The contribution of vision and of sidedness to responses of the ankle musculature to continuous movement of the base of support.

Eight healthy women were subjected to continuous sinusoidal movements of the base of the platform on which they stood. Each subject was tested once in the anterior-posterior (AP) and once in the lateral movement direction. In the first 30 s of each test, subjects' eyes were open while during the following 30 s their eyes were closed. Bilateral EMG recordings from the Medical Gastrocnemius (MG) and the Tibialis Anterior (TA) muscles were continuously taken from the moment subjects became used to the imposed movement. Results revealed that during AP movements the MG and TA muscles were reciprocally activated and deactivated, thereby continuously stabilizing the ankle joint in advance of the imposed anterior or posterior perturbation. Similarly, during lateral movements each MG muscle was activated in the homologous side of the movement trajectory. These basic postural sets were neither affected by elimination of vision nor by sidedness. However, absence of vision caused decrease of the ratio between the IEMG of the MG muscle to that of the TA muscle during AP movements. This decrease resulted from decrease in the activity of the MG muscles and/or from increased activity of the TA muscles while vision was eliminated. The latter phenomenon occurred to a greater extent in the right leg.