UBC Theses and Dissertations
The influence of sensorimotor loop delays in maintaining upright stance McKendry, Geoffrey James
Sensorimotor delays are inherent to the control of standing balance. Increases in sensorimotor delays observed during healthy aging and in people with Multiple Sclerosis (MS) have been theorized to reduce stability. The aim was to determine if balance stability declines with increasing artificial delays in balance control. Furthermore, I sought to determine whether there is an association between estimates of sensorimotor loop delays and the artificial delays at which people become unstable. Thirty healthy participants (19 to 75 years) and three participants with MS were recruited. A rotating platform elicited balance responses to pitch rotations. Surface electromyography recorded muscle activity from soleus (Sol), medial gastrocnemius (mGas), and tibialis anterior (TA) to determine onset latencies of balance responses, providing estimates of sensorimotor loop delays in balance control. A robotic balance simulator introduced artificial delays (50-400 ms) between the participants’ motor commands and resulting whole-body movements. Participants balanced for up to 20 seconds or until a ‘virtual fall’ occurred (exceeding 3° posterior or 6° anterior). Logistic regression analysis using the number of ‘virtual falls’ at each delay was performed to obtain a threshold of stability (5% probability of falling). Maintaining balance became more difficult as delays increased: the number of ‘virtual falls’ and standard deviation of angular backboard displacement increased, while the time to fall decreased as delays increased. The mean threshold of stability was 98 ± 38 ms. In response to the tilt perturbations, short and medium latency responses were observed in Sol (50.8 ± 3.2 ms) and TA (98 ± 10 ms), respectively. Long latency responses in TA, Sol and mGas were 147 ± 12 ms, 166 ± 22 ms, and 178 ± 27ms respectively. There were no correlations between thresholds of balance stability on the robot or the evoked-balance responses and age. A weak association between long latency responses in TA and thresholds of stability was observed. My results support the postulation that balance control becomes unstable as delays in balance control increase. The weak correlation between estimates of sensorimotor delays and thresholds of stability on the robot, however, suggests these measures target different pathways involved in standing balance.
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