Neurophysiological effects of arm weight support: Implications for stroke rehabilitation
Arm weight support may be used as an adjuvant to increase training dosage and improve movement quality during upper limb stroke rehabilitation. However, the underlying neurophysiological effects of weight support are not well understood. The aim of this thesis was to investigate the neurophysiological effects of arm weight support in healthy adults and chronic stroke patients. Four experiments examined the effects of weight support on muscle activation and corticomotor excitability across the upper limb using multiple gradations of supportive force. Transcranial magnetic stimulation and electromyography were employed during different movement tasks. For muscles that generate shoulder abduction (antigravity) torque, muscle activity responded linearly to gradations of supportive force. For distal muscles, a trend between nonessential tonic muscle activity and the degree of weight support provided evidence in support of a common neural drive to the upper limb. Modulation of corticomotor excitability was muscle-specific and discontinuous with respect to linear gradations of supportive force. Therefore, weight support may interact with thresholds in multiple modulatory mechanisms. During a separate rhythmic movement task, an improvement in biceps brachii selectivity with greater weight support was revealed by task-dependent modulation of corticomotor excitability preceding agonist or antagonist contractions. The results indicate that weight support may interact with excitatory mechanisms linking muscle representations as well as local inhibitory circuits. In a comparison of sitting and standing postures, there were small but significant differences in corticomotor excitability across the upper limb. During a reaching task, patients with moderate-severe upper limb impairment were able to hit more targets with greater weight support. Muscle activity tended to decrease with more supportive force; however, the response depended on impairment severity. Weight support had an influence on corticomotor excitability in control, mild, and moderate-severe impairment groups. The pattern of modulation was not consistent and likely reflects individual differences in lesion extent and location. Several novel findings contribute to our understanding of upper limb control. Arm weight support appears to have direct mechanical effects and indirect neurophysiological effects. Chronic stroke patients respond to changes in weight support at behavioural and neurophysiological levels. The responses vary with the severity of motor impairment.
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