Postural responses of normal geriatric and hemiplegic patients to a continuing perturbation.

The electromyographic (EMG) activity of the gastrocnemius and tibialis anterior muscles was recorded from normal geriatric subjects and from hemiplegic patients subjected to a continuous back-and-forth movement of the platform on which they stood. The platform oscillated at a fixed frequency of 0.5 Hz. Movement amplitude was the largest each patient could withstand without support of the hands. Standing on the moving platform continued for 5 min. EMG recordings continued for 60 s, commencing 2 min after movement onset. Analysis of the EMG, in normal subjects revealed a typical pattern of reciprocal activity in the tibialis anterior and gastrocnemius, in synchrony with the back-and-forth movements of the platform. The cyclic activation of each muscle started before movement of the platform could cause its stretch. The normal movement-induced pattern of leg muscle activation was greatly disturbed or totally absent in hemiplegic patients. In such patients, various patterns of tonic activation of one or both muscles were observed in the sound as well as the afflicted leg. Occasional periodic activation of one muscle or the other was observed. However, such activation was not well synchronized with movement of the platform and did not last long. Based on the above observations, we concluded that the ability to adapt to a rhythmic platform movement by production of anticipatory phasic muscular contractions is severely disrupted in hemiplegics.