Afferent feedback in the control of human gait.

Sensory activity contributes to motor control in two fundamentally different ways. It may mediate 'error signals' following sudden external perturbations and it may contribute to the pre-programmed motoneuronal drive. Here we review data, which illustrate these two functions of sensory feedback in relation to human walking. When ankle plantarflexors are unloaded in the stance phase there is a sudden decrease in the sensory activity in muscle and tendon afferents from the active muscles. This decrease in sensory activity results in a drop in EMG activity recorded from the soleus muscle, which demonstrates that the sensory activity contributes importantly to the activation of the muscles. Data suggests that a spinal pathway from gr. II muscle afferents is responsible for this positive feedback contribution to the motoneuronal drive during walking. When cutaneous nerves from the foot are stimulated in the early swing phase of walking a late reflex response may be observed in the tibialis anterior muscle. This reflex may help to ensure that the foot is lifted effectively over an obstacle. Data suggest that this reflex response is at least partly mediated by a transcortical reflex pathway. It seems to be important that reactions to external perturbations are integrated at a supraspinal level during human walking.