Influence of cerebrovascular accident on elongated and passively shortened muscle responses after forward sway.

The purpose of this study was to examine the effects of a rapid length change in the elongated and shortened ankle muscles of subjects with hemiplegia during forward body sway. The onset and amplitude of electrical activity in the ankle muscles of four standing hemiplegic subjects with mild stretch reflex abnormalities and four healthy subjects were studied. Each subject stood on a platform controlled by a hydraulic servomechanism and experienced an unexpected posterior horizontal platform movement that induced a forward body sway. The activity of the gastrocnemius (agonist) and tibialis anterior (antagonist) muscles were monitored bilaterally using surface electromyography. In comparison with the healthy subjects, the hemiplegic subjects showed a greater disassociation between agonist and antagonist activation, a larger frequency of response defaults in the antagonist, and an increase in nonparetic (left) limb agonist amplitude during the 200-msec electromyographic integration period. We concluded that the sequelae of cerebrovascular disease may hamper the initiation of a passive shortening response in the tibialis anterior muscle during forward sway. This finding is functionally significant because the response deficit described seems to be exaggerated in some cases and reduced in others.