Responses of ankle musculature of healthy subjects and hemiplegic patients to sinusoidal anterior-posterior movements of the base of support.

The responses of the medical gastrocnemius and tibialis anterior muscles to continuous sinusoidal perturbation of the base of support were studied in two groups of subjects, forty-two volunteers aged 20 to 84 years and 17 hemiplegic patients aged 52 to 81 years. EMGs were recorded while subjects stood on a platform oscillating in the anterior-posterior direction. Movement amplitude was adjusted to the maximum the subject could sustain without assisted support within the limits of the instrument. In healthy subjects, two basic activation modes were indicated: (a) a reciprocal contraction pattern in which activity of the medial gastrocnemius and tibialis anterior muscles was concentrated in the anterior and posterior half of the oscillation, respectively. This pattern prevailed in the younger subjects whose maximal movement amplitude reached the highest limit of the instrument; (b) a less discrete pattern, characterized by tonic contraction and periods of coactivation of the two muscles, was seen in older subjects who had difficulties in maintaining stance on the moving platform. The disruption of the discrete reciprocal response mode in this group of patients was related to decline in function of postural mechanisms with aging and demanded additional stiffening of the ankle join, presumably a manifestation of reduced automatism in favor of closer CNS modulation. The typical response mode in the sound leg of the hemiplegic patients was variable coactivation of the two muscles. In the afflicted leg, the establishment of a motor set was impaired: Low tone was associated with negligible muscular activity, although with more elevated tone, low level uniform cocontraction was evident.