Heteronymous Ia-afferent connections in the upper limb following stroke.

Synergistic arm movement patterns are common following stroke and may arise through enhanced spinally mediated reflex connections between muscles. Our goal was to investigate the excitability of heteronymous Ia-afferent pathways in people with chronic stroke. Responses to tendon taps of the flexor carpi radialis (FCR) muscle were recorded in FCR, biceps brachii (BB), and middle deltoid (MD) of 13 people with stroke and 13 controls. Heteronymous reflexes were elicited in BB and MD in some, but not all, stroke and control subjects. The prevalence and size of the heteronymous responses were not significantly different between groups. Homonymous reflex responses in FCR were significantly larger in the stroke group. We found that the excitability of heteronymous Ia-mediated pathways from FCR to BB and MD muscles is not enhanced following stroke, despite exaggerated homonymous reflexes, and they are therefore not likely to contribute to coactivation of forearm and more proximal upper limb muscles.