Title: A comparison of the NTV-evoked response to static and dynamic stretch reflex sensitivity during normal and perturbed stance
Thesis Supervisors: Dr. Mark Carpenter, Dr. Tim Inglis
Committee Member: Dr. Romeo Chua
Chair: Dr Eli Puterman
Abstract: Proprioceptive feedback from muscle spindles around the ankle joint is critical for balance control. The muscle spindles lie in parallel with muscle fibres and encode length changes in the muscle, including static (i.e. amplitude) and dynamic (i.e. velocity) components of muscle stretch. Further, the spindles’ sensitivity to dynamic and static components of stretch can be functionally modified based on contextual demands.
Following rapid stretch, spindle feedback evokes a short latency response (SLR), which scales to stretch velocity, and a medium latency response (MLR), which scales to stretch amplitude. Assessing the slope of scaling for SLR-velocity and MLR-amplitude could be indicative of underlying changes in spindle dynamic and static sensitivity. While the slope of SLR-velocity scaling in ankle muscles during standing has been demonstrated to modulate with height-related arousal, the slope of MLR-amplitude scaling has not yet been characterized during standing. Further, Noisy Tendon Vibration (NTV) is another method of evoking spindle-mediated responses during standing, but it is not yet known to what extent these responses relate to spindle dynamic and static sensitivity.
The purpose of this thesis was to assess scaling of SLR-velocity and MLR-amplitude, using unilateral tilts to stretch ankle muscles while standing, and to assess scaling of the NTV-evoked response to NTV amplitude. Additionally, to compare changes in the slope of scaling between all three responses when participants stood in a condition where they could randomly be pushed to perturb balance. Key findings of this thesis include demonstrating MLR-amplitude scaling during standing and characterizing this response by showing it was less linear than SLR-velocity scaling. No changes we observed in slope of scaling between perturbation conditions for any of SLR-velocity, MLR-amplitude, or NTV-evoked response to NTV. Thus, we could not compare the change in slope between the three methods, and the reliance of NTV on spindle dynamic or static sensitivity could not be inferred.