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Frequency characteristics of lower limb muscle responses to proprioceptive perturbations evoked by Achilles tendon vibration during standing and the influence of age and stroke Mildren, Robyn Lynne


There are many structures within the nervous system that, as a whole, are responsible for the control of movement and balance. The spinal cord plays an important role in sensorimotor processing, it integrates sensory signals from the periphery as well as signals from the brain to control muscle activation. The purpose of this thesis was to characterize the short latency (spinally mediated) lower limb muscle responses to proprioceptive perturbations during standing, and examine how they are influenced by ageing and chronic stroke. Chapter 2 develops an innovative methodology to characterize muscle responses to proprioceptive perturbations during standing. Here, we examined the association between noisy (10-115 Hz) suprathreshold Achilles tendon vibration and ongoing triceps surae muscle activity. We observed responses in soleus across a broad frequency bandwidth (~10-80 Hz). Consistent responses were obtained with short trial durations (<60 s); furthermore, responses did not habituate, and the stimulus did not noticeably perturb standing balance. Chapter 3 demonstrates differences in single motor unit responses to noisy and sinusoidal vibration between two plantar flexor muscles – soleus and medial gastrocnemius. These experiments revealed soleus motor units had stronger responses relative to medial gastrocnemius, and single motor units showed minimal non-linear phase locking. Chapter 4 illustrates how cutaneous feedback from the foot sole interacts with the soleus vibration responses. Foot sole cutaneous stimuli were found to modulate the vibration responses in a spatially organized manner, where heel stimuli suppressed and metatarsal stimuli enhanced responses. Finally, Chapters 5 and 6 examine the influences of age and chronic stroke on the characteristics of the soleus vibration responses. We found a narrowing of the frequency bandwidth, and a decrease in gain, amplitude, and scaling of soleus responses with age. On the affected side post-stroke, we found evidence of altered soleus vibration responses, along with changes in postural control and mechanical admittance of the muscle-tendon unit. Collectively, these studies contribute to our understanding of the proprioceptive system and how changes associated with ageing and stroke may contribute to impaired balance and falls. These findings have implications for rehabilitation strategies and the development of neuroprostheses.

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