Title: “A Comparison of the NTV-Evoked Response to Static and Dynamic Stretch Reflex Sensitivity During Normal and Perturbed Stance”
Thesis Supervisors: Dr. J. Timothy Inglis, Dr. Mark Carpenter
Committee Member: Dr. Romeo Chua
Chair: Dr. Tania Lam
Abstract: Proprioceptive feedback from muscles around the ankle joint is critical for bipedal balance control, as length changes sensed in these muscles can be indicative of whole-body sway. These length changes are encoded by muscle spindles, which lie in parallel with skeletal muscle fibres, and respond to both static (i.e. amplitude) and dynamic (i.e. velocity) components of muscle stretch. Further, the sensitivity of the muscle spindles to static and dynamic components of muscle stretch is functionally modifiable and depends largely on the situational context.
One of the outcomes of the spindles’ neuromechanical response to the amplitude and velocity of muscle stretch is a characteristic pattern of muscle responses following rapid stretch. The short latency response (SLR) to muscle stretch has been shown to increase with stretch velocity, while the medium latency response (MLR) increases with stretch amplitude. Previous research by Horslen et al. (2018) used unilateral support surface tilts to elicit these responses in ankle muscles during standing, and inferred that changes in the gain of the SLR to stretch velocity and the MLR to stretch amplitude reflect changes in spindle dynamic and static sensitivity, respectively. While Horslen et al. indirectly investigated changes in spindle sensitivity with arousal, there is also evidence showing that spindle sensitivity can change in the absence of changes in arousal. Another method, Noisy Tendon Vibration (NTV), has been shown to reliably elicit spindle-mediated responses during standing. However, it is not yet known if the elicited response is primarily reliant on spindle static or dynamic sensitivity.
The purpose of the proposed thesis is to compare changes in (1) SLR scaling to stretch velocity, (2) MLR scaling to stretch amplitude, and (3) NTV-response scaling to NTV amplitude, in two different postural contexts that do not increase arousal: standing quietly and standing in anticipation of balance perturbations.
The findings of this thesis will provide insight on how spindle dynamic and static sensitivity may be modulated by postural context during bipedal standing. Additionally, it will further our understanding of how the NTV-evoked response relates to spindle dynamic and static sensitivity. This may lead to more widespread use of NTV to assess spindle sensitivity. This would be useful, as NTV can be assessed during standing without causing any noticeable disruption of balance.