering visual cues such as eyes open and closed are examples where postural task may be quantified. Tools such as the National Aeronautics !! 23!and Space Administration Task Load Index (NASA-TLX) (Hart and Staveland, 1988) may be used to measure subjective mental workload of a task. Thus it is important to quantify task workload in order to adequately compare across conditions in dual task paradigms.  1.6.4 Types of task Dault et al examined the effects of three types of working memory (WM) tasks (central executive, phonological loop and two levels of difficulty for visuospatial sketch pad, VSSP) under different challenges to postural control (sitting, standing, and tandem stance) (Dault et al., 2001a). They reported no change among the different WM tasks or different levels of difficulty of the VSSP on postural control. Moreover, they reported no change in the performance of WM tasks during the different postural positions. Overall, the addition of any WM task, regardless of task type or difficulty resulted in a change in postural sway characterized by decreased amplitude and increased frequency, which they suggested might indicate a tighter control strategy utilized by the CNS in order to minimize the amount of sway. Conversely, in young adults with ankle injury, the modified manikin test2, a task which stresses the VSSP, was observed to significantly affect postural time to boundary measures (time series of the directional CoP data points to reach the border of the foot), compared to tasks which tested the central executive and phonological loop (Burcal et al., 2014). It appears that the type of task may not be as important as whether or not the task interferes structurally (VSSP tasks) or is functionally linked to the postural task in order to facilitate completion of a goal (see also section 1.4).  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!2!The!modified!manikin!test!requires!subjects!to!identify!the!hand!of!a!rotating!stick!figure!holding!a!white!circle!as!quickly!and!accurately!as!possible.!!! 24!1.6.5 Focus of attention and instruction The specific instructions (or not) given to subjects during dual task paradigms are also critical to the analysis of how attention contributes to task performance. A study that examined the effects of three types of suprapostural tasks on postural control reported decreases in postural sway when instructions to focus on either the suprapostural or postural task were given. Moreover, they found the greatest improvement in postural performance when instructions to focus on the suprapostural task were given (Burcal et al., 2014). However, one study that did not give any explicit instructions found that postural sway was unaffected by suprapostural tasks in both controls and vestibular patients (Yardley et al., 2001). When task difficulty was manipulated, increased postural sway was observed when instructions were given to provide equal weighting of attention to both the postural and suprapostural task (Redfern et al., 2004) while another trial found no changes in postural sway between different types of tasks and no change in suprapostural task performance when no instructions were given (Dault et al., 2001a).  However focus of instruction alone cannot account for all the findings. Some studies did instruct subjects to consciously intervene in their postural control by minimizing sway and found no change in the vertical projection of CoG, which indexes the performance of the postural task (Vuillerme et al., 2007). Other studies did not give sway minimization instructions and still found significant effects of the suprapostural task on postural control (Pellecchia, 2003). The variability among the data may lie in which task the focus of attention is on and the type of instructions given.  Giving explicit instructions allows researchers to address the capability of subjects to flexibly allocate attention between postural and suprapostural tasks (Siu and Woollacott, !! 25!2007). In vestibular patients, instructions to focus on the balance task increased postural sway during the dual task condition compared to the no instructions condition (Andersson et al., 2003). Conversely, instructions to focus on the balance task decreased anteroposterior sway compared to focusing on the reaction time task in healthy young individuals (Remaud et al., 2013). However, another study reported an increase in postural control (higher time to boundary means for mediolateral and anteroposterior as well as mediolateral standard deviation) with instructions, regardless of foci (balance or suprapostural task) relative to a no instruction condition (Burcal et al., 2014).   Focusing attention on balance might benefit people with SCI who are learning to stand again. One study used a virtual reality environment to provide online feedback about CoP displacement during standing in people with SCI. After 12 sessions of training, participants showed significant improvement in dynamic balance performance (Sayenko et al., 2010). Another study that supported increased attention to postural control examined explicit attentional focus on dribbling performance in soccer players. Researchers found that novices seem to benefit from online step-by-step instruction (Beilock et al., 2002). These findings are consistent with the concept that when a movement becomes a well-learned skill, the focus of attention may no longer benefit its performance (Baumeister, 1984; Wulf and Prinz, 2001; Beilock et al., 2002). This is exemplified in a study examining focus of attention (awareness of hands) on skilled performance where subjects were asked to perform a roll up task. The task consisted of a ball that was controlled and balanced on 2 rods held in each hand. The subject’s goal was to move the rods apart and drop the ball into one of the holes on the platform below the !! 26!rods. There was reduced performance (decreased points in one minute) when attention was focused on the hand movements (Baumeister, 1984). Similarly, by focusing attention away from balance, a decreased level of interference in postural control strategies may lead to improved postural stability (Fraizer and Mitra, 2008).   The type of instruction may also play a role in explaining some of the variability in results (McNevin and Wulf, 2002; Wulf et al., 2004) . Instructions that direct the participant’s attention to the effects of their movement (external focus) are more beneficial to postural performance than directing attention to their own movement (internal focus) (Wulf and Prinz, 2001; Wulf et al., 2004). For example, Wulf et al. asked subjects to stand on a rubber disc while holding a pole horizontally with elbows flexed to 90 degrees. They were asked to either minimize movements of the feet (postural task, internal focus), minimize movements of the disc (postural task, external focus), hold hands still (suprapostural task, internal focus), or hold pole still (suprapostural task, external focus) (Wulf et al., 2004). They reported that the magnitude of sway (RMS error) on the postural task was greater with internal (hands and feet) than with external (disc and pole) focus of control.   There is further evidence that providing internal focus instructions such as “minimizing sway” could degrade the postural performance during a concurrent task compared to no instruction at all (Wulf et al., 1998). A “constrained action hypothesis” was proposed to account for such observations (Wulf and Prinz, 2001; McNevin et al., 2003). It proposed that subjects who internally focus on body movements would actively intervene in the !! 27!maintenance of a stable posture. Trying to consciously control one’s movement would constrain the motor system and interfere with the automatic processes needed for the normal regulation of a well-developed movement skill (Baumeister, 1984; Wulf and Prinz, 2001; Beilock et al., 2002). Conversely, focusing on the external movement effect would allow the motor system to more naturally self organize, unconstrained by interference caused by conscious control. Support for the constrained action hypothesis was found in a study where subjects were asked to increase active attention to postural control and reduce their body sway while standing on a force platform and respond as fast as possible to an auditory stimulus (Vuillerme et al., 2007). It was reported that the internal focus on body movements compared to no instruction resulted in an increased level of neuromuscular activity in the lower limb yet no change in postural performance. The authors concluded that internal focus of instructions hampered the efficiency for controlling posture, and is consistent with the notion that reinvestment of postural control will have detrimental effects on balance (Beilock et al., 2002; Donker et al., 2007).  1.6.6. Perceptual factors and postural control Although postural control is normally attributed to physiological factors, psychological perceptual factors such as fear of falling and balance self-confidence have been attributed to postural control changes (Carpenter et al., 2006, Huffman et al., 2009). One study investigating the effect of elevated postural threat (standing on a 3-meter platform) on postural control measures reported that task specific changes in self-efficacy (confidence), state anxiety (fear) and blood pressure were related to changes in postural strategy, specifically a stiffening strategy characterized by a decrease in amplitude and an increase in the frequency of postural adjustments (Carpenter et al., 2006). While fear of !! 28!falling and balance self-confidence are associated, they are separate constructs with different effects on postural performance (Carpenter et al., 2006; Li et al., 2002). For example, in quiet standing, state anxiety was highly correlated to changes in amplitude and mean position of CoP measures whereas balance efficacy was correlated to changes in frequency (Carpenter et al., 2006). Another study reported that self-efficacy acts more as a mediator to the effects of fear of falling on functional balance measures such as the Berg Balance Scale (Li et al., 2002). Perceptual factors of postural control appear to reflect the cognitive features of upright stance and should therefore be considered in dual task paradigms involving balance-impaired populations.  1.7 Rationale Postural control is a complex integration of multiple neural systems involving cognition, vestibular, visual and sensorimotor processing. It is a dynamic interaction among context and task specific neural behaviors. Researchers have been able to use dual task paradigms to examine the effects of attention on postural control. To-date there has been a great deal of focus of dual task interactions in other populations while there has been little research on people with SCI. With improvements in acute care and rehabilitation, people with SCI are increasingly able to improve their mobility and standing balance. However, considering that nearly all activities of daily living involve some form of dual tasking of balance and a suprapostural task, the importance of how attention is allocated during these behaviors cannot be overlooked.  !! 29!The overall objective of this study was to explore the effects of attentional requirements of postural control in people with SCI. We selected two tasks from the phonological loop because we were not trying to probe different parts of the brain with this study, but rather capture the heterogeneous nature of our SCI cohort. Changes in reaction time have been shown to be affected by increased balance requirements from sitting to standing to walking in healthy young subjects (Lajoie et al., 1993). This method could give an indication as to how people with SCI will allocate their attention to balance without needing to rely on sway measures, since we anticipated that SCI subjects would not be able to stand for the recommended minimum stance duration for force plate analysis (Carpenter et al., 2001; van der Kooij et al., 2011). For higher-functioning individuals with SCI, the mathematical task provides information about how attentional resources are allocated, while also providing an opportunity for detailed analysis of changes in sway characteristics in subjects who are able to stand for at least 60 seconds at a time.  Both of the suprapostural tasks we selected have been shown to effectively degrade postural stability in healthy older and young adults (Pellecchia, 2003; Swanenburg et al., 2009; Van Daele et al., 2010; Remaud et al., 2013). They both test the phonological loop of working memory. Because they do not test visual or visuospatial inputs, they were not anticipated to cause any structural interference during quiet stance (Kerr et al., 1985; Maylor et al., 2001).  !! 30!We hypothesized that attentional processes have a greater role in postural control of people with SCI compared to able-bodied controls. The specific aims of this project were to: 1) identify the effect of an auditory reaction time task on standing balance performance in people with SCI and able-bodied controls (Experiment 1)  2) compare the effect of a cognitively demanding mathematical dual task vs. a single task on postural control in people with SCI and able-bodied controls (Experiment 2)  3) explore if the addition of a backwards counting task would result in higher movement reinvestment and lower Berg Balance scores in people with SCI 4) examine if dual task costs of performance measures such as reaction time, total standing time, error ratio, number of words uttered, movement reinvestment and CoP parameters would be associated with greater deficits in activities of daily living, self perceived performance and satisfaction in daily activities, and functional balance clinical measures in SCI subjects.  Hypothesis 1 People with SCI would show greater differences in performance between single and dual task conditions than able-bodied controls. Specifically, people with SCI would show greater differences between single and dual task conditions in reaction time (ΔRT) (Hypothesis 1a), quiet standing time (ΔST) (Hypothesis 1b), error ratio (ΔER) (Hypothesis 1c), total number of words uttered (ΔTot) (Hypothesis 1d), movement reinvestment (ΔMSRS) (Hypothesis 1e) and CoP parameters (ΔCoP) (Hypothesis 1f), !! 31!compared to able-bodied controls. The addition of a math task dual task would result in higher movement reinvestment and lower Berg Balance scores in people with SCI (Hypothesis 1g).  Hypothesis 2 We hypothesized that among people with SCI, postural or suprapostural task performance under dual task conditions would be associated with greater deficits in clinical measures. Specifically we hypothesized that dual task costs of reaction time, standing time, error ratio, total number of words uttered, movement reinvestment and CoP parameters would be associated with greater deficits in balance function (Hypothesis 2a), decreased independence (Hypothesis 2b), and decreased satisfaction and perceived performance in activities of daily living (Hypothesis 2c).    !! 32!2 METHODS 2.1 Participants Nine adult subjects with motor incomplete SCI (ASIA impairment scale C [n=5] and D [n=4]) and eight age- and sex-matched able-bodied controls (AB) were recruited to participate in this study (Table 1 & 2). Inclusion criteria included individuals who had completed at least 2 weeks of hospital rehabilitation involving sitting and standing balance and who were medically stable. Subjects had to be able to stand for at least 30 seconds unassisted without the use of hand-held aids though the use of orthoses was permitted. Exclusion criteria included presence of cognitive or visual impairments, inability to follow instructions, orthopedic or neurological problems (other than SCI), or cardiovascular dysregulation (e.g. orthostatic hypotension) that could interfere with postural control.   All procedures were approved by the institutional research ethics board and all participants provided written informed consent prior to any experimental procedures.  2.2 Postural conditions 2.2.1 Sitting For the sitting condition, subjects sat supported in a chair, eyes focused on a target 2 meters in front with hands in their lap.   !! 33!2.2.2 Standing Subjects were asked to stand barefoot in their preferred position on two Bertec force platforms (Bertec FP 4550, Bertec Corporation, Columbus, Ohio, USA), looking straight ahead at a target 2 meters away and with their arms by their sides. Two force plates were chosen so that future analysis might focus on percentage of weight bearing per limb.  Subjects stood between two parallel bars so that they could grab them in case of balance loss. Analog signals from the force plates were streamed to a data acquisition system at 100 Hz using custom-written software in Labview 8.6. Subjects’ performance was videotaped for off- line analysis to measure standing time.  For all standing trials, the subject was asked to focus on balance with instructions to “try to keep your balance as best as you can”. To ensure consistent foot placement between trials, outlines of the feet were traced on the force platform using adhesive tape and landmarks were identified. Commencement of data collection occurred approximately 10-20 seconds after achieving standing balance in order to account for transient effects seen in the first 20 seconds of quiet stance (Carpenter et al., 2001).  2.3 Experimental tasks 2.3.1 Reaction time task (Experiment 1) For the reaction time task, participants wore a noise-cancelling headset with a microphone (Monster, Inspiration, Brisbane, CA, USA) attached to a computer. A custom-written software written in Matlab (Mathworks, Natick MA) was used to produce a 50-ms auditory stimulus (1000Hz) that was delivered at random intervals between 1-3 seconds. Participants were instructed to respond verbally with the word “top” as fast as !! 34!possible to the auditory stimulus. Single task reaction time task performance was recorded while subjects were seated in a chair. For the dual task condition, subjects were asked to perform the reaction time task while standing. Participants completed four 30-second trials of each condition.   2.3.2 Math task (Experiment 2) Subjects were asked to count out loud and backwards by 3’s starting from a random number provided by the researcher. Single task math task performance was measured in sitting (sit and count). For the dual task condition, subjects were asked to stand and perform the postural and math task concurrently with eyes open. We collected data for three 2-minute trials, or for SCI subjects, for as long as they could stand unassisted without touching the parallel bars or moving their hands from their sides since disturbing balance and touching can change the focus of attention during a dual task (McIlroy et al., 1999; Maki et al., 2001; Stoffregen et al., 2007).  2.4 Supplementary experiment The SCI subjects exhibited little difficulty with standing postural control, so we asked subjects to complete the standing and math task protocol with their eyes closed in order to challenge their postural control further (LeMay et al., 2103). For this condition, subjects first achieved quiet standing, and then closed their eyes. Data was collected until the subject opened their eyes or until their hands left their sides or touched the parallel bars.    !! 35!In all, there were 7 different experimental trials combining: a) 2 suprapostural tasks: math task and reaction time task b) 2 vision conditions (for the math task only): eyes open (EO), eyes closed (EC) c) 2 postural conditions: stand and sit The presentation of these 7 conditions was randomized (Table 3).   Consequently, each subject (as able) performed a total of 15 math and 8 reaction time task trials with a total of 16 standing trials between the two tasks. Rest intervals of 10 minutes were provided between trials to decrease the effects of fatigue while providing the time to complete the clinical and functional measures.   2.5 Measures 2.5.1 Reaction time Reaction time (in milliseconds) was measured from the start of the auditory stimulus to the start of the verbal response. The start of the verbal response was defined as the time when the audio signal exceeded the threshold of 5 standard deviations above the mean ambient room noise. This was also confirmed visually (Figure 1). Reaction times that were longer than 3 standard deviations above the mean were discarded and the remaining reaction times were averaged for each condition (Remaud et al., 2012).       !! 36!Figure 1 Reaction time processing  Figure 1: Raw audio signal from the microphone during a representative reaction time trial. Reaction time was measured as the latency, in milliseconds, from the start of the stimulus (red ×) to the start of the verbal response (green ×). The start of the verbal response was defined by the time when the audio signal exceeded 5 standard deviations above the mean ambient room noise (horizontal red line).  2.5.2 Standing time Standing time (seconds) was measured from the single- and dual-task conditions of the math task trials. (Thus, the maximum standing time possible was 2 minutes.) Standing time was defined by the start of the trial until the subject was seen to lose their balance (e.g. arms moved from their sides, touched the parallel bars, or when they opened their eyes).   !! 37!2.5.3 Math task error ratio and total words uttered Subjects were audiotaped for off-line analysis and the recordings were reviewed to track the number of correct and incorrect responses. Error ratios (ER) were calculated as the ratio of the number of incorrect responses to the total number of words uttered (Pellecchia, 2003). The total number of words uttered included any corrections made. Average error ratios and total words uttered over the allotted 2-minute trials were calculated for each subject and then averaged across all participants. Note that for the standing eyes closed condition, not all the SCI participants were able to complete the allotted 2-minute counting time. Therefore, for this condition, the first 60-seconds of counting were used for the analysis. Three subjects were unable to stand with their eyes closed for at least 60 seconds, so their data were excluded from this part of the analysis. Participants whose first language was not English were permitted to count in their preferred language, which was then translated by the author.   2.5.4 Movement Specific Reinvestment Scale The tendency to switch from automatic to conscious control of movement was measured by the Movement Specific Reinvestment Scale (MSRS) (Masters & Maxwell, 2008). The MSRS consists of 10 items that relate to concerns about the style of movement and conscious attention to the process of movement; movement self-consciousness, and conscious motor processing respectively. It has been shown to have acceptable test–retest reliability (r =0.76 and 0.67, respectively) and internal reliability (r =0.71 and 0.78, respectively) (Masters et al., 2008). We modified the questionnaire to reflect standing balance state specific changes by substituting the words “movement” with “standing” !! 38!(Appendix A). Increased reinvestment corresponds to a higher score out of a total possible score of 36.  2.5.5 Center of Pressure (CoP) Signals from the force plates were processed offline using a dual-pass Butterworth filter with a low pass cut off frequency of 5 Hz using custom-written routines in Matlab (Mathworks, Natick, MA). When a 60-second standing bout was achieved, root mean squared (RMS), mean power frequency (MPF) and CoP mean velocity (MVEL) in the mediolateral (ML) and anteroposterior (AP) directions were calculated. MVEL (mm/s) is the average velocity of the CoP in the AP and ML directions and is associated with the amount of corrections made in regulating postural steadiness (Prieto et al., 1996). RMS (mm) estimates the size of the path traveled by the CoP over the support surface (Rocchi et al., 2004) and has been related to the effectiveness of the postural control system (Prieto et al, 1996). MPF (Hz) is proportional to the ratio of the MVEL to the mean distance (the average distance from the mean CoP) (Prieto et al., 1996). MPF is associated with stiffness of the postural control system and may be indicative of a tighter control of postural sway (Carpenter 2001, Winter 1998 from Remaud 2012). Both MVEL and RMS were shown to have good test-retest reliability in postural-suprapostural dual task paradigms (Moghadam et al., 2011). No CoP measures were analyzed if the trial lasted less than 60 seconds since important low frequency components of the signal may be missed (Carpenter et al., 2001, van der Kooij 2011).  !! 39!2.5.6 Perceptual measures  Subjective mental workload was assessed using the National Aeronautics and Space Administration Task Load Index (NASA-TLX) (Hart and Staveland, 1988). We used the online version NASA-TLX beta (Sharek, 2011). An overall workload score is calculated based on a weighted average of six subscales; Mental Demand, Physical Demand, Temporal Demand, Own Performance, Effort, and Frustration. Each contributes differently to the overall workload score depending on the task being evaluated. Subjects were asked to rate each subscale from very low to very high corresponding to how much each subscale contributed to their overall subjective workload after each block of sitting and standing trials (Appendix B). The overall workload score was then calculated based on a numerical scale of 0-100 (0 least amount of workload, 100 highest amount of workload) (Hart, 1998).  Balance self-confidence was assessed prior to each standing trial, while fear was assessed after each trial on a scale of 0-100% (0 least confident/fearful, 100 most confident/fearful). We asked them to “rate your confidence from 0-100% with 0% being least confident and 100% being completely confident on how confident you feel you can maintain your balance and stand”. For perceived fear we asked “please rate how fearful you were of falling during the standing trial from 0-100% with 0% being not fearful and 100% being completely fearful” (Huffman et al., 2009).    !! 40!2.5.7 Clinical and functional measures (SCI subjects only)  The Berg Balance scale (BBS) was used to assess functional balance. We also asked the subject to perform the BBS while subtracting backwards in 3s out loud (BBS-DT). The Berg Balance Scale is comprised of 14 items, testing the ability to maintain positions, assessing different transfers and the ability to maintain balance while voluntary tasks are performed (Appendix C). Each item is scored 0-4 with a score of 0 indicating the lowest level of function and 4 the highest, giving a total score of 56. The scale has shown good concurrent validity in people with SCI (Lemay and Nadeau, 2009). The internal consistency of the scale is very good and it has been shown to possess very good inter-rater (0.98) and intra-rater (0.97) intraclass correlation coefficients (Berg, 1989; Berg et al., 1995).  The level of independence in activities of daily living was assessed using the Spinal Cord Independence Measure (SCIM). Total inter-rater agreement on the various individual SCIM items range from 72-99%. Total agreement was higher than 85% (Kappa coefficients: 0.66-0.98). Correlation of the SCIM is excellent with the Functional Independence Measure (Spearman’s r =0.80) and the Walking Index for Spinal Cord Injury (Spearman’s r =0.97) (Catz et al., 1997). The total SCIM score ranges from 0-100. Higher scores reflect higher levels of independence (Appendix D).    Self-perceived performance and satisfaction in daily activities related to balance and postural control were assessed using the Canadian Occupational Performance Measure (COPM) (Law et al., 1990) (Appendix E). The assessment consists of an interview focused on identifying activities that a participant wants, needs or is expected to perform. !! 41!Adequate test re-test reliability in adults with impairment of one or more activities of daily living has been reported (ICC =.67 performance and .69 satisfaction) (Eyssen et al., 2005-get). Scores range from 1-10, where 1 indicates poor performance and low satisfaction and 10 indicates good performance and high satisfaction.  The Montreal Cognitive Assessment (MoCA) was used to assess mild cognitive dysfunction. It assesses different cognitive domains: attention and concentration, executive functions, memory, language, visuoconstructional skills, conceptual thinking, calculations, and orientation. A range of 19-25.2 indicates mild cognitive impairment while a score of 26 or above is considered normal out of a total possible score of 30 points (Nasreddine et al., 2005) (Appendix F).  2.5.8 Dual-task cost To assess the effects of the postural and suprapostural conditions during dual tasking, a relative measure of change, the dual task cost (DTC), was calculated for each parameter (Siu and Woolacott 2007, McCullough). Dual task costs for outcome measures were calculated as follows:   Dual task cost = [dual task performance-single task performance] x 100 single task performance  For our outcome measures, with the exception of number of words uttered, a positive value represents a decrease in performance under the dual task condition (presence of a dual task cost) whereas a negative value corresponds to an improvement in performance in the dual task condition. !! 42! A summary of the sequence of data collection for the seven conditions is presented in Table 4.  2.6 Statistical analysis All data were analyzed using SPSS statistical software (version 21 for Mac, SPSS Inc, Chicago, IL, USA). The critical value for significance for all statistical tests was set at an alpha value of 0.05 and trends were defined at alpha of 0.10. Reported values are mean ± standard deviation.  2.6.1 Subject characteristics Subject characteristics were summarized using descriptive statistics. An independent t-test was performed to determine whether there were any significant differences in age, height, or weight between the SCI and AB control subjects.  2.6.2 Dual task effects on primary and secondary performance measures A two-way repeated measures of analysis of variance (ANOVA) (group x task) was used to elucidate the between groups (SCI, AB) and task effects (single vs. dual task) of primary measures: reaction time task (eyes open only), standing time, math task performance (error ratio and total number of words uttered) and movement reinvestment, for both eyes open and eyes closed conditions. Normality of distribution was checked using the Shapiro-Wilk test. When the sphericity assumption in the repeated measures ANOVA was violated (Mauchly’s Test), a Greenhouse-Geisser correction was used. !! 43!  We also performed a repeated measures two-way multivariate analysis of variance (MANOVA) on the CoP measures. Because their raw data were not normally distributed, a log transformation was performed on the following secondary dependent measures: MVEL, RMS and MPF in the AP and ML directions, after which normality was then established. Where statistical significance was established for the multivariate tests, we divided the alpha level of 0.05 by the number of CoP parameters tested (6) to establish an alpha level of 0.008 for the univariate tests. This was to reduce the possibility of Type 1 error. We used the data for a 120 second bout for eyes open since all SCI subjects could stand for the maximum amount of time, and 60 seconds for eyes closed.   The effect size (d) for the DTC of the primary and secondary performance measures were calculated with a power of 0.95 using G* Power (Version 3.1). An effect size of 0.2-0.5 is considered small, 0.5-0.8 medium and ≥0.8 large (Cohen, 1977).   For SCI subjects only, the Wilcoxon signed rank test was used to compare single and dual task performance between BBS vs. BBS-DT.  2.6.3 Dual task effects on perceptual measures A two-way repeated measures ANOVA (group x task) was performed on fear, confidence, and mental workload (NASA-TLX) for the reaction time task and eyes closed math task. A 2 x 3 way ANOVA was performed to evaluate the NASA-TLX across the three math task (eyes open) conditions (sit and count, stand and stand and count). !! 44!The effect size (d) of the DTC of perceptual measures was calculated with a power of 0.95.  2.6.4 Associations between performance and functional measures (SCI only) There was no statistical difference between the BBS and BBS-DT, therefore only the BBS was used for correlation analysis. To assess if functional measures were related to balance performance, the dual task cost of primary performance measures (standing time, reaction time and error ratio, total number of words uttered, movement reinvestment) was compared to current clinical indicators of function: BBS, SCIM and COPM performance and satisfaction using Spearman’s coefficient.   2.6.5 Associations between chronicity, cognitive impairment and performance measures (SCI only) To determine if the number of years post-injury (chronicity) was related to the DTC of primary performance measures, Pearson’s product moment-correlation was used. Spearman’s coefficient was used to associate cognitive impairment (MoCA) with DTC primary performance measures. ! !!! 45!3 RESULTS  3.1 Subjects characteristics In total, 17 subjects (9 SCI and 8 AB controls) participated in the study. Parametric tests  revealed no statistically significant difference between the two groups for age, height, or weight, p >.05 (Table 1).  3.2 Reaction time task performance (Experiment 1) There were no statistically significant effects of task, group, or interaction effects for the reaction time task (Table 5A). Overall, the SCI group tended to have longer reaction times in both conditions (p < 0.10). There was also a trend for reaction times to be longer during the standing (dual task) for both groups (p < 0.10) (Table 5A; Figure 2A). The DTC of reaction time for both groups were almost identical (SCI: 10.90 ± 23.87%; AB: 10.67 ± 15.02%; d = 0.01).  For perceived mental workload, there were no statistically significant differences between groups. There were no main effects for task or interaction effects (Table 5B). Moreover, both groups rated the mental demand of the reaction time task quite low (Table 5B; Figure 2B). The effect size was also quite small for the DTC of mental workload (SCI: 42.84 ± 84.58%; AB: 101.55 ± 360.68%; d = 0.22).   !! 46!Figure 2 Reaction time and perceived mental workload between SCI and AB groups A) Reaction time     B) Subjective mental workload   Figure 2A: Comparison for reaction time in milliseconds between SCI and AB groups for the single task of sitting and reaction time, and the dual task of standing and reaction time. Figure 2B: Subjective mental workload for the reaction time task. Both groups performed equally well and rated the mental workload the same p>0.05.   3.3 Math task performance (Experiment 2) There were no statistically significant main or interaction effects for math task performance (error ratio and total number of words uttered) between the SCI and AB groups and between single and dual task conditions for either eyes open (Table 6; Figure 3A) or eyes closed (Table 6; Figure 3B). For eyes open, the effect size for the DTC of total words uttered and error ratio respectively, were small (SCI: -3.85 ± 14.04%; AB: 1.64 ± 13.90%: d = 0.39), (SCI: -1.85 ± 101.87%; AB: -17.26 ± 44.93%; d = 0.20). For eyes closed, the effect size for the DTC of total words uttered was moderate (SCI: -5.42 ± 11.70%; AB: 2.34 ± 17.58%: d = 0.53) while the DTC of error ratio was small (SCI: 54.07 ± 318.05%; AB: -18.50 ±67.35%; d = 0.32).  Reaction time (msec)ABSCI4003002001000NASA-TLXsingle task1007550250dual taskABSCI!! 47!Four of the 9 SCI subjects could not complete the dual task of standing and counting with eyes closed for the allotted 60 seconds of standing time (Subjects 1, 2, 4 and 6) (Table 2), therefore their data were not included in eyes closed math task performance analysis.  Figure 3 Math task performance between SCI and AB groups  ! Figure 3: Comparison between math task performance (error ratio [left panel] and total number of words uttered [right panel]) during the single task of count and sit (white bars) and the dual task of count and stand (grey bars) for A) eyes open, counting for 120 seconds and B) eyes closed, counting for 60 seconds. The SCI group performed equally as well as the AB group in all conditions (p>0.05). !!  !! 48!3.4 Standing time performance (math task) All SCI subjects were able to stand for the maximum trial duration of 2 minutes during the eyes open condition for both single and dual task conditions.  Seven of the nine SCI subjects were able to stand with eyes closed. For total standing time with eyes closed, significant interactions between task and group were observed (F1,15 =5.4, p =.034, partial η2 =.266) (Table 7). SCI subjects stood for 21% shorter duration during dual task (stand and count) than the single task (stand) compared to AB controls (Figure 4). The effect size was large for the DTC of standing time (SCI: -26.15 ± 25.64%; AB: 0 ± 0%; d = 1.44).  Figure 4 Standing time eyes closed between SCI and AB groups !Figure 4: Comparison between SCI and AB groups for the single task of standing (red line) and the dual task of standing and counting (grey line). There was a significant interaction effect (p <0.05). Total standing duration with eyes closed was shorter among the SCI subjects in the dual task condition compared to the AB controls.  ABSCITotal standing time eyes closed (sec)120100806040dual task (stand + count)single task (stand) !! 49!3.5 Movement reinvestment Movement reinvestment as measured by the MSRS revealed statistically significant differences between groups for eyes open (F1,15 =72.6, p <.001, partial η2 =.829) and eyes closed (F1,12 =47.6, p <.001, partial η2 =.799) conditions (Table 8). SCI subjects reinvested 252% more conscious attention to postural control compared to AB subjects when standing with eyes open (SCI =27.11 ± 6.44; AB =7.69 ± 6.84) (Figure 5A). Similarly, with eyes closed, SCI subjects reinvested 234% more attention to postural control compared to AB subjects (F1,12 =47.6, p <.001, partial η2 =.799; SCI =25.42 ± 7.29; AB =7.63 ± 6.31) (Figure 5B). It is interesting to note there was little difference in the mean MSRS scores between eyes open and eyes closed for SCI subjects, indicating they did not appear to reinvest more conscious control of posture in the eyes closed condition (EO =27.11 ± 6.42; EC =25.42 ± 7.30). We also observed a trend towards a single task effect with eyes closed (p =.061). There were no other significant main effects of task or any interaction effects in either the eyes open or eyes closed condition.   The effect size for the DTC of MSRS was moderate for eyes open and large for eyes closed respectively (SCI: 5.36 ± 15.31%; AB: -4.70 ± 13.30%; d = 0.70), (SCI: -1.0 ± 4.89%; AB: -15.21 ± 16.46%; d = 1.17).   !! 50!Figure 5 Movement reinvestment between SCI and AB groups A) Eyes open     B) Eyes closed   Figure 5: Comparison between SCI and AB groups for the amount of conscious control of posture for the single task of standing and the dual task of standing and counting in A) eyes open and B) eyes closed conditions. SCI subjects reinvested more conscious control to posture than AB subjects in both vision conditions (p <0.05).  Higher MSRS scores indicate higher reinvestment.   ABSCIMSRS scoreABSCIsingle task (stand)dual task (count + stand)403020100MSRS score403020100* *!! 51!3.6 Postural performance Representative center of pressure data (CoP) from a single SC and AB subject are plotted in Figure 6 showing combined CoP displacement for single and dual task conditions.  Figure 6 CoP displacement for an SCI and AB subject     ! Figure 6: Representative CoP displacement plots for the single task of standing and the dual task of standing and counting for a single SCI and AB subject. Increased sway represents decreased postural control. There were no differences between single- and dual-task conditions in any CoP sway parameters (velocity, root mean square, mean power frequency) in either the eyes open or eyes closed condition. There were significant group differences between SCI and AB subjects in CoP velocity and root mean square (p<0.05).  A two-way repeated measures MANOVA revealed a statistically significant multivariate main effect for group for both the eyes open and eyes closed conditions (Table 9) (EO: Wilks’ λ=.24, F1,15 =5.2, p =.011, partial η2 =.759; EC: Wilks’ λ=.12, F1,12 =8.2, p =.007, partial η2 =.877); there were no significant multivariate effects of task or interaction effects. Given the significance of the between-groups multivariate test, the univariate SCI subject       AB subject Single task  Dual task !! 52!main effects were examined. Significant univariate main effects for group were obtained for all CoP measures except MPF AP and ML for eyes open and eyes closed (Table 9).  SCI subjects’ mean velocity in AP (SCI =14.35 ± 5.24 mm/s; AB =6.75 ± 5.57 mm/s) and ML (SCI =12.22 ± 5.98 mm/s; AB =3.77 ± 6.35 mm/s) directions was 113% and 224% higher, respectively, than those of controls for eyes open. The sway area (RMS) in AP (SCI =9.20 ± 2.97 mm; AB =4.80 ± 3.13 mm) and ML (SCI =7.96 ± 3.79 mm; AB =2.60 ± 4.04 mm) directions was 92% and 206% more, respectively, than AB controls for CoP parameters collected over a 120 second period.  In the eyes closed conditions, SCI subjects’ mean velocity was 221% and 214% higher in the AP (SCI =30.65 ± 17.66 mm/s; AB =9.54 ± 15.30 mm/s) and ML (SCI =17.36 ± 8.04 mm/s; AB =5.52 ± 6.96 mm/s) directions, respectively, compared to controls. Sway area in the AP (SCI =11.55 ± 3.14 mm; AB =4.96 ± 2.73 mm) and ML (SCI =10.08 ± 6.14 mm; AB =2.99 ± 5.31 mm) directions was 133% and 237% more, respectively, than AB controls for the same CoP parameters collected over a 60 second period.  For eyes open, the effect size for the DTC of MVEL was small; MVELAP (SCI: 22.03 ± 27.99%; AB: 62.27 ± 103.34%; d = 0.53), MVELML (SCI: 48.91 ± 76.55%; AB: 75.98 ± 94.04%; d = 0.32). The effect size was moderate for the DTC of RMS; RMSAP (SCI: 10.0 ± 26.27%; AB: 47.69 ± 68.91%; d = 0.72), RMSML (SCI: 42.12 ± 97.16%; AB: 101.07 ± 99.74%; d = 0.60). The effect size was large for the DTC of MPFAP (SCI: 14.83 ± !! 53!19.75%; AB: 52.06 ± 59.0%; d = 0.85) and small for the DTC of MPFML (SCI: 9.74 ± 37.24%; AB: -.90 ± 41.57%; d = 0.27).  Though there were no significant task effects, we observed that both groups increased sway during dual task performance under both vision conditions (Table 9).   3.7 Berg Balance Scale (SCI subjects only) The Wilcoxon signed rank test revealed no statistically significant difference between BBS and BBS dual task, z =-.108, p =.914.   3.8 Subjective mental workload (NASA-TLX) For the math task, eyes open, there were no statistically significant main or interaction effects (Figure 7A). There was a trend towards a main effect for group (p =.084) and a group x task effect (p =.053) (Table 10).  For eyes open, the effect size was large for the DTC of standing and counting performance respectively (SCI: 66.03 ± 153.54%; AB: 5894.11 ± 9550.72%; d = 0.86), (SCI: 34.47 ±40.59%; AB:-4.53 ±47.65%; d = 0.88).   For the math task eyes closed, we could only assess standing effort since we did not include a baseline of sitting and counting with eyes closed. There was a significant main effect of group (F1,13 =7.7, p =.016, partial η2 =.372) and task (F1,13 =5.3, p =.039, partial η2 =.288) (Figure 7B). The effect size was small for the DTC of standing performance !! 54!(SCI: 15.3 ± 36.26; AB: 191.77 ± 369.22; d = 0.67). Upon closer analysis, it can be seen that the task effects are largely driven by the AB control group and show great variability (Table 10).   Figure 7 Subjective mental workload for the math task between SCI and AB groups A) Eyes open     B) Eyes closed !!!!!!!! Figure 7: Subjective mental workload measured by the NASA-TLX for math tasks in A) eyes open and B) eyes closed conditions. Higher scores (maximum 100) indicate higher mental effort. The SCI group reported all the tasks significantly more effortful than the AB control group for eyes open (*, p < 0.05) and for eyes closed (§, p < 0.10).   3.9 Fear We observed a statistically significant difference between groups for fear ratings during the math task for both the eyes open and closed conditions (EO: F1,15 =5.3, p =.036, partial η2 =.262; EC: F1,13 =13.0, p =.003, partial η2 =.500) (Table 11). SCI subjects were 4193% more fearful than AB controls during eyes open (SCI =13.31 ±15.91; AB =.31 ± 16.88)  and 3490% more fearful during eyes closed (SCI =33.75 ± 25.75; AB =.94 ± 24.09) (Figure 8, top panel). There were no other statistically significant main effects for task or interaction effects. The effect size for the DTC of fear was small for eyes open ABSCINASA-TLX100806040200*__________ABSCINASA-TLX100806040200count + standstandcount + sit§________!! 55!(SCI: 17.75 ± 11.50%; AB: -4.17 ± 11.79%; d = 0.28) and moderate for eyes closed (SCI: 19.31 ± 64.39%; AB: -6.25 ± 17.68%; d = 0.54).  3.10 Confidence The two-way ANOVA revealed a main effect of group for both eyes open and eyes closed (EO: F1,15 =9.9, p =.007, partial η2 =.398; EC: F1,13 =27.1, p <.001, partial η2 =.676) (Table 12). SCI subjects were only 13% less confident than AB controls when performing the math task with eyes open (SCI =88.83 ± 10.02; AB =100 ± 10.64). However, during the eyes closed condition, they were 81% less confident than AB controls (SCI =54.0 ± 23.92; AB =98.0 ± 22.38) (Figure 8, right panel). The effect size was small for the DTC of confidence for both eyes open (SCI: 5.76 ± 29.06%; AB: 0.0 ± 0.0%; d = 0.28) and eyes closed (SCI: -1.09 ± 41.52%; AB: -1.02 ± 2.89%; d = 0.002).   !! 56!Figure 8 Fear and confidence for the math task between SCI and AB groups A) Eyes open     B) Eyes closed   Figure 8: Fear and confidence ratings for the math task under A) eyes open and B) eyes closed conditions for the single task of standing and the dual task of standing and counting. There were significant main effects for group (p <0.05) for both fear and confidence. SCI subjects were more fearful and less confident than AB controls in both vision conditions.  3.11 Correlations The results of the non-parametric analysis for the BBS vs. BBS-DT revealed there was no effect of counting on the BBS. Therefore, we include only the BBS in our correlation analysis. Plots for correlation analysis are shown in Appendix G.  ABSCIABSCIsingle task (stand)dual task (count + stand)ABSCIABSCIFear0255075100Fear0255075100Confidence0255075100Confidence0255075100****!! 57!3.11.1 Associations between performance and functional measures (SCI subjects only) Better balance function (higher BBS scores) was associated with increased math task error ratio (r8 =.772, p =.025, CI 95% [.15, .96]) and decreased standing time (r6 =.868, p =.025, CI 95% [.99, .19]) during the dual task compared to single-task condition. Even when the outlier for the error ratio was removed, the relationship remained significant (r7 =.788, p =.035).  COPM performance was positive correlated with total words uttered (r9 =.740, p =.023, CI 95% [.15, .94]), indicating that higher self reported performance was associated with an increase in total number of words uttered.  The SCIM was not associated with any of the dual-task performance measures.  3.11.2 Associations between chronicity, cognitive impairment and performance measures (SCI only) We did not find any significant correlations between chronicity, MoCA and dual-task performance measures.   !! 58!4 DISCUSSION In this study, we used three approaches to evaluate the attentional requirements of maintaining balance (quiet stance) in people with SCI compared to age-matched AB controls. We used a reaction time task, a math task (counting backwards by 3s), and analyzed the relationship of dual-task performance on clinical measures of balance and daily function to evaluate the extent to which secondary task demands interfere with standing balance.  The reaction time task revealed only a trend towards slightly longer reaction times during the dual task condition in both groups, and indeed, subjects did not find the reaction time task particularly effortful.   We found a significant DTC (dual task cost) in total standing time only with eyes closed between SCI and AB controls during the math task condition. People with SCI stood for a shorter duration when they tried to stand and count backwards, compared to AB controls. This appears to be related to SCI subjects’ perceived mental workload, where they rated standing more effortful than AB controls. We did not observe a significant effect on math task performance, although SCI subjects reinvested significantly more conscious control to posture than AB controls. Overall, the SCI subjects had larger sway velocities and amplitudes and had higher movement reinvestment scores compared to the AB control group. This result appears to correspond to the higher ratings for fear, lower confidence and higher subjective workload in people with SCI.  !! 59!There was no effect of the counting task on Berg Balance Scale scores, and only the total standing time with eyes closed was significantly correlated to the BBS.  There was also a significant relationship between the total number of words uttered during the math task and the COPM performance score.  4.1 SCI subjects characteristics Our SCI cohort was relatively high functioning. Four of the nine subjects were assessed as AIS D, which is defined as an incomplete injury where most key muscles below the neurologic level have a muscle grade greater than or equal to 3/5 (ability to move against gravity) (Kirshblum et al., 2011). The remaining subjects were AIS C, which is an incomplete injury where motor function is preserved below the neurologic level, and most key muscles below the neurologic level have muscle grade less than 3/5 (active full-range movement against gravity) (Kirshblum et al., 2011). Overall our SCI group had better postural control during quiet standing during both eyes open and eyes closed compared to those reported by a study investigating the influence of vision on quiet standing in people with SCI (LeMay et al., 2013). The SCI cohort in that study was very similar to our study in age, height, weight and AIS level. However, the mean time since injury for their group was less than one year whereas the mean time post-injury of the subjects in our study was nearly 12 years. In addition, the median Berg Balance Scale (BBS) score for our SCI group was 43/56, indicating that they would be considered as low falls risk (Stevenson, 2001). Moreover, every subject’s Spinal Cord Independence Measure score was well over the median normative scores for that particular injury level indicating a high level of independence and function in activities of daily living (Aidinoff !! 60!et al., 2011). Finally, all our SCI subjects could stand for the maximum allotted 2 minutes with their eyes open while 7 of the 9 participants could stand for at least 60 seconds with eyes closed. Taken together, the overall lack of significant findings could partially be attributed to this high functioning SCI group when compared to AB controls.  4.2 Reaction time tasks during standing do not elicit dual-task effects  Although we did not observe significantly different dual task cost reaction times between groups, there was a trend for longer reaction times in the standing dual task condition compared to the sitting single task condition in both groups. Our AB control results are in accordance with two previous studies, which reported similar reaction times during sitting and standing in a group of healthy young subjects (Lajoie et al., 1999; Remaud et al., 2013). Previous work investigating reaction times between SCI and AB controls similarly found no difference between sitting and standing, but did find significant differences between standing and the more difficult single support phase of walking (Lajoie et al., 1999). It appears that the reaction time task in sitting and standing was not sufficiently mentally demanding as our NASA-TLX results indicate that the mental effort was quite low and both groups rated the mental effort of the reaction time task to be the same.  4.3 Math task performance is not affected by standing Overall, our SCI group performed equally as well as the AB controls on the math task in both task and vision conditions. In general, the math task performance by the subjects in our study are consistent with that reported in the literature. One study comparing the !! 61!performance between a counting task (count backwards by 3s) and a visuospatial task reported an accuracy rate of 87-91% and a rate of 27-28 words in a 60-second standing trial for healthy young subjects (Stoffregen et al., 2007). Another study reported similar findings for healthy young subjects with ankle injury (Burcal et al., 2014). The overall accuracy rate for our AB group was 95% and 28 words in a 60-second trial, which is comparable to that reported in other studies (Stoffregen et al. 2007; Burcal et al., 2014). We found that the AB and SCI subjects performed equally as well in the math task in both single- and dual-task conditions, despite the finding that the SCI subjects tended to find all the tasks more effortful.   Dual task research has shown that math task performance may be affected by a more difficult postural task condition. For example, math task performance deteriorated in the more difficult standing task that included calf vibration in healthy young subjects and in the no vision condition in healthy elderly subjects (Andersson et al., 2002; Swanenburg et al., 2009). In this study, the addition of the eyes closed condition gave us an opportunity to examine dual-task effects in a more difficult task condition, however, it also limited the data that could be included in the analysis of the math task performance. In the eyes closed condition, math task performance analysis was based only from subjects who could stand for at least 60 seconds with eyes closed (in order to normalize the timeframe for calculating error ratio and total words uttered). We performed an ad hoc analysis on the two subjects who could not stand for at least 60 seconds with eyes closed while performing the dual task (the other subjects could not stand at all with eyes closed). We found the mean error ratio for the two subjects to be higher (0.17) than the average mean !! 62!(0.03) for the entire SCI group. This, together with the finding of significant interaction effects in total standing with eyes closed, reveals that there was a competition for resources with eyes closed and the addition of the math task increased processing resource requirements. This illustrates that only during the more difficult eyes closed condition, and when we were able to include the two lower functioning subjects were we able to see an effect of the math task.    4.4 People with SCI have greater postural sway and movement reinvestment during quiet standing Standing balance in people with SCI may be compromised due to various levels of sensory loss, muscle weakness and spasticity (LeMay et al., 2013), which could lead to greater movement reinvestment and postural sway. As expected, our SCI subjects had less postural control (higher sway velocity and amplitude) during quiet standing compared to AB controls. These results are consistent with a study that compared the mean velocity and amplitude between SCI and AB controls (Lemay et al., 2013).   The Movement Specific Reinvestment Scale (MSRS) evaluates the amount of conscious control devoted to movement, in this case, standing. As expected, we observed higher MSRS scores in our SCI subjects. The modified MSRS we used asked subjects to evaluate the state of their level of conscious attention with regards to the standing tasks and found that SCI subjects reinvested significantly more (over 200%) attention to postural control than AB in both vision conditions. These results are in accordance with a study that compared the amount of conscious reinvestment of stroke subjects with !! 63!matched controls (Orrell et al., 2008). They reported that the total MSRS scores of stroke subjects were nearly double that compared to controls. Increased conscious control to posture could be viewed as an adaptive strategy by people with reduced postural control in order to maintain quiet standing.   Despite the overall higher propensity to reinvest, compared to the AB group, the SCI group had nearly identical MSRS scores between single and dual tasks and between eyes open and eyes closed conditions. They did not increase their attention to posture during the dual task condition as expected. This is corroborated by the nearly identical NASA-TLX scores between single and dual tasks which indicate that the SCI subjects felt that the effort of standing and standing and counting were nearly the same during the eyes open condition.   4.5 Postural sway and movement reinvestment was not affected by dual-tasking  Neither postural sway nor movement reinvestment scores were affected by the dual-task condition. Sway velocity, amplitude, and frequency components did not change between single- and dual-task conditions in either the SCI or AB group. The math task was not expected to produce a dual task effect for our AB group since quiet standing with eyes open should be easy for them to perform. Indeed, previous studies have reported an increase in stiffness of the postural control system in AB subjects only during more difficult dual task conditions such as tandem standing on see saws while performing a modified Stroop task (Dault et al., 2001).   !! 64!Although we added an eyes closed condition to provide more challenge to maintaining quiet stance, we still did not see a significant dual-task effect on sway parameters in SCI. There was a general pattern for sway to be higher during the dual-task condition compared to the single task in both the eyes open and the more difficult eyes closed condition. One possibility is that the effects of articulation could have contributed to the slight increase in sway we observed during the dual task. Consideration must be made when involving verbal responses since depending on the level of the lesion, SCI can lead to impairment of the respiratory system (Winslow and Rozovsky, 2003). Disorders in respiration could account for some of our results since many subjects in our SCI group had high lesions. Indeed, they uttered slightly less words than AB controls in both vision conditions. Changes in respiration and speech have been shown to have an effect on postural control (Bouisset and Duchêne, 1994). Researchers have found that tasks requiring verbal articulation resulted in a more pronounced increase in postural sway frequency and sway path than those tasks that did not (Dault et al., 2003). Our AB controls uttered more words than the SCI group and we observed several AB subjects swaying rhythmically during the counting task. It is difficult to ascertain if the slight increase in postural sway we observed was due to the effects of articulation in the SCI subjects, or an attenuation of the differences in dual task effects between groups (due to the rhythmic swaying in some AB subjects). Regardless, the effects of articulation should always be considered when including a verbal response in dual task paradigms.      !! 65!4.5.1 Perceptual measures affect postural sway and movement reinvestment in SCI !Perceptual measures may influence the amount of resources devoted to processing tasks with regards to postural control (Maki and McIlroy, 1996). Psychological factors such as increased fear and decreased confidence have been reported to be associated with increased reinvestment (Huffman et al., 2009). In able-bodied subjects, an increased threat to posture such as standing on a high platform resulted in a reinvestment of postural control (Huffman et al., 2009). Other research also supports an increase in cortical response scaled to balance perceptions such as fear and confidence during a postural threat (Adkin et al., 2008). Another study proposed that the addition of a math task of counting backwards by 3s increased the level of arousal in comparison with the standing task and that this could cause a person to sharpen their performance (Andersson et al., 2002). Our SCI were significantly more fearful and less confident than our AB controls overall, but not more so between single and dual tasks. This lack of difference in perceptual measures between single and dual tasks could partially explain why there was no change in performance during the dual task for CoP parameters and MSRS ratings. However, the differences in perceptual measures between groups could explain why we observed significant group effects between SCI and AB subjects for movement reinvestment and sway for both eyes open and closed conditions.  The significant effect of perceptual measures on postural sway and movement reinvestment is also evident when comparing the mental effort of standing between SCI subjects and AB controls. SCI subjects tended to find all the tasks more effortful than AB controls especially in the eyes closed condition. Similarly, SCI subjects were !! 66!significantly more fearful and less confident than AB controls when standing. These findings intersect with the main group effects we observed in CoP performance and movement reinvestment scores and could explain why we saw a significant difference in performance between SCI and AB controls.     4.5.2 Lack of findings due to task parameters and high functioning SCI group !Our inability to detect a dual-task effect of the math task could be because the suprapostural task was not perceptually linked to the environment. Previous studies have compared the variability of postural sway between a visuospatial task and a math task of counting backwards by 3s. The able-bodied young participants in this trial were asked to stand with feet together on a firm surface. Although there was no difference in subjective mental workload between the two tasks, it was found that postural sway was reduced with the visuospatial task (considered to be perceptually linked to the environment), whereas the math task (considered to be independent of the environment) did not have an effect on postural control (Stoffregen et al., 2007). Similarly, an earlier study exploring five different cognitive tasks, each testing different aspects of working memory, found no significant difference between sit and count (single task) and stand and count (dual task) in two groups of older adults, but reported that visuospatial tasks such as Brooks spatial task did affect postural stability. They suggested that tasks utilizing the visuospatial sketchpad reduces the ability to use visual information in the control of postural stability (Maylor and Wing, 1996). In keeping with this approach, the eyes closed condition we used in our study should have sufficiently increased resource requirements similar to tasks utilizing the visuospatial sketchpad. !! 67! In our study, it became apparent that quiet standing was not challenging enough for our SCI group, so we introduced an eyes closed condition in order to create an increase in resource requirements for postural control. The eyes closed condition appeared to be quite challenging for our SCI subjects, as there was a general increase in velocity and sway area in this condition.  This seems plausible since SCI subjects would presumably have to rely more on visual inputs to maintain postural control given that proprioceptive inputs could be severely compromised (Lemay et al., 2013). Moreover, the SCI subjects rated the mental effort of standing significantly higher than AB controls in the eyes closed condition. Yet, we found no significant differences between math task performance and postural sway despite the significant finding that subjects found standing effort more challenging during the dual task compared to the single task. It is conceivable that since the math task was not perceptually linked to the environment, we would not see an effect on postural control.  Despite our findings, there have been many studies that have reported a dual task effect when combining a counting backwards task with quiet standing. In one study, four different types of tasks in elderly adults (>75 years old) who were at risk for falls during quiet standing were compared; no task, repeating a number out loud, counting backwards by 7s out loud and counting backwards silently. It was found that there were significant decreases in postural control with the counting backwards out loud task, demonstrating a dual-task effect in this population with compromised balance function (Swanenburg et al., 2009). Another study reported significant increases in CoP path length only with the !! 68!most difficult counting backwards by 3s task, compared to the less difficult tasks of digit reversal (reverse the order of a pair of digits) and 2-bit classification (combine 2 digits into a single double digit number). In this case, healthy young subjects were asked to stand on a foam pad to challenge their postural control as these three levels of suprapostural tasks of increasing difficulty were tested (Pellecchia, 2003). The researchers suggested that the integration of the suprapostural and postural tasks becomes a single higher order skill and that task integration becomes more challenging as the attentional demands of the component tasks increase. Both trials employed challenging postural tasks that were able to create a dual task effect with the addition of the math task for their population under study.   We feel that the eyes closed condition was sufficiently challenging, as corroborated by our NASA-TLX results and significant findings on standing time. However, our lack of significant dual-task effects on math performance and CoP parameters may have been hampered by our attempt to normalize the data by only selecting those subjects who could stand for at least 60-seconds, thereby inadvertently limiting our SCI group to those who were high functioning. This is underscored by the fact that our standing time results, which included the two lower functioning subjects revealed a significant dual task cost between our SCI and AB groups. Indeed, our protocol revealed that the math task was not challenging enough for subjects who could stand for at least 60-seconds eyes closed, yet was too difficult for those who could not stand for the minimum 60-seconds.   !! 69!4.6 Influence of dual task postural task on clinical measures We were interested in determining whether functional measures commonly used in people with SCI were sensitive to the changes in dual task cost postural control performances such as reaction time, standing time, error ratio, total number of words uttered, and movement reinvestment. Larger dual task costs, or bigger changes between single task and dual task imply that the dual task condition was more challenging and this might be more related to functional measures than the single task condition.  We found a strong correlation between the BBS with standing time eyes closed and perceived performance in daily activities (COPM) with number of words uttered. Better balance function was related to longer dual task standing time, and better self-perceived performance was associated with higher number of words uttered during the math task. This is not surprising given that we found a significant dual-task effect in SCI on standing time with eyes closed, but not with any other dual-task performance measure. However, we did not find that adding a math task influenced the BBS scores significantly in our cohort. When we reviewed the data between BBS and BBS-DT scores, we observed only a 1-point difference between the two measures for nearly all the subjects. This would seem to indicate that the math task of counting backwards by 3s did not sufficiently require an increase in resource processing during the BBS test for our SCI cohort.  Our preliminary work in associating clinical outcomes such as the BBS, SCIM and COPM with dual tasks has not been previously studied. Other tests such as the Timed Up and Go (TUG) combined with counting backwards by 3s have already been included in !! 70!the Mini BesTEST (Horak et al., 2009). The Stops Walking while Talking Test (Lundin-Olsson et al., 1997) evaluates the ability for subjects to simultaneously walk and talk without stopping. Clinical measures offer the advantage of being quick, cost effective and easy to apply in both the research and clinical setting. Further research is needed to identify the type of dual tasks that would be challenging yet appropriate to track functional dual-task effects in people with SCI.  4.7 Cognitive function and dual tasking in SCI It has been reported that people with SCI may develop mild cognitive dysfunction over time even without a concurrent head injury. Studies using standard neurophysiological tests have identified performance impairments in a range of cognitive functions that span memory, attention and executive function (Lazzaro et al., 2013). The use of scalp-recorded event related potentials (ERPs) have provided preliminary evidence for processing changes in people with SCI compared to able-bodied controls in later cognitive stages involving selective attention, resource allocation and processing speed (Cohen et al., 1996; Ament et al., 1995). This could have potential implications when attempting to perform a secondary cognitive task while standing particularly in persons with SCI whose postural stability is challenged with spasticity, clonus, weakness and sensory loss. Our SCI group had a median MoCA score of 27 (MoCA score ≥26 is considered normal) and we found no significant correlations between the MoCA and dual task cost of performance measures. These results might not be surprising since researchers have found that global measures of cognitive function such as the MoCA are not independently associated with clinical measures of balance function such as the BBS !! 71!(Muir-Hunter et al., 2014). Instead, specific measures of executive function, such as the Trail Making Test, have been found to be more associated with the dual task TUG in elderly adults with mild cognitive impairment (Muir-Hunter et al., 2014). Further research might include tasks that are sufficiently demanding for people with SCI, so that it can be ascertained if mild cognitive deficits might be related to a decrease in dual task balance performance.   !! 72!5 CONCLUSIONS AND FUTURE DIRECTIONS Attempting to perform more than one task at a time is a daily challenge for people with diminished postural control. Coupled with having to focus their attention on both or either task, postural control may be detrimentally affected. Our study has demonstrated that total standing time during eyes closed is adversely affected with the addition of a math task, and that this is related to balance function as measured by the Berg Balance Scale. Perceptual measures such as increased fear, decreased confidence and increased perceived mental workload correspond to increases in postural sway and conscious control of standing in subjects with SCI. However, they do not appear to be significantly affected by the addition of a dual task. It appears that for our SCI cohort, standing on a firm surface with eyes open and performing a concurrent suprapostural task of minimal mental workload would not greatly affect their postural control any more than the single task of standing quietly.   With the exception of standing time with eyes closed, the effect sizes of the differences in dual task cost between groups of most of our performance measures were very small, with large amounts of variability, consistent with the idea that our postural and secondary tasks were not challenging enough. Consequently, we would have required a much larger samples size in order to yield statistically significant effects in our outcome measures.  Our SCI group was living and working in the community and physically active, and so generalizability of our findings may be limited to only the higher-functioning strata of the SCI population. Given that our math task did not appear to sufficiently challenge the !! 73!processing requirements of our SCI cohort particularly with eyes open, future directions could be aimed at developing tasks that better challenge their postural control system and provide a suitable suprapostural task. However, caution must be exercised when selecting a task in order to achieve a dual task effect as consideration of the ecological validity of such a task is equally important. For example, dual task situations that are encountered on a daily basis which challenge vision and postural control (visuospatial tasks), might be more appropriate for people with SCI since they rely more on vision to maintain postural control due to proprioceptive deficits. Another consideration would be to challenge the postural environment such as altering the support surface, since our protocol revealed that the eyes closed condition is suitable only for high functioning subjects. Moreover, one could argue that the eyes closed condition is not as ecologically valid as using a visuospatial task or challenging the postural environment. This might enable us to include those people with SCI who cannot manage the eyes closed condition. In conclusion, further research needs to be undertaken in understanding the effects of a dual task on postural control, which might have potential implications for the addition of a suprapostural task in rehabilitation protocols in people with SCI.   !! 74!6 TABLES Table 1 Descriptive statistics of subject characteristics and clinical evaluations (N=17) Variable SCI AB controls Number of subjects 9 8 Agea,b (years) 51.7 (11.3) 49.0 (11.3) Heighta,b (cm) 174.3 (8.8) 177.6 (10.4) Weighta,b (kg) 78.1 (9.0) 78.5 (13.6) Sex 8M, 1F 7M,1F Clinical Measure Median (inter-quartile range)   BBS 43 (30-52)   BBS-DT 35 (29-51)   SCIM 73 (65-79)   COPM performance 3 (3-4)   COPM satisfaction 3 (3-3)   MoCA 27 (25-28)   Mean (standard deviation) a Normal distribution (Shapiro-Wilk, p>.05) b No significant difference between groups (T-test, p>.05) Abbreviations: BBS, Berg Balance Scale; BBS-DT, Berg Balance Scale dual task; SCIM, Spinal Cord Independence Measure; COPM, Canadian Occupational Performance Measure; MoCA, Montreal Cognitive Assessment !! 75!Table 2 Subject characteristics and clinical outcomes of individual SCI subjects (n=9) Subject Age (years) Gender Height (cm) Weight (kg) Level AIS Post injury (years) BBS BBS-DT SCIM COPM Perf COPM Satis MOCA 1 37 M 180 88 C6-7 C 21 29 29 81 3.0 3.0 26 2 33 M 175 70 C5-6 C 11 10 11 69 2.4 1.6 24 3 59 F 155 59 L4-5 D 2.5 52 NT 61 3.0 1.2 28 4 59 M 175 86 C5-6 C 6 NT NT 70 3.4 3.8 26 5 63 M 178 80 C4-5 D 2 50 49 73 2.8 2.6 24 6 51 M 170 75 T3 C 12 35 34 76 8.0 8.0 27 7 56 M 175 84 C3-5 D 38 52 53 80 5.0 1.8 27 8 64 M 173 82 T12 D 1 55 51 77 2.8 1.0 29 9 44 M 188 79 C1-2 C 12 32 35 61 3.6 3.0 28 Mean (SD) 51.78 (11.34) 8M, 1F 174.33 (8.86) 78.11 (9.05)  5C 4D 11.72 (11.75)                    Median (SD) 43 (45) 35 (42) 73 (20) 3 (6) 3 (7) 27 (5) All subjects sustained motor incomplete spinal cord injuries Abbreviations: BBS, Berg Balance Scale; BBS-DT, Berg Balance Scale dual task; SCIM, Spinal Cord Independence Measure; COPM, Canadian Occupational Performance Measure; MoCA, Montreal Cognitive Assessment, NT, not tested.  Higher scores indicate higher function: Total scores: BBS=56, SCIM=100, COPM=10. MoCA=30.  !! 76!Table 3 Experimental conditions Task Single task (count) Single task (stand) Dual task  Reaction time  NA    Sit and reaction time   Stand and reaction time  Math   Sit and count   Stand   Stand and count   Math eyes closed  NA  Stand eyes closed   Stand eyes closed and count          !! 77!Table 4 Summary of sequence of data collection Task  Single task Count Single task Stand Dual task  pre-task measures:  Confidence Confidence Math task: Sit and count Stand Stand and count  post-task measures: NASA-TLX NASA-TLX Fear MSRS NASA-TLX Fear MSRS  pre-task measures:  Confidence Confidence Math eyes closed task: NA Stand eyes closed Stand eyes closed and count  post-task measures:  NASA-TLX Fear MSRS NASA-TLX Fear MSRS  pre-task measures:     Reaction time task: NA Sit and reaction time Stand and reaction time  post-task measures:  NASA-TLX SCIMa COPMa NASA-TLX MoCAa a for SCI subjects only Abbreviations: NASA, National Aeronautics and Space Administration Task Load Index; MSRS, Movement Specific Reinvestment Scale; SCIM, Spinal Cord Independence Measure; COPM, Canadian Occupational Performance Measure; MoCA, Montreal Cognitive Assessment   !! 78!Table 5 A  Reaction time task performance Reaction time (msec) AB SCI Task Effect Group x Task Effect Single task 222.77 (18.66) 270.86  (77.41)  F1,15=3.45 P =.083  F1,15=.001 P =.981 Dual Task 245.34 (28.89) 292.86  (77.40) Group Effect F1,15 = 3.36 P =.087   Mean (standard deviation)  N=9 SCI, N=8 AB subjects Sit and reaction time=single task; stand and reaction time=dual task Not significant at p>.05 (Two-way ANOVA)    Table 5 B  Subjective mental workload for reaction time task  AB SCI  Task Effect Group x Task Effect NASA-TLX Single Task  6.54 (10.07)  7.37 (2.09)     F1,14=2.46 P =.139  F1,14=.67 P =.428 Dual-Task 7.67 (11.61) 10.94 (8.53) Group Effect F1,14 =.243 P =.630  Mean (standard deviation) SCI=8, AB=8 subjects Sit and reaction time=single task; stand and reaction time=dual task NASA-TLX score from 0-100 Not significant p>.05 (Two-way ANOVA)    !! 79!Table 6 Math task performance for eyes open and closed Math task Eyes open Eyes closed Total words uttered AB SCI Task Effect Group x Task Effect AB SCI! Task Effect! Group x Task Effect!Single task (sit and count) 56.38 (21.27) 55.66 (18.08) F1,15=.25 P =.622 F1,15=.79 P =.388 28.71 (11.48) 28.17 (9.28)! F1,12=2.68 P =.128! F1,12=2.03 P =.179!Dual task (stand and count) 57.17 (20.24) 52.81 (15.21)   28.42 (10.80) 26.25 (8.13)! ! !Group Effect F1,15 =.082  P =.779    F1,12 =.18  P =.676! ! !Error ratio AB SCI Task Effect Group x Task Effect AB SCI! Task Effect! Group x Task Effect!Single task (sit and count) .05 (.05) .05 (.05) F1,15=.06 P =.809 F1,15=.77 P =.394 .05 (.05) .05 (.06)! F1,12=.63 P =.444! F1,12=.51 P =.490!Dual task (stand and count) .05 (.07) .04 (.05)   .05 (.08) .04 (.04)! ! !Group Effect F1,15 =.072  P =.793    F1,12 =.09  P =.770! ! !Mean (standard deviation)  Eyes open: N=9 SCI, N=8 AB subjects; Eyes closed: N=6 SCI, N=8 AB subjects Sit and count=single task; stand and count=dual task Eyes open counting for 120 seconds; eyes closed counting for 60 seconds Not significant at p>.05 (Two-way ANOVA)!! 80!Table 7 Standing time performance eyes closed Standing time  (sec) AB SCI Task Effect Group x Task Effect Single task 120.00  (0.00) 77.42  (50.32)  F1,15=5.44 P =.034*  F1,15=5.44 P =.034* Dual Task 120.00  (0.00) 61.28  (50.70) Group Effect F1,15 = 8.29 P =.011*   Mean (standard deviation)  N=9 SCI, N=8 AB subjects Stand=single task; stand and count=dual task *Significant at p<.05 (Two-way ANOVA)  Table 8 Movement reinvestment performance for math task, eyes open and closed MSRS AB SCI Task Effect Group x Task Effect Eyes open Single task  7.88  (1.81)  26.78 (7.07)  F1,15=.07 P =.796  F1,12=.89 P =.362 Dual-Task 7.50  (2.14) 27.44  (5.53) Group Effect F1,15 =72.66 P <.001*   Eyes closed  Single Task  8.38  (2.26)  25.50  (7.06)  F1,12=4.26 P =.061  F1,12=2.73 P =.125 Dual-Task 6.88  (1.46) 25.33 (7.20) Group Effect F1,12 =47.69 P <.001*   Mean (standard deviation) Eyes open: SCI=9, AB=8 subjects; Eyes closed: SCI=6, AB=8 subjects Stand=single task; stand and count=dual task MSRS scores from 1-36, higher scores indicate increased reinvestment *Significant at p<.05 (Two-way ANOVA)  !! 81!Table 9 Center of pressure postural performance eyes open and closed          Eyes open         Eyes closed CoP parameter AB (n=8) SCI (n=9) Group effect AB (n=8) SCI (n=6) Group effect MVELAP (mm/s) Single task Dual task  5.24 (.74) 8.26 (4.9)  13.31 (5.75) 15.38 (4.30) F=37.28 P<.001*  8.92 (2.88) 10.15 (2.60)  30.85 (19.96) 30.45 (15.51) F=36.71 P<.001* MVELML (mm/s) Single task Dual task  2.81 (.58) 4.72 (2.16)  10.30 (5.26) 14.15 (8.51 F=34.26 P<.001*  3.99 (.97) 7.04 (6.66)  14.47 (4.63) 20.25 (11.08) F=33.24 P<.001* RMSAP (mm) Single task Dual task  3.83 (.89) 5.77 (3.34)  8.98 (2.80) 9.41 (2.48) F=20.84 P<.001*  4.91 (1.36) 5.01 (1.92)  11.02 (2.16) 12.08 (3.63) F=31.43 P<.001* RMSML (mm) Single task Dual task  1.79 (.67) 3.41 (1.88)  6.86 (3.07) 9.06 (5.76) F=27.72 P<.001*  1.87 (.96) 4.12 (4.68)  7.56 (1.97) 12.60 (9.61) F=24.55 P<.001* MPFAP (Hz) Single task Dual task  .17 (.06) .24 (.05)  .23 (.08) .26 (.08) F=1.33 P=.268  .30 (.11) .35 (.12)  .45 (.25) .43 (.19) F=1.85 P=.199 MPFML (Hz) Single task Dual task  .37 (.22) .30 (.04)  .28 (.11) .28 (.05) F=.78 P=.391  .45 (.16) .40 (.15)  .33 (.07) .32 (.10) F=2.41 P=.146 Mean (standard deviation) for CoP measures taken over 120 seconds standing eyes open and 60 seconds for eyes closed Eyes open: N=9 SCI, N=8 AB subjects; Eyes closed: N=6 SCI, N=8 AB subjects Stand=single task; stand and count=dual task *Significant at p<.05 (Two-way repeated measures MANOVA, univariate main effects) Abbreviations: MVEL, mean velocity; RMS, root mean square; MPF, mean power frequency in the AP, anteroposterior and ML, mediolateral directions  !! 82!Table 10 Subjective mental workload for math task, eyes open and closed NASA-TLX AB SCI Task Effect Group x Task Effect Eyes open Single task (sit and count)   25.33  (24.59)  29.29  (17.91)  F2,30=2.41 P =.130a  F2,30=3.88 P =.053a Single task (stand)  4.92  (8.59) 34.26  (22.73) Dual-Task (stand and count)  22.81  (22.07) 35.54  (21.33) Group Effect F1,15 = 3.44 P =.084   Eyes closed  Single Task (stand)   7.96 (12.63)  39.52  (20.36)  F1,13=5.27 P =.039   F1,13=.877 P =.366  Dual-Task (stand and count)  21.90  (23.53) 45.38  (25.37) Group Effect F1,13 =7.71 P =.016    Mean (standard deviation) Eyes open: SCI=9, AB=8 subjects; Eyes closed: SCI=7, AB=8 subjects Sit and count=single task count; stand=single task stand; stand and count=dual task NASA-TLX score from 1-100 *Significant at p<.05 (Eyes open 2x3 way ANOVA; Eyes closed two-way ANOVA)  a Greenhouse-Geisser correction !! 83!  Table 11 Fear ratings for math task, eyes open and closed Fear AB SCI Task Effect Group x Task Effect Eyes open Single task  .38  (1.06)  13.94  (22.13)  F1,15=.017 P =.899  F1,15=.011 P =.917 Dual-Task .25  (.71) 12.67  (21.89) Group Effect F1,15=5.34 P =.036*   Eyes closed  Single Task  1.25  (3.54)  34.79  (29.55)  F1,13=.088 P =.772  F1,13=.025 P =.876 Dual-Task .63  (1.77) 32.71  (28.00) Group Effect F1,13=12.99 P =.003*   Mean (standard deviation) Eyes open: SCI=9, AB=8 subjects; Eyes closed: SCI=7, AB=8 subjects Stand=single task; stand and count=dual task Rated from 0-100 (0-least fearful, 100-most fearful)  *Significant at p<.05 (Two-way ANOVA)    !! 84!Table 12 Confidence ratings for math task eyes open and closed Confidence AB SCI Task Effect Group x Task Effect Eyes open Single task  100  (0.00)  88.33  (17.07)  F1,15=.03 P =.865  F1,15=.03 P =.865 Dual-Task 100  (0.00) 89.33  (6.34) Group Effect F1,15 =9.92 P =.007*   Eyes closed  Single Task  98.50  (3.51)  55.57  (24.57)  F1,13=.50 P =.491  F1,13=.14 P =.720 Dual-Task 97.50  (4.63) 52.43  (25.54) Group Effect F1,13 =27.07 P <.001*   Mean (standard deviation) Eyes open: SCI=9, AB=8 subjects; Eyes closed: SCI=7, AB=8 subjects Stand=single task; stand and count=dual task Rated from 0-100 (0-least confident, 100-most confident)  *Significant at p<.05 (Two-way ANOVA)    !!! 85!REFERENCES Adkin AL, Campbell AD, Chua R, Carpenter MG. 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Interference between postural control and mental task performance in patients with vestibular disorder and healthy controls. J Neurol Neurosurg Psychiatry 71: 48–52, 2001.   !! 94!APPENDICES Appendix A: The Movement Specific Reinvestment Scale-Modified © Masters, Eves & Maxwell (2005)   Name: ____________________________ Date: _____ Age: _____ Hand: L / R   Directions: Below are a number of statements about your movements. The possible answers go from ‘strongly agree’ to ‘strongly disagree’. There are no right or wrong answers so circle the answer that best describes how you feel for each.  1. I reflect about my standing a lot  strongly     moderately     weakly        weakly     moderately     strongly disagree     disagree           disagree     agree        agree                 agree  2. I am always trying to think about my standing when I carry it out  strongly     moderately     weakly        weakly     moderately     strongly disagree     disagree           disagree     agree        agree                 agree  3. I’m self-conscious about the way I look when I am standing  strongly     moderately     weakly        weakly     moderately     strongly disagree     disagree           disagree     agree        agree                 agree  4. I’m aware of the way my body works when I am standing  strongly     moderately     weakly        weakly     moderately     strongly disagree     disagree           disagree     agree        agree                 agree  5. I’m concerned about my style of standing  strongly     moderately     weakly        weakly     moderately     strongly disagree     disagree           disagree     agree        agree                 agree  6. I am concerned about what people think about me when I am standing  strongly     moderately     weakly        weakly     moderately     strongly disagree     disagree           disagree     agree        agree                 agree    !! 95!Appendix B: NASA-TLX         !! 96!Appendix C: The Berg Balance Scale  Description:  14-item scale designed to measure balance of the older adult in a clinical setting.  Equipment needed: Ruler, 2 standard chairs (one with arm rests, one without). footstool or step, stopwatch or wristwatch, 15 ft walkway  Completion:   Time: 15-20 minutes  Scoring:  A five-point ordinal scale, ranging from 0-4. “0” indicates the lowest level of function and “4” the highest level of function. Total Score = 56  Interpretation: 41-56 = low fall risk    21-40 = medium fall risk    0 –20 = high fall risk Criterion Validity:  “Authors support a cut off score of 45/56 for independent safe ambulation”.    Riddle and Stratford, 1999, examined 45/56 cutoff validity and concluded: • Sensitivity = 64% (Correctly predicts fallers) • Specificity = 90% (Correctly predicts non-fallers) • Riddle and Stratford encouraged a lower cut off score of 40/56 to assess fall risk  Comments: Potential ceiling effect with higher level patients.  Scale does not include gait items  Minimal Detectable Change:   “A change of 4 points is needed to be 95% confident that true change has occurred if a patient scores within 45-56 initially, 5 points if they score within 35-44, 7 points if they score within 25-34 and, finally, 5 points if their initial score is within 0-24 on the Berg Balance Scale.”  Donoghue D; Physiotherapy Research and Older People (PROP) group, Stokes EK. (2009). How much change is true change? The minimum detectable change of the Berg Balance Scale in elderly people.  J Rehabil Med. 41(5):343-6.  Norms:  Lusardi, M.M. (2004). Functional Performance in Community Living Older Adults.  Journal of Geriatric Physical Therapy, 26(3), 14-22. !! 97!  Berg Balance Scale   Name: __________________________________ Date: ___________________  Location: ________________________________ Rater: ___________________  ITEM DESCRIPTION      SCORE (0-4)  Sitting to standing      ________ Standing unsupported     ________ Sitting unsupported                                                                   ________ Standing to sitting      ________ Transfers       ________ Standing with eyes closed     ________ Standing with feet together     ________ Reaching forward with outstretched arm   ________ Retrieving object from floor     ________ Turning to look behind     ________ Turning 360 degrees      ________ Placing alternate foot on stool   ________ Standing with one foot in front    ________ Standing on one foot      ________  Total  ________   GENERAL INSTRUCTIONS !! 98!Please document each task and/or give instructions as written. When scoring, please record the lowest response category that applies for each item.  In most items, the subject is asked to maintain a given position for a specific time. Progressively more points are deducted if: • the time or distance requirements are not met • the subject’s performance warrants supervision • the subject touches an external support or receives assistance from the examiner  Subject should understand that they must maintain their balance while attempting the tasks. The choices of which leg to stand on or how far to reach are left to the subject. Poor judgment will adversely influence the performance and the scoring.  Equipment required for testing is a stopwatch or watch with a second hand, and a ruler or other indicator of 2, 5, and 10 inches. Chairs used during testing should be a reasonable height. Either a step or a stool of average step height may be used for item # 12.  Berg Balance Scale  SITTING TO STANDING INSTRUCTIONS: Please stand up. Try not to use your hand for support. (    ) 4 able to stand without using hands and stabilize independently (    ) 3 able to stand independently using hands (    ) 2 able to stand using hands after several tries (    ) 1 needs minimal aid to stand or stabilize (    ) 0 needs moderate or maximal assist to stand  STANDING UNSUPPORTED INSTRUCTIONS: Please stand for two minutes without holding on. (    ) 4 able to stand safely for 2 minutes (    ) 3 able to stand 2 minutes with supervision (    ) 2 able to stand 30 seconds unsupported (    ) 1 needs several tries to stand 30 seconds unsupported (    ) 0 unable to stand 30 seconds unsupported  If a subject is able to stand 2 minutes unsupported, score full points for sitting unsupported. Proceed to item #4.  SITTING WITH BACK UNSUPPORTED BUT FEET SUPPORTED ON FLOOR OR ON A STOOL INSTRUCTIONS: Please sit with arms folded for 2 minutes. (    ) 4 able to sit safely and securely for 2 minutes (    ) 3 able to sit 2 minutes under supervision (    ) 2 able to able to sit 30 seconds (    ) 1 able to sit 10 seconds (    ) 0 unable to sit  without support 10 seconds  !! 99!STANDING TO SITTING INSTRUCTIONS: Please sit down. (    ) 4 sits safely with minimal use of hands (    ) 3 controls descent by using hands (    ) 2 uses back of legs against chair to control descent (    ) 1 sits independently but has uncontrolled descent (    ) 0 needs assist to sit  TRANSFERS INSTRUCTIONS: Arrange chair(s) for pivot transfer. Ask subject to transfer one way toward a seat with armrests and one way toward a seat without armrests. You may use two chairs (one with and one without armrests) or a bed and a chair. (    ) 4 able to transfer safely with minor use of hands (    ) 3 able to transfer safely definite need of hands (    ) 2 able to transfer with verbal cuing and/or supervision (    ) 1 needs one person to assist (    ) 0 needs two people to assist or supervise to be safe  STANDING UNSUPPORTED WITH EYES CLOSED INSTRUCTIONS: Please close your eyes and stand still for 10 seconds. (    ) 4 able to stand 10 seconds safely (    ) 3 able to stand 10 seconds with supervision  (    ) 2 able to stand 3 seconds (    ) 1 unable to keep eyes closed 3 seconds but stays safely (    ) 0 needs help to keep from falling  STANDING UNSUPPORTED WITH FEET TOGETHER INSTRUCTIONS: Place your feet together and stand without holding on. (    ) 4 able to place feet together independently and stand 1 minute safely (    ) 3 able to place feet together independently and stand 1 minute with supervision (    ) 2 able to place feet together independently but unable to hold for 30 seconds (    ) 1 needs help to attain position but able to stand 15 seconds feet together (    ) 0 needs help to attain position and unable to hold for 15 seconds  REACHING FORWARD WITH OUTSTRETCHED ARM WHILE STANDING INSTRUCTIONS: Lift arm to 90 degrees. Stretch out your fingers and reach forward as far as you can. (Examiner places a ruler at the end of fingertips when arm is at 90 degrees. Fingers should not touch the ruler while reaching forward. The recorded measure is the distance forward that the fingers reach while the subject is in the most forward lean position. When possible, ask subject to use both arms when reaching to avoid rotation of the trunk.) (    ) 4 can reach forward confidently 25 cm (10 inches) (    ) 3 can reach forward  12 cm (5 inches) (    ) 2 can reach forward 5 cm (2 inches) (    ) 1 reaches forward but needs supervision (    ) 0 loses balance while trying/requires external support !! 100! PICK UP OBJECT FROM THE FLOOR FROM A STANDING POSITION INSTRUCTIONS: Pick up the shoe/slipper, which is place in front of your feet. (    ) 4 able to pick up slipper safely and easily (    ) 3 able to pick up slipper but needs supervision  (    ) 2 unable to pick up but reaches 2-5 cm(1-2 inches) from slipper and keeps balance independently (    ) 1 unable to pick up and needs supervision while trying (    ) 0 unable to try/needs assist to keep from losing balance or falling  TURNING TO LOOK BEHIND OVER LEFT AND RIGHT SHOULDERS WHILE STANDING INSTRUCTIONS: Turn to look directly behind you over toward the left shoulder. Repeat to the right. Examiner may pick an object to look at directly behind the subject to encourage a better twist turn. (    ) 4 looks behind from both sides and weight shifts well (    ) 3 looks behind one side only other side shows less weight shift (    ) 2 turns sideways only but maintains balance (    ) 1 needs supervision when turning (    ) 0 needs assist to keep from losing balance or falling  TURN 360 DEGREES INSTRUCTIONS: Turn completely around in a full circle. Pause. Then turn a full circle in the other direction. (    ) 4 able to turn 360 degrees safely in 4 seconds or less (    ) 3 able to turn 360 degrees safely one side only 4 seconds or less (    ) 2 able to turn 360 degrees safely but slowly (    ) 1 needs close supervision or verbal cuing (    ) 0 needs assistance while turning  PLACE ALTERNATE FOOT ON STEP OR STOOL WHILE STANDING UNSUPPORTED INSTRUCTIONS: Place each foot alternately on the step/stool. Continue until each foot has touch the step/stool four times. (    ) 4 able to stand independently and safely and complete 8 steps in 20 seconds (    ) 3 able to stand independently and complete 8 steps in > 20 seconds (    ) 2 able to complete 4 steps without aid with supervision (    ) 1 able to complete > 2 steps needs minimal assist (    ) 0 needs assistance to keep from falling/unable to try    STANDING UNSUPPORTED ONE FOOT IN FRONT INSTRUCTIONS: (DEMONSTRATE TO SUBJECT) Place one foot directly in front of the other. If you feel that you cannot place your foot directly in front, try to step far enough ahead that the heel of your forward foot is ahead of the toes of the other foot. (To !! 101!score 3 points, the length of the step should exceed the length of the other foot and the width of the stance should approximate the subject’s normal stride width.)  (    ) 4 able to place foot tandem independently and hold 30 seconds (    ) 3 able to place foot ahead independently and hold 30 seconds (    ) 2 able to take small step independently and hold 30 seconds (    ) 1 needs help to step but can hold 15 seconds (    ) 0 loses balance while stepping or standing  STANDING ON ONE LEG INSTRUCTIONS: Stand on one leg as long as you can without holding on. (    ) 4 able to lift leg independently and hold > 10 seconds (    ) 3 able to lift leg independently and hold  5-10 seconds (    ) 2 able to lift leg independently and hold ≥ 3 seconds (    ) 1 tries to lift leg unable to hold 3 seconds but remains standing independently. (    ) 0 unable to try of needs assist to prevent fall   (    )   TOTAL SCORE (Maximum = 56)   !! 102!Appendix D: Spinal Cord Independence Measure Adapted from Catz A et al. SCIM - spinal cord independence measure: a new disability scale for patients with spinal cord lesions, Spinal Cord, 35: 850-856, 1997; Appendix. Used with permission from Nature Publishing.  Below is Version 1 – May 1996, Raanana, Israel. SCIM now has a version III out (http://www.rehabmeasures.org/Lists/RehabMeasures/Attachments/967/SCIM.pdf), with slight changes in wording of ratings and an additional item (transfers: ground-wheelchair).  Spinal Cord Independence Measure Worksheet:  Patient Name: _______________________________    Date for Round 1: _______________________         Date for Round 2:_______________________ Date for Round 3: _______________________                Date for Round 4: _______________________  Self-care 1. Feeding:  0 = needs parenteral, gastrostomy or fully assisted oral feeding  1 = eats cut food using several adaptive devices for hand and dishes  2 = eats cut food using only one adaptive device for hand; unable to hold cup  3 = eats cut food with one adaptive device; holds cup 4 = eats cut food without adaptive devices; needs a little assistance (e.g., to open containers)  5 = independent in all tasks without any adaptive device  R1:   R2: R3: R4:    2. Bathing (soaping, manipulating water tap, washing)  0 = requires total assistance  1 = soaps only small part of body with or without adaptive devices 2 = soaps with adaptive devices; cannot reach distant parts of body or cannot operate a tap 3 = soaps without adaptive devices; needs a little assistance to reach distant parts of body 4 = washes independently with adaptive devices or in specific environmental setting  5 = washes independently without adaptive devices !! 103!R1:  R2: R3: R4:    3. Dressing (preparing clothes, dressing upper and lower body, undressing)  0 = requires total assistance 1 = dresses upper body partially (e.g. without buttoning) in a special setting (e.g. back support) 2 = independent in dressing and undressing upper body. Needs much assistance for lower body.  3 = requires little assistance in dressing upper or lower body 4 = dresses and undresses independently, but requires adaptive devices and/or special setting  5 = dresses and undresses independently, without adaptive devices  R1:  R2: R3: R4:   4. Grooming (washing hands and face, brushing teeth, combing hair, shaving, applying makeup)  0 = requires total assistance  1 = performs only one task (e.g. washing hands and face) 2 = performs some tasks using adaptive devices; needs help to put on/take off devices 3 = performs some tasks using adaptive devices, puts on/takes off devices independently 4 = performs all tasks with adaptive devices or most tasks without devices 5 = independent in all tasks without adaptive devices  R1:  R2: R3: R4:    Respiration and Sphincter Management 5. Respiration  0 = requires assisted ventilation  2 = requires tracheal tube and partially assisted ventilation 4 = breathes independently but requires much assistance in tracheal tube management 6 = breathes independently and requires little assistance in tracheal tube management 8 = breathes without tracheal tube, but sometimes requires mechanical assistance for breathing  10 = breathes independently without any device !! 104!R1:  R2: R3: R4:    6. Sphincter management – Bladder  0 = indwelling catheter 5 = assisted intermittent catheterization or no catheterization, residual urine volume > 100 cc  10 = intermittent self-catheterization  15 = no catheterization required, residual urine volume < 100 cc  R1:  R2: R3: R4:    7. Sphincter management – Bowel 0 = irregularity, improper timing or very low frequency (less than once in 3 days) of bowel movements 5 = regular bowel movements, with proper timing, but with assistance (e.g. for applying suppository)  10 = regular bowel movements, with proper timing, without assistance  R1:  R2: R3: R4:    8. Use of toilet (perineal hygiene, clothes adjustment before/after, use of napkins or diapers)  0 = requires total assistance  1 = undresses lower body, needs assistance in all the remaining tasks 2 = undresses lower body and partially cleans self (after); needs assistance in adjusting clothes and/or diapers 3 = undresses and cleans self (after); needs assistance in adjusting clothes and/or diapers 4 = independent in all tasks but needs adaptive devices or special setting (e.g. grab-bars)  5 = independent without adaptive devices or special setting  R1:  R2: R3: R4:    Mobility (room and toilet) !! 105!9. Mobility in bed and action to prevent pressure sores  0 = requires total assistance  1 = partial mobility (turns in bed to one side only)  2 = turns to both sides in bed but does not fully release pressure  3 = releases pressure when lying only  4 = turns in bed and sits up without assistance 5 = independent in bed mobility; performs push-ups in sitting position without full body elevation  6 = performs push-ups in sitting position R1:  R2: R3: R4:    10. Transfers: bed-wheelchair (locking wheelchair, lifting footrests, removing and adjusting arm rests, transferring, lifting feet)  0 = requires total assistance  1 = needs partial assistance and/or supervision  2 = independent R1:  R2: R3: R4:    11. Transfers: wheelchair-toilet-tub (if uses toilet wheelchair – transfers to and from; if uses regular wheelchair – locking wheelchair, lifting footrests, removing and adjusting arm rests, transferring, lifting feet)  0 = requires total assistance 1 = needs partial assistance and/or supervision, or adaptive device (e.g. grab-bars)  2 = independent R1:  R2: R3: R4:   Mobility (indoors and outdoors) 12. Mobility indoors (short distances)  0 = requires total assistance 1 = needs electric wheelchair or partial assistance to operate manual wheelchair  2 = moves independently in manual wheelchair  3 = walks with a walking frame  4 = walks with crutches  5 = walks with two canes  6 = walks with one cane  7 = needs leg orthosis only  8 = walks without aids !! 106!R1:  R2: R3: R4:    13. Mobility for moderate distances (10-100 meters)  0 = requires total assistance 1 = needs electric wheelchair or partial assistance to operate manual wheelchair  2 = moves independently in manual wheelchair  3 = walks with a walking frame  4 = walks with crutches  5 = walks with two canes  6 = walks with one cane  7 = needs leg orthosis only  8 = walks without aids R1:  R2: R3: R4:     14. Mobility outdoors (more than 100 meters)  0 = requires total assistance 1 = needs electric wheelchair or partial assistance to operate manual wheelchair  2 = moves independently in manual wheelchair  3 = walks with a walking frame  4 = walks with crutches  5 = walks with two canes  6 = walks with one cane  7 = needs leg orthosis only  8 = walks without aids R1:  R2: R3: R4:     15. Stair management  0 = unable to climb or descend stairs  1 = climbs 1 or 2 steps only, in a training setup 2 = climbs and descends at least 3 steps with support or supervision of another person 3 = climb and descends at least 3 steps with support of handrail and/or crutch and/or cane  4 = climbs and descends at least 3 steps without any support or supervision  !! 107!R1:  R2: R3: R4:    16. Transfers: wheelchair-car (approaching car, locking wheelchair, removing arm and foot rests, transferring to and from car, bringing wheelchair into and out of car)  0 = requires total assistance  1 = needs partial assistance and/or supervision, and/or adaptive devices  2 = independent without adaptive devices R1:  R2: R3: R4:    Round 1: Self-care subscale score (0-20): ____________________ Respiration and sphincter management subscale score (0-40): _____________________ Mobility subscale score (0-40): ____________________ Total Score: __________________  Round 2: Self-care subscale score (0-20): ____________________ Respiration and sphincter management subscale score (0-40): _____________________ Mobility subscale score (0-40): ____________________ Total Score: __________________  Round 3: Self-care subscale score (0-20): ____________________ Respiration and sphincter management subscale score (0-40): _____________________ Mobility subscale score (0-40): ____________________ Total Score: __________________  Round 4: Self-care subscale score (0-20): ____________________ Respiration and sphincter management subscale score (0-40): _____________________ Mobility subscale score (0-40): ____________________ Total Score: __________________    !! 108!Appendix E: Canadian Occupational Performance Measure  !!!! !!! 109!Appendix F: Montreal Cognitive Assessment  !!!!! 110!Appendix G: Correlations between performance and clinical measures  !!!!!! !!                     0 20 40 60 -25 25 75 BBS Reaction time (% change) r = -.371 p = .3650 50 100 -25 25 75 SCIM Reaction time (% change) r = -.192 p = .6200 20 40 60 -75 -50 -25 0 BBS Standing time (% change) r = .868 p = .0250 50 100 -75 -50 -25 0 SCIM Standing time (% change) r = -.206 p = .6580 20 40 60 -100 0 100 200 300 BBS Error ratio (% change) r = .772 p = .0250 50 100 -100 0 100 200 300 SCIM Error ratio (% change) r = .140 p = .7200 20 40 60 -30 -15 0 15 30 BBS Total number of words r = .491 p = .2170 50 100 -30 -15 0 15 30 SCIM Total number of words r = -.109 p = .781!! 111!                                  0 20 40 60 -25 -5 15 35 55 BBS MSRS r =- .458 p = .2540 50 100 -25 -5 15 35 55 SCIM MSRS r =- .424 p = .2550 5 10 -25 25 75 COPM Reaction time (% change) Perfor r = -.008, p = .983 Satis r = .251, p = .515 0 15 30 -25 25 75 MoCA Reaction time (% change) r = -.119 p = .7610 5 10 -75 -50 -25 0 COPM Standing time (% change) Perfor r = -.542, p = .209 Satis r = -.075, p = .873 0 15 30 -75 -50 -25 0 MoCA Standing time (% change) r = -.510 p = .2430 5 10 -100 0 100 200 300 COPM Error!ratio!(%!change)!Perfor r = .088, p = .822 Satis r = -.341, p = .369 0 15 30 -100 0 100 200 300 MoCA Error ratio (% change) r = .305 p = .4240 5 10 -30 -15 0 15 30 COPM Total number of words Perfor r = .740, p = .023 Satis r = .092, p = .814 0 15 30 -30 -15 0 15 30 MoCA Total number of words r = .610 p = .081!! 112!                       Associations between clinical measures of function, cognition and chronicity with the dual task cost of primary performance measures. Boxes in bold are significant p< 0.05.  0 5 10 -25 -5 15 35 55 COPM MSRS Perfor r =-.114, p =.770 Satis r =.210, p =.587 0 15 30 -25 -5 15 35 55 MoCA MSRS r =- .315 p = .4090 20 40 -25 25 75 Chronicity Reaction time (% change) r = -.213 p = .5820 20 40 -75 -50 -25 0 Chronicity Standing time (% change) r = -.053 p = .9110 20 40 -100 0 100 200 300 Chronicity Error ratio (% change) r = -.266 p = .4890 20 40 -30 -15 0 15 30 Chronicity Total number of words r = .176 p = .6510 20 40 -25 -5 15 35 55 Chronicity MSRS r = -.088 p = .821

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