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Effect of the biceps Brachii tendon on elbow flexor force steadiness in men and women Lizu, Afruna 2015

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     EFFECT OF THE BICEPS BRACHII TENDON ON ELBOW FLEXOR FORCE STEADINESS IN MEN AND WOMEN by Afruna Lizu M.B.B.S., The University of Chittagong, Bangladesh, 2010  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in  THE COLLEGE OF GRADUATE STUDIES  (Interdisciplinary Studies)   THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan)   November 2015 © Afruna Lizu, 2015ii  ABSTRACT Tendons are connective tissue that transmit force to the skeleton. Therefore, mechanical and material properties of the tendon, such as stiffness, strain, stress and Young’s Modulus likely influence force production and control. Men are steadier than women in maintaining isometric elbow flexion force, but the cause of this sex-related difference in force steadiness (FS) has not been established. To-date most FS studies have centred on absolute force as well as motor unit activity, with limited attention given to the interaction between muscle and tendon. The purpose of this study was to determine whether the mechanical properties of tendon differ between men and women, and whether this contributes to men being steadier than women. Ten men (23 ± 4years) and 10 women (22 ± 3years) who were healthy and recreationally active participated in a single session to evaluate maximal force, steadiness and tendon properties. Submaximal forces of 2.5%, 5%, 10%, 20%, 40% and 60% maximum voluntary contraction (MVC) were executed and FS as well as tendon properties were measured during these efforts. Contractions were randomized within and between individuals for the submaximal isometric elbow flexion contractions. All measures were undertaken during the 5 second steady state contraction executed in a neutral wrist position. Results indicated that men were significantly stronger (p < 0.001), steadier (p < 0.001) and the BB tendon was stiffer (p = 0.03) in men than women. Tendon stiffness and CV of force were strongly correlated (r2 = 0.60; r = 0.77; p < 0.001) for women but moderately correlated (r2 = 0.21; r = 0.46; p < 0.001) for men. Therefore, our results suggest that having a more compliant BB tendon is likely a contributing factor of less stability in the elbow flexors of women compared to men.   iii  PREFACE Ethics approval for this research was granted by the University of British Columbia’s Behavioural Research Ethics Board on March 5, 2015. The ethics approval certificate number for the current study is H14-00165. To date, the research included in this thesis has not been published in full. Preliminary results were presented at the European Congress of Sports Sciences (ECSS) on June 27, 2015.                                  iv  TABLE OF CONTENTS ABSTRACT ............................................................................................................................. ii PREFACE ............................................................................................................................... iii TABLE OF CONTENTS ........................................................................................................ iv LIST OF TABLES ................................................................................................................... vi LIST OF FIGURES ............................................................................................................... vii LIST OF ABBREVIATIONS .............................................................................................. viii ACKNOWLEDGEMENTS .................................................................................................... ix DEDICATION .......................................................................................................................... x CHAPTER 1: INTRODUCTION ............................................................................................ 1 1.1 Neuromuscular Organization of Voluntary Movement .................................................... 1 1.2 Structural and Mechanical Properties of Tendon .............................................................. 4 1.3 Integrated Functional Interaction between Muscle and Tendon ....................................... 8 1.4 Contribution of Biceps Brachii Muscle and Its Tendon to Movement ........................... 12 1.5 Isometric Force and Force Steadiness ............................................................................. 13 1.6 Sex difference in force steadiness ................................................................................... 15 1.7 Summary of Literature .................................................................................................... 16 CHAPTER 2: PURPOSE AND HYPOTHESES ................................................................. 17 2.1 Purpose ........................................................................................................................ 17 2.2 Hypotheses .................................................................................................................. 17 CHAPTER 3: METHODOLOGY ........................................................................................ 18 3.1 Participants ...................................................................................................................... 18 3.2 Experimental Protocol .................................................................................................... 18 3.3 Experimental Setup ......................................................................................................... 21 3.4 Assessment ...................................................................................................................... 21 3.4.1 Resting BB muscle and tendon length and CSA measurement ............................... 21 3.4.2 Maximal Voluntary contractions (MVC) and Voluntary Activation ....................... 22 3.4.3 Measurement of tendon elongation and CSA during the steadiness task ................ 23 3.5 Data Analysis .................................................................................................................. 25 3.5.1 Muscle and Tendon Properties at rest ...................................................................... 26 v  3.5.2 Mechanical Properties of tendon .............................................................................. 27 3.5.3 Voluntary Activation and Force Steadiness ............................................................. 30 3.6 Statistical Analysis .......................................................................................................... 30 CHAPTER 4: RESULTS ....................................................................................................... 32 4.1 Participant Characteristics .............................................................................................. 32 4.2 Resting BB Muscle and Tendon ..................................................................................... 32 4.3 Tendon’s Mechanical and Structural Properties during Steadiness Task ....................... 34 4.4 Steadiness ........................................................................................................................ 42 4.5 Correlation and Regression Analysis .............................................................................. 43 CHAPTER 5: DISCUSSION ................................................................................................. 46 5.1 Sex differences in mechanical properties of BB tendon ................................................. 46 5.2 BB tendon during different submaximal forces .............................................................. 50 5.3 Force Steadiness and Mechanical Properties .................................................................. 51 CHAPTER 6: CONCLUSIONS ............................................................................................ 54 6.1 Implications ..................................................................................................................... 54 6.2 Strengths ......................................................................................................................... 55 6.3 Limitations and Future Directions .................................................................................. 56 REFERENCES ....................................................................................................................... 58 APPENDICES ......................................................................................................................... 71 Appendix A: Copyright Approval for Figure 1.0 ................................................................. 71 Appendix B: Copyright Approval for Figure 1.2 .................................................................. 72 Appendix C: Ethics Approval ............................................................................................... 73 Appendix D: Consent Form .................................................................................................. 74       vi  LIST OF TABLES Table 4.0: Participant characteristics……………………………………………….. 32   vii  LIST OF FIGURES Figure 1.0: Diagram of the multi-unit structure of the tendon…………………………. 6 Figure 1.1: Schematic of force production from the CNS to the interaction of mechanical elements in the periphery …………………………………….. 10 Figure 1.2: Length-tension relationship of a sarcomere………………………………... 11 Figure 3.0: Experimental protocol……………………………………………………… 20 Figure 3.1: Representative visual feedback for 40% submaximal force………………... 25 Figure 3.2: Representation of BB muscle and tendon data analysis at resting condition. 27 Figure 3.3: Representative images of tendon displacement at 60% MVC in a male participant.…………………………………………...……………………… 28 Figure 3.4: Representation of measurement of the mechanical properties of tendon…... 29 Figure 4.0: Sex differences in tendon and muscle at rest (A) Tendon CSA, (B) muscle CSA, (C) muscle thickness, (D) tendon length, and (E) muscle length.…... 33 Figure 4.1: Tendon displacement across force levels…………………………………... 34 Figure 4.2: Tendon force between men and women across force levels……...………... 35 Figure 4.3: Tendon CSA for the main effects of (A) sex and (B) force levels…………. 36 Figure 4.4: Tendon strain for (A) men and women and, (B) force level ……………… 38 Figure 4.5: Tendon stress across force levels…………………………….…………...… 39 Figure 4.6: Tendon stiffness for (A) men and women and, (B) force levels…………… 41 Figure 4.7: Young's Modulus across force levels…………………………..…………... 42 Figure 4.8: Force steadiness between men and women across force levels………...….. 43 Figure 4.9: Relationship between CV of Force (%) and tendon mechanical properties... 45  viii  LIST OF ABBREVIATIONS αMN: Alpha motor neuron  ANOVA: Analysis of variance B-Mode: Brightness-mode  BB: Biceps Brachii  Ca2+: Calcium  CNS: Central Nervous System CSA: Cross-sectional area CV: Coefficient of variation  FS: Force steadiness  LH: Long head  mm: millimeter  MTJ: Muscle-tendon junction  MG: Medial gastrocnemius MU: Motor unit  MUDR: Motor unit discharge rate  MUDRV: Motor unit discharge rate variability  MVC: Maximal voluntary contraction N: Newton Na+: Sodium RTL: Resting tendon length SH: Short head SD: Standard deviation  US: Ultrasound VL: Vastus lateralis µm: micrometer  ix  ACKNOWLEDGEMENTS First of all, I would like to thank Almighty Allah for giving me the strength and courage to complete this project. My deepest gratitude goes to my supervisor Dr. Jennifer Jakobi, who had always been there to help me as a mentor. Thank you for sharing your knowledge with me and for challenging me and always encouraging me to do more than I thought I was able to. Without your guidance, knowledge and care throughout my academic journey the completion of this research would not have been possible. I would also like to extend immense gratitude to my committee members, Dr. Chris McNeil and Dr. Paul van Donkelaar for their valuable time. I would also like to thank all the participants who volunteered their time and effort to make this study possible. I am indebted to my amazing colleagues of my research groups, Dr. Brad Harwood, Noelannah Neubauer, Sharmin Arefin, Carey Simpson, Kaylee Larocque, Rowan Smart, Brian DoHyun Kim, Nikita Bouwmeister and Dylan Melady. It was a great pleasure to work with such a wonderful research group. This research would not have been possible without their help and support. Special thanks to Sharmin and Rowan for being my hands throughout all of my data collection. I really appreciate all of the long hours you put in with me. I would like to thank my dear husband Golam Kabir for all his support, love and especially taking good care of our daughter Ruqayaah Kabir which helped me to concentrate on my work. Finally, I am thankful to my family members for their unconditional love, support and sacrifices throughout my entire life. x  DEDICATION          To My Family………….     1  CHAPTER 1: INTRODUCTION 1.1 Neuromuscular Organization of Voluntary Movement The production of voluntary movement for a desired outcome is not a simple procedure. It requires development, processing and integration of information generated and received in the brain for the transmission of signals down to the peripheral nervous system and inevitably the muscle. More specifically, voluntary action requires the movement command to be initiated and then processed through a hierarchy where information from the primary motor cortex generates the movement plan. Multiple excitatory and inhibitory electrochemical signals from the primary motor cortex become integrated and inevitably are sent down the descending tracts to the grey matter of the spinal cord. Within the spinal cord and brain, voluntary control depends on a continuous exchange of descending information with somatosensory, visual and postural signals generated throughout the body. A small, albeit fundamental, anatomical component of the transfer of information to the peripheral motor system is the alpha motor neuron (αMN) which lies in the ventral horn grey matter of the spinal cord. When electrochemical inputs converge on the cell body of the αMN and the threshold potential reaches approximately -55mV an action potential is generated and conducted down the axon to the muscle. In the peripheral nervous system, motor commands are transmitted to the extrafusal muscle fibres via axons of an αMN. The axon of the αMN splits to innervate multiple muscle fibres. A single αMN and all of the muscle fibres that are innervated is termed a motor unit (MU) (Ghez & Krakauer, 2000; Sherrington, 1925). Each αMN normally innervates more than one muscle fibre but each muscle fibre is only innervated by one αMN (Masakado, 1994). The meeting area of nerve terminals and muscle fibre is called the neuromuscular junction. The neuromuscular 2  junction consists of the presynaptic plasma membrane of the motor nerve, the synaptic cleft between the nerve and muscle, and postsynaptic membrane of the motor end plate which is located on the sarcolemma of the muscle fibre. The nerve terminals are filled with numerous synaptic vesicles containing the neurotransmitter acetylcholine. The action potential travels through the cell membrane and down to the nerve terminals, and causes the opening of voltage-gated calcium (Ca2+) channels and permits the entry of Ca2+ into the terminal. Ca2+ ions initiate fusion of the vesicles to the membrane of the terminal button leading to the release of acetylcholine into the synaptic cleft by means of exocytosis (Weber et al., 1998). Acetylcholine crosses the synaptic cleft and binds to acetylcholine receptors embedded in the motor endplate membrane of the muscle fibre leading to the opening of voltage-gated Na+ channels. Inward flow of Na+ current depolarizes the postsynaptic membrane of the muscle fibre and propagation of the action potential continues along the muscle membrane as well as down into the muscle through a series of transverse tubules. Eventually Ca2+ is released from the sarcoplasmic reticulum and binds to troponin-tropomyosin complex on the actin filaments of the muscle fibre. A conformational change in troponin causes the tropomyosin to move, exposing active sites on the actin filament. Which initiates cross-bridge formation, cycling between actin and myosin and generates muscle contraction (McArdle et al, 2010). Contraction of numerous individual muscle fibres creates the global muscle contraction. The contraction and movement of muscle generates afferent feedback that interacts with descending commands to adapt and control muscle contraction for human movement. Two important sensory organs, the muscle spindle and the golgi tendon organ both influence the motor neuron in regulating MU discharge rate (MUDR) for controlling force output. Muscle spindles are composed of 3-10 intrafusal muscle fibres, embedded in parallel direction between 3  extrafusal muscle fibres. The nuclear bag and nuclear chain intrafusal fibres are proprioceptors that are responsible for revealing information about the static length and rate of length change (velocity) of muscle. Muscle spindles have both sensory (Group Ia and Group II afferent) and motor (gamma motor neurons) innervations. When a muscle is stretched, Group Ia afferents act in response to the velocity and degree of stretch and send this information to the spinal cord and excite alpha (efferent) motor neurons monosynaptically, while Group II afferents convey information about the degree of stretch to the brain. The Group II afferent axons enter the CNS in the dorsal root and synapse with the second order neurons which ultimately transmit the information to the brain. Overall, the net input causes agonist muscles to contract. An additional and important pathway working with the muscle spindle is an excitatory pathway operating through gamma motor neurons. These neurons have small diameter axons that originate in the ventral horn of the spinal cord and are responsible for causing shortening of the polar regions of the intrafusal fibres. This eventually leads to increasing the discharge rate of the sensory endings and ultimately, these efferent neurons help to maintain optimal sensitivity of the spindle at all muscle lengths during muscle contraction (Pearson & Gordon, 2000). Golgi tendon organs are found at the muscle-tendon junction (MTJ) and provide further information about muscle forces. The golgi tendon organ is most sensitive to changes in contractile force (from low to high force) and relays inhibitory signals to motor neurons of the same muscle, and excitatory signals to antagonist muscles via Group Ib afferents. Thus, they act as a protective mechanism to guard the muscle from overstretching (McComas, 1996). Finally, these aspects of the neuromuscular system are integrated with the mechanical properties of the musculotendinous unit to produce the desired movement. 4  1.2 Structural and Mechanical Properties of Tendon Tendons are anatomic structures, interposed between muscles and bones, which allow force transmission from the attached muscle to the skeleton to accomplish desired movements (Józsa & Kannus, 1997). Being connective tissue, tendons mainly consist of collagens, proteoglycans, glycoproteins, cells and water (Silver et al., 2003). Collagen is the most ample protein in the extracellular matrix with type I collagen forming the major component in the tendon. Indeed, due to its organizational characteristics it occupies a vital role for the tensile strength of tendons (Wang et al., 2012). Two other important ground substances, proteoglycans (e.g. aggrecan & decorin) and glycoproteins (e.g. tenascin-C, fibronectin and elastin) also play an important role in mechanical stability due to their viscous properties (Watkins, 2010). Particularly, elastin causes length recovery after mechanical stretching (Józsa & Kannus, 1997; Pins et al., 1997; Wang et al., 2012). Additional to collagen and ground substances, tendons also contain different types of cells, such as tenocytes and tenoblasts, chondrocytes, synovial cells and vascular cells. Tendon cells; tenocytes and tenoblasts are oriented to the axis of the tendon between the collagen fibre bundles with the main function of producing extracellular matrix components (i.e. collagen, fibronectin and proteoglycans). Histologically, tendons are multi-unit structures. The smallest structural unit is the fibril which is composed of long and stiff rod-like collagen molecules. A group of collagen fibrils form a collagen fibre, which is the basic unit of a tendon. At the next level of tendon structure, several collagen fibres are covered by a thin layer of connective tissue termed endotenon which forms a primary fibre bundle (sub fascicle). Again, the endotenon covers the primary fibre bundles to form secondary fibre bundles (fascicles) and then tertiary fibre bundles. The ultimate structural level is the tendon unit which is formed by the tertiary fibre bundles and also surrounded by a 5  connective tissue, the epitenon (Wang, 2006) (Figure 1.0). In the unloaded resting conditions, the collagen fibres are characterized by a wavy pattern known as crimp, mostly due to the cross-linking of proteoglycan fibres. However, the wavy construction becomes invisible when the tendon is stretched due to the straightening of the collagen fibres (Elliott, 1965; Józsa & Kannus, 1997). 6   Figure 1.0: Diagram of the multi-unit structure of the tendon. Smallest structural unit is the fibril which is composed of long and stiff rod-like collagen molecules. A group of collagen fibrils form a collagen fibre. Several collagen fibres are covered by the endotenon, to form a primary fibre bundle (sub fascicle). Primary fibre bundles form secondary fibre bundles (fascicles) and then tertiary fibre bundles. Finally conjunction of the tertiary fibre bundles form tendon units. Fibre bundles are surrounded by a connective tissue, endotenon whereas a tendon unit is also surrounded by a connective tissue called the epitenon (Wang, 2006, J. Biomech., 39:1563-1582, p.1565. License number: 3625800455654).  Previously, it has been impossible to measure directly in humans in vivo the mechanical properties of the tendon. Most of the values of mechanical properties of tendon have been gained from measures of isolated animal or human tendon by applying tensile loads (Bennett et al., 1986; Lieber et al., 1991). However, studying isolated tendon is inappropriate when trying 7  to understand the physiological role of human tendon in force generation as it functions within a musculotendinous unit, rather than in isolation. With advances in ultrasound (US) technology with real-time brightness-mode (B-Mode) over the past two decades, the ability to investigate the in vivo tendon mechanical and structural properties has become available.  Mechanical properties of tendon refers to how it behaves in response to an applied force. The common parameters to illustrate the mechanical properties of a tendon are stress, strain, stiffness and Young’s Modulus. Stress is estimated as the ratio of tendon force and the cross sectional area, and the strain as the quotient from the tendon elongation and resting tendon length (Heinemeier & Kjaer, 2011). Young's Modulus is the linear slope of the stress-strain relationship. Therefore, the stress-strain relationship represents the genuine material characteristics of the tendon (Heinemeier & Kjaer, 2011). A steeper slope of stress-strain relationship accounts for a higher Young's Modulus suggesting less tendon elongation at a given tendon force and, therefore a higher stiffness of the tendon (Heinemeier & Kjaer, 2011). Stiffness is calculated by using a linear regression between tendon force and tendon elongation (Onambele et al., 2007) and it indicates the ability of the tendon to transfer force rapidly and effectively. Thus, a stiffer tendon requires less time to be stretched than a compliant tendon (Maganaris et al., 2006; Wilkie, 1949). The specific mechanical properties of tendons depend on the muscles involved, the intensity of the contraction and the training status of the individual (Ishikawa & Komi, 2008). For example, Kubo et al. (2001) reported, an in vivo increase in human patellar tendon stiffness and Young's Modulus following twelve weeks of resistance training. Kongsgaard et al. (2007) reported that resistance training induced region specific hypertrophy of the patellar tendon. More recently, Malliaras et al. (2013) also observed increased stiffness following resistance training which 8  used high contraction intensities. Similarly, Albracht & Arampatzis (2013) found that with an exercise intervention of running the Achilles tendon increased in stiffness, which was accompanied by an improvement of running economy. These studies suggest that the extent of the adaptation is related to the pattern of applied loading (e.g., duration, intensity, repetitions, and sets) and the duration of the intervention. Furthermore, the tendon’s mechanical and morphological properties also varies depending on sex and age (Benjamin et al., 2008). Several studies have suggested decreased stiffness and Young's Modulus in older adults compared with young adults (Narici et al., 2008; Onambele et al., 2006). Magnusson et al. (2003) also demonstrated that the tendon may change its cross-sectional area with ageing, as old women had an ~ 22% greater Achilles tendon cross-sectional area compared with young women.  There is limited research on characterization of tendon properties between men and women, but mechanical properties seem to be influenced by sex, with lower stiffness and Young's modulus values reported for the tendon-aponeurosis of women compared to men (Kubo et al., 2003; Onamble et al., 2007). Although, the mechanisms are not clearly established, it has been suggested that estrogen is responsible for the sex difference in the structural and mechanical properties of tendon (Onamble et al., 2007; Zazulak et al., 2006). In addition, men show greater increases in collagen synthesis following a training intervention compared with women (Magnusson et al., 2007; Miller et al., 2007; Sullivan et al., 2009), and the level of estrogen was proposed as the primary reason for compromising the collagen synthesis in women (Magnusson et al., 2007). 1.3 Integrated Functional Interaction between Muscle and Tendon Muscle and tendon work as a unit within the musculoskeletal system according to Hill’s classical model (Hill, 1938) which indicates that muscle compliance is dependent upon the 9  anatomical aspects of parallel and series elastic components. Parallel elastic components include connective tissue structures (muscle membranes; the epimysium, perimysium and endomysium) that bind muscle fibres and fascicles together as well as intra fibrillar proteins such as titin (Kellermayer et al., 1997;  Magid & Law, 1985; Roberts, 2002; Wang et al., 1993). Series elasticity contributes mostly within the tendon and to a lesser extent within the cross-bridges themselves through the actin-myosin filaments (Linari et al., 1998; Roberts, 2002). Therefore, the changes in the length of the muscle fibre due to the active development of force is influenced by tendon. Thus, tendon inevitably contributes to changes in fascicle lengths through individual sarcomere lengths changing. Length changes of the sarcomere have been observed in animal and human studies during isometric contractions (Coirault et al., 1997; Ito et al., 1998; Muhl, 1982). For example, Muhl (1982) reported that the muscle fibres of rabbits shortened during isometric contractions which Coirault et al. (1997) supported by demonstrating in  the hamster diaphragm that muscle decreased ~5% its resting length  during an isometric contraction, but the whole muscle length did not change. Therefore, for a desired outcome the contraction dynamics of muscle fibres interact with the tendon mechanics and facilitate force transmission to the bone during locomotor performance (Figure 1.1). 10   Figure 1.1: Schematic of force production from the CNS to the interaction of mechanical elements in the periphery. CNS, Central Nervous System  The degree of tendon elongation depends upon the mechanical properties as well as dimensions (length and/or cross-sectional area) of the musculotendinous unit (David et al., 1978; Reeves, 2006). Thereby, the mechanical properties of a tendon affect the functional properties of the entire musculotendinous unit. Ultimately, it affects force-length and force-velocity-relationships of the muscle fibres of the linked unit due to its non-rigid spring like properties (Alexander, 2002; Ettema et al., 1990; Fukunaga et al., 2002). In the length-tension relationship of muscle, the plateau region illustrates optimal overlap of the actin and myosin at which force production is maximal. Shortening or lengthening of the sarcomere on the ascending or descending limb of the force-length curve beyond this small range results in decreased force production (Gordon et al., 1966). Therefore, during muscle contraction the length-tension curve can be shifted by the action of a series compliant tendon when stretched. However, the extent to which contractile properties of skeletal muscles are modified by the tendon's mechanical property remains unknown. Fukunaga et al. (2002) suggested that during ankle bending by plantar flexion (Kubo et al., 2000; Sakuma et al., 2011), walking (Fukunaga et al., 2001), running (Lichtwark et al., 2007) and jumping (Ishikawa et al., 2005) the elasticity of tendon 11  may account for a rightward shift of the actual working length of the sarcomeres (Figure 1.2). Thus, allowing the muscle fibres to contract around the plateau region of the length-tension curve and in this way the force potential of the muscle fibres was improved.  Figure 1.2: Length-tension relationship of a sarcomere. Tendon compliance allows the muscle fibres to contract around the plateau region of the sarcomeres length-tension curve. This improves the force potential of the whole muscle (Fukunaga et al. 2002, Exerc. Sport Sci. Rev., 30:106-110, p.110, License number: 3625801012737). MG: Medial gastrocnemius, VL: Vastus lateralis, µm: micrometer  Furthermore, according to Hill's (1938) force–velocity relationship, muscle force declines with increasing shortening velocity. A greater increase in tendon stiffness can reduce the shortening velocity of the muscle fibres. Thus, the velocity of force transmission to the bone increases. Ankle bending (MG) Pedaling (VL) Walking (MG) Sarcomere length (µm) Relative force 12  During force development, the elastic property of tendon can affect muscle fascicle geometry. For example, in the human gastrocnemius, fibre pennation angle and physiological cross-sectional area varied over the path of an isometric contraction from resting to highest contraction as muscle fibres shortened against a compliant tendon (Narici et al., 1996). The authors found from rest to highest force, at a fixed ankle joint angle of 110°, pennation angle increased and fibre length decreased.  Moreover, Rack and Ross (1984) studied the long flexor muscle and tendon of the thumb, while the other muscles of the thumb were anaesthetised and observed that the elastic properties of the tendon cause a reduction in the ability of the thumb to maintain position against a vibrating surface. Similarly, Ward et al. (2006) reported that high stiffness of the digital flexor tendons might render their muscle spindles quite sensitive to length changes at the fingertips and thereby suitable for fine positional control of movement. Thus, it is likely that the mechanical properties of a tendon impact the ability to produce a steady force; however, this has not been investigated. 1.4 Contribution of Biceps Brachii Muscle and Its Tendon to Movement The Biceps Brachii (BB) muscle is a powerful supinator and flexor of the elbow as well as flexor of the shoulder (Tortota, 2005). It is a large two-headed muscle located on the anterior surface of the arm. The two heads include the long head (LH) and short head (SH) and they are integral in movement of the shoulder and elbow. The LH of BB arises from the superior glenoid labrum and supraglenoid tubercle of the scapula and inserts on the radial tuberosity of the radius. Whereas, the SH originates from the tip of the coracoid process of the scapula and attaches distally on the bicipital aponeurosis. The distal BB tendon is formed approximately 7 cm above the elbow joint from the SH and LH of the BB muscle (Skaf et al., 1999; Stevens et al. 2012). It has no tendon sheath and flattens as a strap paratenon-lined extrasynovial structure. 13  The strength of the BB muscle is greater in men than women; sex-related differences in muscle strength is one of the most well established phenomenon contributing to ability of men to maintain a more consistent isometric force compared with women. Miller et al. (1993) investigated mechanisms of strength differences between men and women in the BB and suggested that a larger cross sectional area (CSA) of muscle and larger muscle fibres of men compared with women was the primary reason for men being stronger than women. Contrary to that, a review by Jones et al. (2008) reported that the relationship between muscle force and muscle cross-sectional area is inconsistent. This inconsistent relationship might exist due to the contribution of the tendon, but this remains unknown. The majority of studies that have evaluated tendon mechanical properties in men and women have focused mainly on lower limbs (Kubo et al., 2006; Maganaris & Paul, 1999; Magnusson et al., 2001; Muraoka et al., 2005; Stafilidis et al., 2005). However, Kulshreshtha et al. (2007) reported that the BB tendon is wider and thicker in men compared with women but the functional relevance of this anatomic difference was not considered in studies that have evaluated in vivo tendon properties in the upper limb (Murata et al., 2009; Ohta et al. 2004).   1.5 Isometric Force and Force Steadiness Isometric contractions are used in a wide variety of situations. Literally, isometric contraction means no change in muscle length during contractions (Ito et al. 1998); however, it is well established that during an isometric contraction muscle fibres shorten and the tendon lengthens, due to the contractile properties of muscle and elastic properties of the tendon. After a few seconds the muscle and tendon achieve an equilibrium until there is no slack in the musculotendinous unit, and the total length of the musculotendinous complex is kept constant. Isometric muscle force offers information about strength and more importantly, the ability to 14  maintain a steady isometric contraction around a desired value provides a quantitative measure of how the neuromuscular system controls force output (Enoka et al., 2003). When a person executes a steady isometric contraction around a given force level, the force output fluctuates (Lippold et al., 1957), which is known as force steadiness (FS). In relative terms, it is typically stated as the coefficient of variation (CV) (standard deviation (SD) / mean) of isometric force. There are certain functional tasks of daily living that demand a muscle to maintain a given load at a constant force level. Therefore, the ability to produce or maintain a force can influence an individual’s ability to execute activities of daily living. For example, reduced steadiness is related to an increased risk of falling (Carville et al., 2007) and a decrease in ability to rise from a chair or climb stairs in older adults (Seynnes et al., 2005). Steadiness varies depending on a variety of factors, such as the muscle group investigated, position, age, sex, intensity of contractions, type of contractions, arousal level and  training (Brown et al., 2010; Harwood et al., 2008; Tracey & Enoka, 2002). With an increase in intensity of contraction (% MVC), the relative measures of variability follow a U-shaped function (Danion & Gallea, 2004; Slifkin & Newell, 1999; Tracy & Enoka, 2002). At low and high force levels, the CV of force is highest or the force output is least stable whereas, at intermediate force levels the CV of force is lowest and force output is most consistent. The majority of force steadiness research has centred on understanding force steadiness between young and old adults (Barry et al., 2007; Bazzucchi et al., 2004; Burnett et al., 2000; Christou et al., 2004; Galganski et al., 1993; Keen et al., 1994; Laidlaw et al., 2000, 1999; Sosnoff & Newell, 2006; Tracy & Enoka, 2002). The findings identify older adults to be less steady than young adults in the first dorsal interosseus muscle (Laidlaw et al., 2000), elbow flexors (Bazzucchi et al., 2004), knee extensors (Tracy & Enoka, 2002), tibialis anterior (Dewhurst et al., 2007) and quadriceps of older adults with a history of falling (Carville et al., 2007). However, the results of several studies regarding the underlying 15  cause for differences in FS are equivocal. It has been suggested MU activity and most notably MU discharge rate variability (MUDRV) contributes to a decline in force steadiness with increasing age. MU discharge rate (MUDR) refers to the frequency at which a MU discharges an action potential while discharge variability refers to the inconsistency in which the action potentials fires. Tracy et al. (2005) reported a correlation between the decrease in FS and the increase in MUDRV in the first dorsal interosseous. Similarly, Laidlaw et al., (2000) and Moritz et al., (2005) also reported that MUDRV was the best predictor of the force fluctuations in the first dorsal interosseous muscle during isometric contractions. However, Sosnoff & Newell (2006) and Brown et al. (2010) reported that FS decreased as muscle strength declined. 1.6 Sex difference in force steadiness The ability to produce steady isometric contraction is of interest since it influences the performance outcome of motor tasks (Enoka et al., 2003; Hamilton et al., 2004) and predicts functional ability in older women (Seynnes et al., 2005). However, there is limited research available on sex differences in FS, but studies suggest that women are less steady than men in the elbow flexors (Brown, et al., 2010) and first dorsal interosseous (Christou et al., 2004). Yet, the underlying mechanism for the sex differences in force steadiness has not been addressed clearly. Brown et al., (2010) proposed that strength is the primary factor that contribute to men being steadier than women. In that study, force was the only physiological variable documented. More recently, Brown (2011) compared MUDR and MUDRV between young men and women during sustained isometric elbow flexion across 5 to 50 % force levels and reported that men were stronger as well as steadier than women. Strength was a strong predictor of force steadiness but MUDRV was not correlated. Svendson & Madeleine (2010) suggested that men had elevated activity of the BB compared with women and that muscle activation may 16  be different between sexes. Alternatively, Christou et al. (2004) proposed that women exhibited greater 1-to 2-Hz oscillations in force during the steady-state contraction due to a reduced ability to modulate descending drive. Most studies that have investigated the difference between men and women producing stable contractions typically consider the neural aspect of force generation or absolute force (Brown, et al., 2010; Clark et al., 2005; Harwood et al., 2008; Nonaka et al., 2006). However, one potential reason that remains overlooked in understanding force steadiness is the influence of the mechanical properties of the tendon.  1.7 Summary of Literature It is well established that women experience a reduction in ability to perform functional activities earlier in life than men and that steadiness is a good predictor of functional execution (Marmon et al., 2011). Therefore, investigations into sex-related differences in force steadiness are increasing. Studies to date suggest that MUDR, MUDRV and strength partially contribute to differences between men and women. But, the execution of a steady contraction involves the tendon transmitting force between muscle and bone and to do this the inherent mechanical aspects that centre on compliance and stiffness change based upon the force level and the degree of muscle shortening. It is likely that compliance will have an effect on the muscle’s contribution to FS since the tendon acts as a spring in series (Rack & Ross, 1984; Ward et al., 2006). Thus, information about the movement might not be transmitted to the muscle thereby reducing the sensitivity of the spindle. Further, this increase in compliance might contribute to the slow transmission of force and make position control difficult, which would result in reduced FS of women compared with men. To date, the relationships of the tendon’s mechanical properties and force fluctuation during isometric elbow flexion have not been assessed in men and women. 17  CHAPTER 2: PURPOSE AND HYPOTHESES 2.1 Purpose The purpose of this study was to investigate the relationship between the mechanical properties of the tendon relative to FS in men and women. Specifically, stress, strain, Young’s Modulus and stiffness of the BB tendon will be evaluated simultaneously with FS of the elbow flexor muscles over a range of submaximal isometric forces between men and women.  2.2 Hypotheses It was hypothesized that: 1. Strength would be less in women compared with men. 2. Men would produce steadier isometric contractions than women at all submaximal isometric force levels.  3. Women would have a more compliant tendon compared with men across all submaximal forces.  4. There will be strong relationship between the force steadiness and the mechanical properties of tendon in men and women.     18  CHAPTER 3: METHODOLOGY 3.1 Participants Ten men and ten women between 18 to 30 years were recruited from the local community. Participants were screened and excluded from participation if they had 1) surgery to the right arm and forearm, 2) known neurological or cardiovascular disorders, 3) systemic diseases affecting collagenous tissue, 4) active stage of tendinopathy, 5) a history of participation in motor control tasks (e.g., sewing or musicians), 6) history of eating disorder and 7) taken hormonal contraceptives in the prior year. Female participants were tested in the follicular phase (days 1-13) of their menstrual cycle. 3.2 Experimental Protocol Participants undertook a single experimental session at the Neuromuscular Physiology Research Laboratory, at the University of British Columbia Okanagan campus. All participants visited the laboratory at approximately the same time in the morning and all tests were initiated between 7:00 a.m. - 11:00 a.m. to control for hormonal circadian rhythms.  Ethical approval for the study was acquired from the University of British Columbia Behavioural Research Ethics Board (H14-00165). Prior to testing, the procedures, risks and benefits of the study were explained and informed consent was obtained. Each participant filled out the Edinburg Handedness Activity Questionnaire as well as a Background Information Questionnaire regarding hobbies, physical activities and previous employment to determine average level of physical activity at the time of data collection. Subsequently, their anthropometrical measures were recorded (height, weight, age, sex, moment arm, lever length) before starting the physiological assessments. 19  Assessments consisted of resting muscle and tendon length and cross-sectional area (CSA), muscle thickness of the BB at 110° elbow angle. Following resting anatomical measures voluntary muscle activation was assessed with the twitch interpolation technique by applying supramaximal stimulation during and after a maximal voluntary contraction (MVC). The highest MVC was used to calculate submaximal elbow flexion force of 2.5%, 5%, 10%, 20%, 40% and 60% of MVC. The order of contractions were randomized within a series and the recording of CSA and tendon elongation were randomized between subjects. Following the submaximal contractions, the MVC was reassessed to ensure fatigue did not occur as a result of the experimental protocol (Figure 3.0).  20   Figure 3.0: Experimental protocol. Sequence of measuring resting biceps brachii muscle, tendon length and CSA with ultrasonography. Followed by estimation of muscle activation by twitch interpolation technique and finally measurement of tendon elongation and CSA of BB tendon during submaximal steady contractions. BB, Biceps Brachii; CSA, Cross-sectional area; MVC, Maximal voluntary contractions.  Ultrasonography • Resting BB tendon length and CSA measurement • Resting BB muscle length, thickness and CSA measurement  Simulation Protocol (Twitch interpolation technique) • Maximal voluntary contractions (MVC) • Voluntary Activation Force steadiness Protocol • Execution of submaximal steady contractions at 2.5, 5, 10, 20, 40 and 60 %  of MVC • Recording of tendon displacement at the 6 submaximal isometric force levels • Recording of CSA of BB tendon at the 6 submaximal isometric force levels Step 1 Step 2 Step 3 Verification of MVC • To evaluate fatigue Step 4 21  3.3 Experimental Setup Participants sat in a firm high-back custom-designed force dynamometer chair (Don Clarke, University of Windsor; Harwood, Edwards and Jakobi, 2010) with instruction to remain in an upright position with their back resting against the chair. The chair was adjusted to the participant’s height for the hip and knee angle to be placed at 90°. The right elbow was positioned at 110° elbow flexion on a padded support and the forearm on a horizontal supporting plate in the neutral position with the shoulder abducted at 15°. The supporting plate of the force dynamometer chair was adjusted so the participants were able to grasp the handle of the wrist apparatus comfortably. A MLP-150 linearly calibrated force transducer (68kg) (Transducer Techniques, Temecula, CA, USA) with a sensitivity of 266 N/mV was used for detection of upward (flexion) or downward (extension) elbow force. Visual feedback was provided with a 20.5 inch flat screen computer monitor located 1 meter in front of the participant to ensure that constant visual feedback was provided across all contractions. Tendon elongation and CSA images of the biceps tendon were obtained using real-time B-mode US (GELOGIQ E9; General Electric, Fairfield, CT, USA) with a 15 MHz linear US probe (ML6-15) in a custom-built foam cast and fixed with straps and placed perpendicularly to the arm to record CSA while longitudinally to record MTJ elongation. These positions were monitored during all contractions to ensure probe stability through the use of hyperechoic markers placed under the probe.  3.4 Assessment 3.4.1 Resting BB muscle and tendon length and CSA measurement Resting BB tendon length and CSA and total muscle length, thickness and CSA were assessed at an elbow angle of 110º. Clear Aquasonic US transmission gel (Parker Laboratories, Inc., 22  New Jersey USA) was applied for acoustic contact between the probe and the skin. Two to three images were recorded for each variable at resting condition and the best image was selected for analysis. All US images were acquired at a rate of 29 frames per second. Total tendon length was recorded from the point where the BB muscle receded into tendon at the MTJ to the insertion on the radial tuberosity of the radius. First, a sagittal scan was taken to trace the BB MTJ at the proximal end of the distal tendon along the belly of the muscle. On the skin surface, the location was marked by placing a 3 mm thick 160mm long echo absorptive marker which provided a dark band shadow with US imaging. A second sagittal scan was taken to determine the insertion point of the tendon on the radius and subsequently a marker was placed on the skin surface in an identical procedure to the proximal mark. Following the sagittal scans a longitudinal scan was taken to obtain the entirety of the tendon length. To obtain CSA measurements of the tendon the US transducer was placed perpendicularly on the tendon and transverse images were acquired at the MTJ level.  In vivo measurements of BB muscle size (total muscle length, thickness and CSA) were also acquired. To record the total muscle length the greater tubercle of the humerus was identified on US and then a longitudinal scan was taken from this bony landmark on the humerus to the MTJ of BB. To obtain CSA measurement of the BB muscle, a longitudinal scan was taken transversely from medial to lateral over the bulk of muscle belly. 3.4.2 Maximal Voluntary contractions (MVC) and Voluntary Activation To assess voluntary activation of the elbow flexors, the twitch interpolation technique was used. Two percutaneous carbon rubber stimulation electrodes (4 × 4.5 cm) were firmly secured by tape over the upper and lower ridges of the BB. A series of single twitches of the elbow flexors 23  were evoked using a constant voltage stimulator (200s pulse width) (DS7AH: Digitimer, LTD. Welwyn Garden City, Hertfordshire, UK) in an increasing manner until the generation of a maximal twitch. This stimulation intensity was then increased further 10% to elicit supramaximal twitch stimuli. Participants were instructed to flex their elbow joint by pulling the wrist apparatus upward as hard and as fast as possible and to maintain this effort for 4–6 seconds. Participants performed approximately three MVCs separated by 2 minutes of rest. Three supramaximal single twitches stimuli with a one second interval were delivered during (interpolated twitch) and following (potentiated twitch) the MVC. Voluntary activation was calculated (see Section 3.5.3) from the highest MVC (Herbert & Gandevia, 1999; Jakobi & Rice, 2002). During all attempts of MVC visual feedback and strong verbal encouragement were provided.  The force signals were recorded with a linearly calibrated V72-25 resistive bridge transducer amplified at 100Hz,  sampled at a rate of 496 Hz with a 16-bit 1401 plus A/D converter (CED, Cambridge, England) and band-pass filtered (10 Hz –20 Hz). 3.4.3 Measurement of tendon elongation and CSA during the steadiness task The highest MVC was used to calculate the target force of the submaximal contractions. Submaximal contractions of 2.5, 5, 10, 20, 40 and 60 % MVC were executed. One series of submaximal contractions was undertaken to record BB tendon elongation while the other series was used to measure tendon CSA. Within each series of contractions and between subjects the submaximal force levels were randomized. Before starting the sub-maximal force steadiness tasks, practice was given to become familiar with the protocol and to finalize probe position. The visual feedback of real-time force output was displayed on the computer monitor. Four vertical cursors were visible on the screen to indicate the phases of the task: ramp up, hold, 24  ramp down and end. The duration between up and hold was set at 2s for 2.5 and 5%, 3s for 10 and 20%, 4s for 40 and 60%. The duration of the steady state effort was 5s for all submaximal contractions. While vertical cursors were used to display when a movement started or changed, a horizontal curser was set for the target force (Figure 3.1). At least one trial of each submaximal target was done depending on the US image quality. If the US image quality was not clear, then a minimum of two or three trials of that specific submaximal target force were performed. 25   Figure 3.1: Representative visual feedback for 40% submaximal force. Ramp up, five seconds of steady state contraction and ramp down. The transitions between ramp up, plateau and ramp down were identified with vertical cursors. The horizontal cursor was used to identify the target force. The vertical axis displays the amount of force in newtons and the horizontal axis the time in seconds.  3.5 Data Analysis For the offline analysis of all US images, the inherent measurement system of the GE LogiQ E9 US (General Electric, Fairfield, CT, USA) was utilized, with the best image of the two- three Time (s) Force (N) 26  attempts selected for analysis. All analysis was performed with the experimenter blinded to the sex of the subject as well as the force level.  The US analysis and FS trials were matched for analysis. The middle three seconds steady state of each isometric submaximal force (2.5%, 5%, 10%, 20%, 40%, and 60% of MVC) were used for both force steadiness analysis and for the measurement of tendon displacements and CSA. The US data were recorded in cm and converted to mm (Maganaris and Paul, 2002).  3.5.1 Muscle and Tendon Properties at rest Resting tendon length (RTL) was measured from the MTJ level to the insertion on the radial tuberosity with a single trace marking. Tendon CSA was also measured from the previously taken image at MTJ level. In a similar manner, the total BB muscle length and CSA were measured at rest (Figure 3.2). Muscle thickness was measured as the linear depth from the upper to lower aponeuroses across the widest part of the muscle.  27  A B   C D   Figure 3.2: Representation of BB muscle and tendon data analysis at resting condition. (A) Total length of BB tendon, (B) CSA of BB tendon, (C) Total length of BB muscle, and (D) CSA of BB muscle of a male participant. The white arrow in each image indicates the specific measurement. MTJ, muscle-tendon junction, BB, biceps brachii, CSA, cross sectional area  3.5.2 Mechanical Properties of tendon To obtain tendon displacement measurements during all six submaximal contractions, two sagittal plane images were captured from the video of a submaximal contraction. The first image was taken before the start of a contraction while the second one was from the steady state of the contraction. For both images, tendon length was measured from the MTJ level to the edge of the image. The difference in tendon length was considered tendon displacement (Figure 3.3).  28   A B    Figure 3.3: Representative images of tendon displacement at 60% MVC in a male participant. (A) Indicative of the resting condition; (B) steady state at 60% MVC. The white arrow in each image identifies the MTJ. Noticeable displacement (1.61 - 0.76 = 0.85 cm = 8.5 mm) of the origin of the BB tendon in the transition from rest to 60% MVC. MTJ, muscle-tendon junction, BB, biceps brachii  Mechanical properties of BB tendon stiffness, stress, strain and Young’s Modulus were calculated via the following equations: BB tendon Stiffness = BB tendon force / tendon displacement. To calculate tendon force the external muscle moment was divided by the external moment arm of the BB tendon. External muscle moment was estimated from the force produced at specific submaximal force levels and the lever arm length. Lever arm length was the radius length of the participant which was measured externally from the lateral epicondyle of the humerus to the styloid process of radius. External moment arm was measured as the perpendicular distance from the lateral epicondyle of the humerus to a linear edge. The linear edge was held across the 29  pre marked MTJ to the insertion point of BB tendon on the skin with the forearm in a neutral wrist position and the elbow at 110°.  Tendon strain = tendon displacement / resting tendon length.  Tendon stress = Tendon force / CSA of the tendon. Tendon strain was calculated by dividing the tendon displacement by the resting tendon length, whereas tendon stress was estimated by dividing the calculated tendon force by the CSA of the tendon for each submaximal force. Finally, to obtain the measure of Young’s Modulus for all submaximal force levels tendon stress was divided by tendon strain (Figure 3.4). Young’s Modulus = Tendon stress / Tendon strain   Figure 3.4: Representation of measurement of the mechanical properties of tendon. CSA: Cross sectional area 30  3.5.3 Voluntary Activation and Force Steadiness Voluntary activation and FS were measured offline using Spike 2 Version 7 software (Cambridge Electronics Design, Cambridge, UK). Voluntary activation was calculated from the highest MVC by using the formula: % activation = [(1 - superimposed twitch/ potentiated twitch) x 100] (Allen et al., 1994; Herbert & Gandevia, 1999; Jakobi & Rice, 2002). FS was calculated as the coefficient of variation (CV) for each submaximal level of contraction (SD / mean). Therefore, it is inversely related to steadiness; the higher the CV of force, the less steady the contraction. 3.6 Statistical Analysis Independent sample t-tests were conducted to evaluate differences in participant characteristics and resting anatomical measurements of BB muscle and tendon (age, height, weight, MVC, lever arm length, moment arm, resting tendon length, tendon CSA, BB muscle length, muscle CSA and muscle thickness).  A 2 (sex) × 6 (force level) repeated-measures analysis of variance (ANOVA) was used to determine differences in the dependent variables of tendon mechanical and morphological properties (tendon displacement, tendon CSA, tendon force, strain, stiffness, stress, Young’s Modulus). Comparison of the coefficient of variation of force (FS) between men and women across all six force levels was also undertaken with a 2 (sex) × 6 (force level) repeated measures ANOVA. Statistical interactions were examined with Tukey Post hoc tests.  Non-linear (polynomial) regression analysis was used to examine the relationship between the dependent variable of FS and the independent variables of tendon mechanical properties (tendon displacement, strain, stress, stiffness, and Young’s Modulus) between men and women. 31  All statistical procedures were performed using Statistical Package for Social Sciences (SPSS) version 20.0. All tables are reported as mean ± standard deviation (SD), and figures are presented as mean ± standard error (SE). An α level of (p ≤ 0.05) was set for statistical significance.   32  CHAPTER 4: RESULTS 4.1 Participant Characteristics Age (F = 0.51), height (F = 0.96), and voluntary activation (F = 0.63) did not differ between men and women (p > 0.05) (Table 4.0). However, men were significantly heavier (F = 0.47) and stronger (F = 2.45) than women, and had a longer moment arm length (F = 0.96, p < 0.001) and lever arm length (F = 2.44) compared to women (p ≤ 0.05) (Table 4.0). Table 4.0: Participant characteristics. Parameters Men Women Age (years) 22.40 ± 3.71 21.78 ± 3.07 Height (m) 1.66 ± 0.12 1.68 ± 0.07 Weight (kg) 74.31 ± 13.18* 63.22 ± 9.95 Lever arm length (mm) 276.6 ± 13.16* 260.55 ± 17.40 Moment arm (mm) 57.4 ± 4.17* 48.55 ± 3.46 Resting tendon length 83.36 ± 7.90 76.36 ± 8.29 Resting muscle length 138.00 ± 11.74 139.88 ± 7.54 MVC (N) 265.74 ± 52.67* 163.74 ± 32.06 Voluntary activation (%) 98.3 ± 3.40 97.2 ± 5.1 *, men greater than women (p ≤ 0.05); mm, millimetre; kg, kilogram; N, Newton; m, meter. 4.2 Resting BB Muscle and Tendon Tendon CSA (F = 2.26) was greater in men compared with women as was muscle CSA (F = 6.40) and muscle thickness (F = 7.40) (p < 0.05). There were no significant differences in the resting tendon length (F = 0.03) and muscle length (F = 0.69) (p > 0.05) (Figure: 4.0). 33  A B SexMen WomenTendon CSA (mm2)051015202530 SexMen WomenMuscle CSA (mm2)0200400600800100012001400 C D SexMen WomenMuscle Thickness (mm)0510152025 SexMen WomenTendon Length (mm)020406080100 E  SexMen WomenMuscle Lengt h (mm)020406080100120140160  Figure 4.0: Sex differences in tendon and muscle at rest (A) Tendon CSA, (B) muscle CSA, (C) muscle thickness, (D) tendon length, and (E) muscle length. *, men significantly greater than women (p < 0.05); black bar, men; grey hatched bar, women. Values are mean ± SE, p<0.05. * * * 34  4.3 Tendon’s Mechanical and Structural Properties during Steadiness Task The sex × force level repeated measures ANOVA for tendon displacement was non-significant for sex. Although there was no interaction (F = 0.24, p = 0.80) or main effect of sex (F = 3.10, p = 0.09), there was a significant main effect of force level (F = 13.85, p < 0.001, power = 0.99). Tendon displacement was lowest at 2.5% MVC compared to all other force levels (p < 0.05). There was also a difference of displacement at 5% MVC from 2.5%, 20%, 40% and 60% and at 60% strain was higher than 2.5%, 5%, 10% and 20% (p < 0.05) (Figure 4.1).  Figure 4.1: Tendon displacement across force levels. *, lowest tendon displacement at 2.5% compared to all other force levels (p < 0.05); #, difference of displacement at 5% MVC from 2.5%, 20%, 40% and 60%; ¶, at 60%, strain was higher than 2.5%, 5%, 10% and 20% (p< 0.05)  mm, millimeter; %, percent; MVC, maximal voluntary contraction. Values are mean ± SE, p<0.05. 35  There was a significant interaction for sex × force level (F = 16.98, p < 0.001, power = 1.00) for tendon force. Men had a greater tendon force than women and across all submaximal targets, tendon force increased significantly (p = 0.001) (Figure 4.2).  Figure 4.2: Tendon force between men and women across force levels. Tendon force for men (black bars) and women (grey bars) at the six submaximal force levels. *, significantly higher than women (p < 0.001), #, tendon force increases with increased force levels from 2.5% MVC to 60% MVC (p < 0.001).  N, Newton; %, percent; MVC, maximal voluntary contraction; black bars, men; grey bars, women. Values are mean ± SE, p<0.05.  Tendon CSA was significantly greater in men than women (F = 11.21, p = 0.004, power = 0.88) (Figure 4.3A) and was significantly higher at 5% compared with relative forces of 10%, 20%, 40%, and 60% (Figure 4.3B).  36  A  B  Figure 4.3: Tendon CSA for the main effects of (A) sex and (B) force level. *, men greater than women (p < 0.05), #, at 5% MVC tendon CSA  was greater than 10% ,20%, 40% and 60% MVC (p < 0.05) %, percent; MVC, maximal voluntary contraction; CSA, tendon cross sectional area; mm, millimeter. Values are mean ± SE, p<0.05. 37   The interaction between sex and force level for tendon strain was non-significant (F = 0.16, p = 0.87). However, there was a significant main effect for sex (F = 4.23, p = 0.05, power = 0.49) and force level (F = 14.48, p < 0.001, power = 0.99). Therefore, women had a higher strain compared to men (Figure 4.4A). Strain at 2.5% MVC compared to 10%, 20%, 40%, and 60% (p < 0.01) was lowest  and at 40% and 60% MVC, was highest compared to all other force levels (p < 0.05). Strain did not differ between 40% and 60% as well as 5% and 10% MVC (p > 0.05) (Figure 4.4).  38  A  B  Figure 4.4: Tendon strain for (A) men and women and, (B) force level. Strain was higher in women (white hatched bar) than men (black bar), lowest strain at 2.5% than all other force levels (p < 0.05); (A) *, significantly lower than women (p < 0.05). (B) #, difference of strain at 5% MVC from 2.5%, 20%, 40% and 60%; ¶, at 60% strain was higher than 2.5%, 5%, 10% and 20% (p < 0.05). mm, millimeter; %, percent; MVC, maximal voluntary contraction. Values are mean ± SE, p<0.05. 39  The interaction between sex and force for tendon stress was non-significant (F = 0.27, p = 0.69) as was the main effect for sex (F = 0.18, p = 0.67), but there was a significant effect for force level (F = 168.90, p < 0.001, power = 1.00). Average stress increased as force increased from 2.5% to 60% (p < 0.05) (Figure 4.5).  Figure 4.5: Tendon stress across force levels. *, significant difference between all force levels (p < 0.05). N, Newton; mm, millimetre; %, percent; MVC, maximal voluntary contraction. Values are mean ± SE, p<0.05.  The interaction for tendon stiffness was non-significant (F = 0.53, p = 0.52); however, there was a significant main effect for sex (F = 5.54, p = 0.03, power = 0.60) and force level (F = 22.56, p < 0.001, power = 0.99). Overall, men had a higher tendon stiffness compared to women (Figure 4.6A). Pairwise comparisons revealed that the average stiffness at 2.5% and 5% MVC were similar (p = 0.69) but were significantly lower compared to 10%, 20%, 40%, and 60% 40  MVC (p < 0.05). Therefore, except 2.5% and 5% MVC, tendon stiffness increased with increasing force level (p < 0.05) (Figure 4.6B). The interaction (F = 0.25, p = 0.67) and main effect for sex was non-significant (F = 0.21, p = 0.64) for Young’s Modulus; however, there was a main effect for force level (F = 16.26, p < 0.001, power = 0.99) (Figure 4.7). The pairwise comparisons revealed that at 2.5% and 5% MVC Young's Modulus was lower than all force levels (p < 0.05); however, Young’s Modulus increased with force from 10% to 60% MVC (p < 0.05).     41  A  B  Figure 4.6: Tendon stiffness for (A) men and women and, (B) force level. Tendon stiffness was higher in men (black bar) than women (white hatched bar) and increased as force increased from 5% to 60%. (B) *, significantly greater than women (p < 0.05). (B) #, at 2.5% and 5% MVC tendon stiffness was lower than all force levels (p < 0.05); ¶, tendon stiffness increased significantly from 10% MVC to 60% MVC (p < 0.05). N, Newton; mm, millimetre; %, percent; MVC, maximal voluntary contraction. Values are mean ± SE, p<0.05. 42   Figure 4.7: Young’s Modulus across force levels. *, at 2.5% and 5% MVC Young's Modulus was lower than all force levels (p < 0.05), #, Young's Modulus increases with  force from 10% to 60% (p < 0.05), Newton; mm, millimetre; %, percent; MVC, maximal voluntary contraction. Values are mean ± SE, p<0.05. 4.4 Steadiness The 2 (sex) × 6 (force level) repeated measures ANOVA for FS revealed a significant interaction (F = 7.92, p = 0.004, power = 0.87). There was a main effect of sex (F = 20.82, p < 0.001, power = 0.99) as females were less steady than males. There was also a main effect for force levels (F = 70.44, p < 0.001, power = 1.00). Pairwise comparisons for force level showed force steadiness increased with force from 2.5% MVC to 40% MVC (p < 0.05), however, at 60% MVC steadiness differed from all force levels except 20% MVC (p > 0.05). (Figure 4.8).  43   Figure 4.8: Force steadiness between men and women across force levels. *, significantly less than men (p < 0.01). );#, CV of force decrease with increased force levels  from 2.5% MVC to 40% MVC (p < 0.05); ¶,at 60% MVC steadiness differed from all forces except 20% MVC (p < 0.05);CV, coefficient of variation; %, percent; MVC, maximal voluntary contraction; black bars, men; grey bars, women. Values are mean ± SE, p<0.05.  4.5 Correlation and Regression Analysis The relationship of force steadiness with tendon displacement, strain, stress, tendon stiffness and Young’s Modulus were evaluated across all submaximal forces for men and women. The relationship between tendon displacement and CV of force (%) for men was strong and significant (r2 = 0.51; r = 0.71; p < 0.001), but was moderate in women (r2 = 0.23; r = 0.47; p < 0.001). Stress and CV of force (%) was highly correlated and significant in men (r2 = 0.62; r = 0.79; p < 0.001) and in women (r2 = 0.69; r = 0.83; p < 0.001). However, the relationship 44  between strain and CV of force (%) for both men (r2 = 0.44; r = 0.66; p < 0.0001) and women (r2 = 0.20; r = 0.44; p < 0.001) was moderate but significant. There was high and significant correlation between tendon stiffness and CV of force (%) in women (r2 = 0.60; r = 0.77; p < 0.001) and the relationship was moderate but significant in men (r2 = 0.21; r = 0.46; p < 0.001). Young’s Modulus and CV of force (%) showed a low and significant correlation for men (r2 = 0.15; r = 0.38; p = 0.003) and no significant correlation for women (r2 = 0.02; r = 0.13; p = 0.36). 45  A B Tendon Displacement (mm)0 5 10 15 20 25 30CV of Force (%)0246810MenWomenMen reg. line, r2=0.51Women reg. line, r2=0.23 Stress (N/mm2)0 10 20 30CV of Force (%)0246810Men, r2 = 0.62Women, r2 = 0.69 C D Strain0 5 10 15 20 25 30CV of For ce (%)0246810Men r2 = 0.44Women r2 = 0.20 Tendon Stiffness (N/mm)0 20 40 60 80 100 120 140CV of Force (%)0246810Men r2 = 0.21Women r2 = 0.60 E  Young's Modulus (N/mm2)0 2 4 6 8 10 12 14CV of  Force (%)01234567Men r2 = 0.15Women r2 = 0.02  Figure 4.9: Relationship between CV of Force (%) and tendon mechanical properties. (A) Tendon Displacement, (B) Stress, (C) Strain, (D) Tendon Stiffness, and (E) Young's Modulus. %, percent; N, Newton; mm, millimetre; black circles, men; open circles, women; black solid line, men regression line; black dotted line, women regression line  46  CHAPTER 5: DISCUSSION This is the first study to assess the contribution of the BB tendon's mechanical properties to FS in men and women. The majority of studies to date have evaluated tendon mechanical properties of men and women in the lower limb (Kubo et al., 2006; Maganaris & Paul, 1999; Magnusson et al., 2001; Muraoka et al., 2005; Stafilidis et al., 2005) or relative to postural balance (Onambele et al., 2006; Ward et al., 2006). However, no study has considered the role of tendon mechanics in FS of the upper limb. The main findings of this study were a) women had a more compliant BB  tendon compared to men when all submaximal force levels were collapsed, b) men had a larger BB tendon CSA than women, c) men developed a higher tendon force at all submaximal forces, d) irrespective of sex, stress, strain and Young’s Modulus increased with increasing level of force, and  e) men were stronger as well as steadier than women from 2.5% to 60% MVC and force stability increased as the contractions became more intense. Overall, mechanical properties of tendon displacement, stress, strain and stiffness in the BB tendon were moderately to highly related with FS in men and women, whereas Young’s Modulus did not demonstrate a relationship with steadiness. Tendon stiffness and CV of force were strongly correlated for women whereas only moderate for men. Therefore, it is likely that the BB tendon mechanical properties influence steadiness of elbow flexors and could be one of the factors related to sex differences between men and women. 5.1 Sex differences in mechanical properties of BB tendon  The mechanical properties of tendon provide a useful index of the functionality of tendon. Tendon force, resting tendon length, tendon displacement, and CSA are baseline tendon parameters that contribute to the mechanical properties of stress, strain, stiffness and Young’s 47  Modulus. The structural property of tendon is represented through quantification of resting tendon length and tendon CSA, tendon force is a measure of the forces exerted on tendons, whereas tendon displacement is the measure of changes in length during transmission of the force. Therefore, measures of these elements were made for the determination of mechanical and structural properties. In the present study, these parameters were lower, except tendon displacement, in women compared to men; however, only tendon force and CSA showed statistically significant differences. The lower tendon force, and greater tendon displacement in women suggests greater deformation of the BB tendon and increased slack in the musculotendinous unit during force transmission compared to men. There is evidence that a larger CSA of the tendon leads to a smaller displacement during force transmission (Heinemeier and Kjaer, 2011). Therefore, the smaller CSA of the BB tendon in women provides additional insight into the underlying cause of greater deformation of the tendon in women compared to men during submaximal force production. During each submaximal force, CSA of the BB tendon was measured to determine tendon stress. The higher the tendon CSA, the lower the stress value and the higher the tendon force, the higher the stress value. Men had a larger BB tendon CSA than women and experienced higher tendon force at all submaximal levels. In the current study, tendon stress did not differ between sexes and this was likely due to the higher tendon force and CSA in men compared to women. Furthermore, irrespective of force levels, tendon strain was greater in women compared to men. These differences suggest that the tendon of women undergoes a greater tendon displacement relative to its resting tendon length compared with men and this occurs when lower tendon forces were developed in the women. Therefore, precise movements that are 48  required for upper extremity steadiness in activities of daily living are hampered in women due to the slackness of the musculotendinous unit.    Young’s Modulus, an index of the materialistic property of the tendon, did not differ between men and women. Young’s Modulus is the ratio of tendon stress and tendon strain, and is thus independent of the CSA and length of tendon. Thereby, Young’s Modulus makes it possible to compare different dimensions of the composition of the tendon such as, crosslinking density, arrangement ratio of type I to type III collagen or sensitivity to cellular signals (Onamble et al., 2007). In this study, the non-significant difference in Young’s Modulus between men and women indicate that the composition of the tendon does not contribute to the observed differences in BB tendon mechanics. Rather, structural differences of tendon CSA and length between men and women could be the reason for observed differences in BB tendon mechanics. Tendon stiffness is a measure of displacement to the applied forces. In this study the stiffness of the BB tendon was significantly lower in women (~42%) compared to men. The observation of a more compliant BB tendon in women is also in accordance with prior studies of the lower limbs, which indicate reduced stiffness of the tendon-aponeurosis in the medial gastrocnemius as well as patellar tendon of women compared to men (Kubo et al., 2003; Onamble et al., 2007). Lower stiffness of the BB tendon in women suggests greater tendon strain compared to men for equivalent relative forces and indicates that there might be slower transmission of force from muscle to bone. Therefore, the less stiff tendon of women compared to men would slow down the rate of force generation, because the muscle would have to shorten further to stretch the tendon and make it effective for transmitting the force of contraction to the skeleton. This decreased shortening to stretch would not only reduce rate of force transmission through the 49  tendon but create a compliant system minimizing effective and smooth transmission of force to the bone. It is well known that tendon stiffness increases with resistance training (Kubo et al., 2001) and decreases with inactivity (Kubo et al., 2004); however, men tend to show a greater increase in collagen synthesis following training compared to women (Magnusson et al., 2007). The physical activity level of the participants is an important factor to be considered.  In this study the women were relatively strong, while the men were slightly weaker compared with prior reports of the BB. Thus, the sex-related differences observed here were likely minimized by the tall and strong women tested in this study.  Muraoka et al. (2005) reported that increases in muscle strength are also accompanied by an increase in tendon stiffness for the purpose of avoiding non physiological levels of strain. In the current study, the elbow flexion forces were significantly higher in men compared with women, which could be one of the factors that contributed to higher BB tendon stiffness in the men, and this in-turn would enhance force steadiness.  There is also evidence that estrogen levels can compromise the collagen synthesis in women both directly and indirectly via associated low levels of relevant growth factors, (Magnusson et al., 2007). Thereby, this could contribute to the decrease in tendon stiffness or increased laxity of ligamentous tissues in women. Magnusson et al. (2007) reported elevated collagen synthesis after an acute bout of exercise for non-users of oral contraceptives in the follicular phase of the menstrual cycle, whereas users of oral contraceptives who have elevated circulating levels of estradiol did not demonstrate any change in collagen synthesis.  In this study, to control for the artificial increase of estrogen and progesterone, the female participants were not taking hormonal contraceptives for at least six months prior to participating. If females who were on 50  hormonal contraceptives were studied it is likely that the BB tendon would already be more compliant, thus we might observe a greater difference in tendon stiffness compared with men. Additional to hormonal control, all women were tested in the follicular phase of the menstrual cycle when force is typically highest (Sarwar et al., 1996). Therefore, in this study potential contributors to increased strength and tendon stiffness were factored into the design to better evaluate tendon properties between men and women.  5.2 BB tendon during different submaximal forces  Functional activities of daily living does not subject the tendon to extreme strain that would cause tendon failure. Thus, it is most likely in activities of daily living that the toe region of the tendon length tension relationship is used (Ito et al., 1998). The toe region is defined as the primary part of the length tension relationship that has a low slope at the onset of length changes (Butler et al., 1978). In an ideal length-tension relationship for tendons, highest force is required; however, in this study we considered the structural and mechanical properties of BB tendon from 2.5 to 60% MVC relative to force steadiness. If we were to measure higher force levels from 60% to MVC, a steeper slope of the stress-strain curve would likely be evident. To my knowledge this is the first study to report tendon mechanical properties in the BB across different submaximal force levels. Limited studies have reported tendon properties in the upper limbs (Murata et al., 2009; Ohta et al., 2004). Ohta et al. (2004) suggested that the average value of stiffness of the BB tendon was 100 N/mm at 80% MVC for men, whereas in this study, it was 53.21 N/mm at 60% MVC for men. Which suggests differences in methodology regarding tasks being performed between studies and this also contributes to varying outcomes of mechanical properties.  51  In this study, irrespective of sex, tendon displacement and strain increased with increasing submaximal force levels from 2.5 to 40% MVC, yet both displacement and strain did not differ between 40% and 60%. This provides an indication that the BB tendon is stretched more easily when force is low and becomes less compliant with increments of force. This aligns with previous reports from animal studies (Ker et al., 1988; Lieber et al., 1991; Loren & Lieber, 1995) as well as with in vivo measures of the human tibialis anterior (Ito et al., 1998). At rest, there is a pattern of the collagen fibres and fascicles of tendon, which fold upon each other, known as crimp, and this organization disappears following the application of tensile force (Elliott, 1965). Therefore, it is likely that at low force levels the crimp is allowing pronounced tendon displacement but as force increases the crimp disappears slowly which causes less difference in tendon displacement and strain. Increased stiffness and Young’s Modulus with increasing level of force may contribute to the prevention of tendon injuries. A greater tendon stiffness would allow physiological levels of strain to be maintained at higher muscle forces, thereby acting as a safety factor for the tendon in injury prevention. For example, if large forces are generated, a reduced stiffness would increase the risk of injury due to rupture. Therefore, the lower BB tendon stiffness of women in the current study suggests the higher chance of injury in activities such as, weight lifting, volleyball or rugby that involves heavy upper body exertion.  5.3 Force Steadiness and Mechanical Properties  Muscle and tendon work as a unit, therefore, it is unlikely that one single factor is responsible for FS rather, a combination of factors between muscle and tendon likely contribute. Studies to date on sex-related differences in force steadiness have been limited to neuromuscular factors. For example, it is well known that the increase in force variability might arise from the 52  organization of the MU pool, recruitment and rate coding strategies or the activation pattern of the MU population (Taylor et al., 2003). Additionally, strength is thought to be a strong predictor of steadiness (Brown et al., 2010). In the literature there is also evidence of MU recruitment and MUDR in the tibialis anterior being related to the compliance of the muscle-tendon complex. Pasquet et al. (2005) reported that higher MUDRs were observed at forces below 10% when the muscle-tendon complex was compliant and from 10 to 35% MVC when the tendon became stiffer, recruitment of additional motor units played a dominant role. This finding provides an indication of how the muscle-tendon complex interacts with the nervous system to control force.  In the current study, as submaximal force increased the ability to hold steady contractions also increased. As well, tendon mechanical properties of stress, strain, Young’s Modulus, and tendon stiffness also increased with force. These results suggest that there is a relationship between tendon mechanical properties and the amount of force, thus suggesting that there is an interaction between tendon and force that influence the ability to control force. To evaluate this effect further, tendon mechanics were related to force steadiness and there was a relationship between tendon displacement, stress, strain and stiffness in the BB tendon with FS for both men and women whereas Young’s Modulus was not related with FS. Additionally, in this study, men were significantly stronger and steadier at all submaximal force levels and had stiffer tendon than women. It is possible that, higher strength of men contributed to FS through increasing tendon stiffness. Further, tendon stiffness and CV of force were strongly correlated for women whereas only moderate for men. Thus, there are factors beyond the tendon, likely neural strategies that are differentially interacting with the mechanics of the system to influence force production in men and women unequally. 53  To control the motor output, as force increases, the nervous system alters the number of active motor units (which is MU recruitment) and also the discharge rates of active MUs along with muscle-tendon mechanics. The central command is constantly modulated by the afferent proprioceptive information during all submaximal contractions performed at different muscle fascicle lengths. Therefore, it is likely that a stiffer BB tendon contributes to FS by rendering the muscle spindle more sensitive to length perturbations of muscle fascicle. More recently, a study by Johannsson et al. (2015) have reported a relationship between steadiness and mechanical properties of human Achilles tendon. The authors suggested that a stiffer tendon might act as a better transducer of the neural adjustments required for controlling force output (Johannsson et al., 2015). Alternatively, with a compliant tendon the muscle will have to shorten further to stretch the tendon and make it effective for transmitting the force of contraction to the skeleton which confounds the nervous system’s ability to control steady force. Therefore the compliant BB tendon of women may not allow appropriate adaptation to changes in muscle fascicles which confounds the nervous system’s ability to control steady isometric elbow flexion force. These results suggest that having a more compliant BB tendon is likely one of the factors of decreased stability in the elbow flexors of women compared to men. Functional tasks, such as inserting a key into a lock, buttoning shirts, tying shoes, using phones or eating with a spoon require a high degree of control to properly execute and FS is associated with execution of functional tasks. Therefore, a compliant tendon of women could be one of the reasons of reduced functional activity at an earlier age compared to men.  54  CHAPTER 6: CONCLUSIONS The overall aims of this study were met. Mechanical and structural properties were recorded from the BB tendon in men and women. The hypothesis that the BB tendon would be more compliant in women than men at all submaximal force levels was not fully accepted. Although, there was no sex difference in stiffness at all submaximal force levels ranging from 2.5% to 60% MVC, when force levels were collapsed, women had a more compliant BB tendon compared to men. FS of the elbow flexors was evaluated in men and women across 2.5% to 60% MVC. The hypothesis that men would be steadier than women between 2.5% to 60% MVC was accepted. The relationship between BB tendon stiffness and FS of the elbow flexors in men and women was evaluated. The mechanical properties of tendon displacement, stress, strain and stiffness in the BB tendon were moderately to highly correlated with FS for both men and women, whereas Young’s Modulus was not. Tendon stiffness and CV of force were strongly correlated for women whereas only moderate for men. Therefore, it is likely that the BB tendon mechanical properties is one of the factors that influence steadiness of elbow flexors and a compliant tendon could be one of the factors contributing to the sex differences in steadiness. 6.1 Implications A strong predictor of functional performance is FS. The results of this study offer an understanding of the relationship between mechanical properties of tendons and FS. Therefore, these may have implications for people wanting to maintain steadiness to complete tasks of daily living or the tasks that require steady control, such as during the shooting component in a biathlon race or painting. It is evident (Brown et al., 2010) that strength occupies a primary role 55  in FS and the results from this study also show that strength contribute to FS through its influence on increasing tendon stiffness.  Although strength training is a known contributor to enhancing FS (Brown et al., 2010; Tracy et al., 2004) and tendon stiffness (Muraoka et al., 2005), the time period  of adaptation differs between muscle and tendon. The adaptation is slower in tendon most likely due to the delayed responses on the transcriptional growth factors in tendon compared to muscle (Heinemeier and Kjaer, 2011). Therefore, specific physical exercises that improve tendon stiffness should be accentuated, such as resistance training, plyometric training for the functional improvement of both men and women very early in life. Moreover, the strong correlation of BB tendon stiffness with steadiness in women compared with men provides insight into the underlying cause of reduced steadiness. Therefore, women should be strongly encouraged to strength train to improve tendon stiffness and enhance force control.  6.2 Strengths  The measurement of BB tendon mechanical properties has not been reported previously, as the recording of this tendon in vivo during muscle contraction is challenging; however, the inter-rater reliability of the measurement within our lab was very high (r = 0.99). Another strength of this study was the participant exclusion criteria. For example, it is evident that estrogen directly and indirectly compromise collagen synthesis in women and this would influence the results (Kjaer et al., 2009; Magnusson et al., 2007). Therefore, women who participated in this study were not taking hormonal contraceptives. The tight control placed on hormonal supplementation and the time of testing strengthens the design, but might limit the extension of results to women on hormonal replacement. Moreover, this study evaluated tendon mechanical properties of the BB tendon relative to force steadiness at six different submaximal force levels. 56  Finally, to make consistent and accurate measurements, analysis was done in a time-locked approach, that is the US recordings and force steadiness analysis was conducted over matched periods which also strengthens the outcome of the study.  6.3 Limitations and Future Directions  Strength is an important predictor of FS and tendon stiffness. Therefore, it would be interesting to investigate FS and mechanical properties of tendon in men and women who are matched for strength. There is also evidence in the literature that fibre type distribution is different between sexes (Staron et al., 2000), future research should consider the contribution of contractile properties and muscle activity to force steadiness in conjunction with evaluation of sex-related differences in force steadiness. Quantification of contractile properties and muscle activity, ranging from MU activity to MU number estimation at the same time would add understanding to force steadiness in men and women. Additionally, force steadiness and tendon properties were quantified at 6 forces with the highest being 60% MVC, thus determination of tendon mechanical properties at higher force level cannot be ascertained from this study. Further pilot work and adaptations of US probes and or technology is necessary to image clearly the BB tendon at high force levels in order to prevent probe shifting as the BB and tendon alter shape at high forces.  This experimental set-up only allowed for the evaluation of the properties of BB rather than all elbow flexors, such as the brachialis and brachioradialis muscles which also contribute to force, yet steadiness was measured as elbow flexion force. Finally, it is well known that the tendons of muscles involved in positional control are stiffer (Ward et al., 2006) and that FS also depends on position. Thus, future research should evaluate the impact of BB tendon’s property on FS in 57  three positions of forearm to gain a better understanding of the contribution of tendon to position dependent force control between men and women. 58  REFERENCES Albracht, K., &Arampatzis, A. (2013). 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Viscoelasticity of the sarcomere matrix of skeletal muscles. The titin-myosin composite filament is a dual-stage molecular spring. Biophysical Journal, 64(4), 1161. Ward, S. R., Loren, G. J., Lundberg, S., & Lieber, R. L. (2006). High stiffness of human digital flexor tendons is suited for precise finger positional control. Journal of Neurophysiology, 96(5), 2815-2818. Weber, T., Zemelman, B.V., McNew, J.A., Westermann, B., Gmachl, M., Parlati, F. et al. (1998). SNAREpins: Minimal machinery for membrane fusion. Cell, 92(6), 759-772. Wilkie, D. R. (1949). The relation between force and velocity in human muscle. The Journal of Physiology, 110(3-4), 249-280. Zazulak, B. T., Paterno, M., Myer, G. D., Romani, W. A., & Hewett, T. E. (2006). The effects of the menstrual cycle on anterior knee laxity. Sports Medicine, 36(10), 847-862.   71  APPENDICES Appendix A: Copyright Approval for Figure 1.0  72  Appendix B: Copyright Approval for Figure 1.2  73  Appendix C: Ethics Approval   74  Appendix D: Consent Form LETTER OF CONSENT  Force Steadiness and Muscle Architecture Principal Investigator:  Dr. Jennifer Jakobi, PhD, Associate Professor Health and Exercise Sciences, UBC Okanagan. Ph: 250.807.9884; Email: jennifer.jakobi@ubc.ca Co-Investigators Sharmin Arefin, MSc Student, Interdisciplinary Graduate Studies Health and Exercise Sciences, UBC Okangan Ph: 250-807-9190; sharmin.arefin@yahoo.com Afruna Lizu, MSc Student, Interdisciplinary Graduate Studies Health and Exercise Sciences, UBC Okangan Ph: 250-807-9190; afrunalizu@gmail.com  You are being invited to participate in a research study looking at force steadiness and muscle and tendon architecture (anatomy). You must be healthy and between the age of 19-90 years. The purpose of this letter is to provide you with the information you need to make an informed decision about participating in this research. Voluntary participation and other pertinent information Your participation in this study is completely voluntary. Should you choose to participate, you will be required to sign the consent form at the end of this information material. However, you are free to withdraw from this study at any point in time if you wish to discontinue your participation without providing any reasons. This information will provide you with all necessary facts regarding the study, so please review it with care before you decide if you are going to participate or not. If you choose not to participate in this study, you will not be penalized in any way, nor do you need to disclose why you have chosen not to participate. You should only agree to participate if you feel happy that you know enough about the study. If you are participating in another study at this time, please inform the lead study investigator to determine if your participation in this study is appropriate.  75  Background Information Ultrasonography is a non-invasive procedure used to measure muscle and tendon structures by taking ‘pictures’ through the skin. This investigation will use Ultrasound to measure changes in the muscle and tendon in healthy persons during stretching and force producing movements.  While your muscle and tendon are measured you will be producing force over a variety of force levels that range from low forces to your best efforts. The measures will be made over a course of 60-120 minutes. The time varies between participants as some images are gathered quickly and others vary between persons anatomy and take longer to record.  Purpose of this study The aim of this research study is to determine the effect of muscle and tendon architecture on force production and whether this differs in males and females across age.  Who can participate? If you are a healthy male or female between the ages of 19 and 90 years old, you are welcome to participate in this study. You must be able to speak and read English fluently. Who should not participate? You should not participate in this study if you are: (1) Unable to ambulate independently, even with the help of a walking-aid; (2) You have severe cognitive impairment or (3) a neurological disorder; (4) You are unable to read or speak English fluently; (5) You have now, or have previously had major orthopaedic surgery. Procedure for this study Should you choose to participate in this study, your participation would involve; completing questionnaires investigating your health and physical activity; performing maximal voluntary contractions and submaximal contractions. Ultrasound will be used to collect data during the duration of this study.  This assessment is approximately 60-120minutes or you can visit over a number of occasions for shorter duration.  1. You will come to the Neuromuscular Physiology Lab in Arts and Science 164 at a predetermined time. Upon arrival you will do a health history and physical activity questionnaire. You do not have to answer any questions that you are uncomfortable answering.  2. The graduate student will take initial measurements of your muscle and tendon using ultrasound. 3. The graduate student will operate the dynamometers to record force.  76  4. The ultrasound will be record throughout the force efforts.   Your responsibilities It is important that you come to the lab dressed in appropriate clothing. Wearing shorts or loose pants and t-shirts are ideal for comfort. Also, please refrain from exercising the day of testing as it could have an effect on the results of the study. Risks and discomforts of participation The risks associated with the proposed research are minimal. Minimal muscle soreness may result from maximal voluntary contractions.  Associated benefits of participation There are no direct benefits to you, except the results of an assessment of your muscles and tendons.    Results could provide information useful to understanding why force control decreases with increasing age between males and females. Will I be paid or do I have to pay to participate? You will not be paid for participating and there is no financial cost to your participation in the described experiments. However, any incurred parking and travel expenses as a result of your participation in this study will be reimbursed, provided receipts are submitted to a member of the research team. You are free to withdraw from the experiment at any point in time. You do not have to provide a reason for withdrawing from the study if you do not wish to do so. What to do if you want to withdraw from this study: Participation in this study is voluntary. You have the right to refuse to participate, refuse to answer any questions or withdraw from the study at any time with no effect on your future (care/academic status/employment). You do not have to provide any reasons to do so. If you choose to enter the study and then decide to withdraw at a later time, all data collected about you during your enrolment in the study will be retained for analysis. This data is to be used for MSc thesis work, which will become a publicly accessible document. No individual will be disclosed in this document and there are no identifiers of person. Data not used in the publication(s) are kept on file, with all data, for approximately 5-years which approximates the funding cycle. What happens if something goes wrong during the study? Any adverse event that should arise will be followed-up thoroughly to ensure your safety and 77  health. Background health history and functional assessment parameters will be collected and stored on a computer, while any clinical symptoms will be recorded in the Investigator’s laboratory book. All data arising from this study will be archived and stored securely by the Investigators through password protected computer systems and remain confidential. You do not waive any legal rights by signing the consent form. The researchers of this study will be readily available if you would like to discuss any problems or concerns that may arise. Following completion of the project you will be provided with a feedback sheet explaining the outcomes and any substantive findings. Can I be asked to leave the study? If you do not adhere to the study guidelines outlined earlier in this study, you will be asked to leave the study. Also, in the rare event that a medical emergency occurs during the study, you will be automatically withdrawn from the study to ensure your safety and well-being. After this study is complete: Results of this project may be published as part of a manuscript, and will be in-part included in graduate student (Sharmin Arefin, Afruna Lizu) theses. Thesis documents are publicly available on the internet. No data will be linked to a specific participant. Your data will be assigned a personal identification number to ensure anonymity in both the analysis and documentation of results. The raw data obtained in this study will only be available to the principle investigator (Dr. Jakobi). As stated earlier, you will be provided with a feedback sheet explaining the outcomes and specific findings of this study. Privacy and confidentiality Your confidentiality will be respected. No information that discloses your identity will be released or published without your explicit consent to the disclosure. However, research records and medical records identifying you may be inspected in the presence of the Investigator or his or her designate by representatives the UBC Behavioural Research Ethics Board for the purpose of monitoring the research. However, no records which identify you by name or initials will be allowed to leave the Investigators' offices. Personal descriptors (i.e. names) will be coded to a numeric value and data will be kept in a locked cabinet in ASC 164 at the University of British Columbia Okanagan. The data will be made available only to members of the research team, and destroyed in 5 years. The master copy will be stored separate from the coded data in ASC 164 at the University of British Columbia Okanagan.  If you have any concerns about your rights as a research participant and/or your experiences while participating in this study you may contact the Research Participant Complaint Line in the UBC Office of Research Services at 1-877-822-8598 or the UBC Okanagan Research 78  Services Office at 250-807-8832. It is also possible to contact the Research Participant Complaint Line by email (RSIL@ors.ubc.ca). CONSENT FORM FOR PARTICIPANTS  I have read and understand the information sheet concerning this project. All my questions have been answered to my satisfaction. I understand that I am free to request further information at any stage. I know that: 1. My participation in the project is entirely voluntary, and I am free to   withdraw from this study at any time without any disadvantage. 2. The data on which the results of the project depend will be retained in secure storage for five years, after which they will be destroyed. 3. I will be required to complete initial assessments and the protocol of approximately 2-hour in duration.   4. The experimental session will involve the following measurements:  Demographic & health history questionnaires, physical activity questionnaires, and measures of strength by performing maximal voluntary contractions.  Assessment of tendon and muscular changes as well as force. 5. The results of the project will be published as part of a thesis and may be published as part of a manuscript and will be available in the University of British Columbia Okanagan Library but every attempt will be made to preserve anonymity. 6. I will receive a signed and dated copy of this consent form.  I agree to take part in this project  ....................................................................................................... Printed name of Subject  .............................................................................................. Signature Date  ............................................................................................. Printed name of principal investigator  .............................................................................................. Signature Date 

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