Thesis Title: Adapting to Sensorimotor Delay in the Control of Standing Balance
Thesis Supervisor: Dr. Jean-Sébastien Blouin
Committee members: Dr. Mark Carpenter, Dr. Lyndia Wu
Defence Chair: Dr. Hyosub Kim
Abstract: When balancing upright, humans must differentiate self-motion generated by their own motor commands from that induced by external events. This dynamic process is required to generate balance-correcting response and must take into account uncertainty in the sensorimotor control of balance induced by sensorimotor noise as well as sensing and actuation delays. In the present study, I characterized how humans adapt their control of standing balance when faced with uncertainty associated with sensorimotor delays. Twenty-two young healthy adults stood upright in a robotic balance simulator that allowed me to manipulate the delays between their self-generated ankle torques and resulting whole-body motion. Participants balanced in the anteroposterior direction with baseline delays (pre- and post-adaptation) and adaption to imposed delays of 250ms for 20 minutes. I observed that the introduction of sensorimotor delays destabilized how participants balanced upright, with most reaching the virtual limits of the balance simulation in the first 5 minutes. Participants also exhibited an increase in lower leg muscles activation (4-12 times pre-adaptation and agonist-antagonist co-contractions (5-13 times pre-adaptation), immediately after the delay was introduced. Through exposure to the imposed delays, participants adapted their control of balance by minimizing whole-body motion variability from 10.64 ± 3.47 to 1.19± 0.64 [°/s]^2, while the end of the adaptation still 15 times greater than no-delay quiet standing. Similarly, through the adaptation, the gradual decrease in the muscle activation (except for soleus) and muscle co-contraction was also observed. Further investigation suggests that the lack of decrease in soleus may be linked to participants tend to lean more forward during the adaptation. When removing the imposed delay (post-adaptation period), the sway variance in the participants whole-body motion quickly return to pre-adaptation values. However, muscle activation and co-activation at post-adaptation remained at levels similar to those observed during the late adaptations. The present findings show that humans adapt their control of balance by increasing muscle activation and co-contraction when faced with imposed sensorimotor delays and exhibit minimal (or brief) after-effects when an imposed delay is removed. The results from this study provide initial insights to help us to understand how humans adapt to the changes in sensorimotor delays they may experience throughout their lifespan or encounter a neurological disorder.