| Summary: | Mechanisms underpinning motor control of standing balance post-stroke remain unclear. Following stroke, ankle plantarflexor muscles demonstrate impairment associated with asymmetrical postural control and decreased balance. Stroke also results in increased attentional demands during challenges to standing balance. This thesis examined motor control impairment post-stroke from medial gastrocnemius motor units to the tri-muscle plantarflexor complex. Investigation of motor, kinematic and kinetic parameters of postural control during external perturbations in standing and associated levels of physiological arousal have furthered understanding of balance impairment post-stroke.
Methods: Medial gastrocnemius motor units were recorded in controls (Chapter 2) and people post-stroke (Chapter 3) in standing as perturbations were sequentially applied at the pelvis under conditions of increased anteriorly-directed challenge. In both studies, motor unit firing rate was calculated during dynamic response to perturbation, and maintenance of steady state between perturbations. Joint kinematics, surface electromyography and movements of the centre of pressure were assessed. In Chapter 4, this methodology was expanded to cross-correlation analysis of electromyography activity of the three plantarflexor muscles with anterior-posterior centre of pressure during steady state. In Chapter 5, attentional demands surrounding timing of external perturbation were manipulated to investigate effects of stroke on physiological arousal and postural reactions.
Results: In healthy subjects, medial gastrocnemius utilized primarily motor unit recruitment to maintain standing with a modest increase in motor unit firing rate only during the dynamic response to external perturbations. The paretic medial gastrocnemius also primarily used motor unit recruitment; however, lacked firing rate modulation during the dynamic response, albeit firing rate was related to kinematic variables of postural control. In people post-stroke, the three plantarflexors demonstrated asymmetrical motor control of postural sway between-legs but symmetry was improved under conditions of increased challenge to standing balance. Finally, knowledge of timing of perturbations did not decrease the heightened anticipatory postural strategy and level of physiological arousal exhibited post-stroke.
Conclusions: This dissertation provides new understanding of motor control of standing balance post-stroke and reveals anticipatory postural strategies adopted post-stroke under conditions challenging balance. These findings implicate the importance of introducing challenge to standing balance post-stroke in the assessment and rehabilitation of postural control post-stroke. === Medicine, Faculty of === Graduate
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