Significance of proprioceptive mechanisms in the regulation of stance.

Compensatory electromyographic (EMG) responses and several biomechanical parameters were studied following impulsive disturbance of the limbs during stance of human volunteers on a treadmill. Treadmill acceleration impulses were backwards or forwards directed, or their initial direction was reversed after 30 ms. Backwards directed impulses were followed by gastrocnemius, forwards directed ones by tibialis anterior EMG responses (latency 65 to 75 ms) whose durations depended on impulse duration. When the direction of the impulse was reversed, the respective antagonistic leg muscles were activated again with a delay of 68 to 75 ms after onset of stretch of these muscles. The behaviour of the EMG responses could best be correlated to the displacement at the ankle joint and may be described in terms of a stretch reflex response. The results indicate that these stretch reflex responses help control of the body's centre of gravity thereby preventing falling. Head movements induced by the impulses showed little correlation with the appearance of the EMG responses, suggesting that the vestibular system is unlikely to be directly involved in the generation of these responses. Vestibular signals may, however, significantly contribute to slow body sway stabilization.