Reflex excitation of muscles during human walking.

Sensory activity may contribute to the control of human walking in two different ways. It may contribute to the pre-programmed drive to the motoneurones and to the reactions to unexpected external perturbations. Some recent findings in relation to these two different roles of sensory activity will be reviewed. When unloading the ankle plantarflexors in the stance phase of walking a drop in the soleus EMG activity is seen at a latency of around 60 ms. This drop is likely caused by the removal of the contribution of Gp II afferents from the ankle plantarflexors to the motoneuronal drive. When stretching plantarflexor muscles in the stance phase three reflex responses are generally observed. These responses may be caused by the spinal monosynaptic Ia reflex pathway, a spinal Gp II pathway and a transcortical reflex pathway, respectively. The reflex responses are modulated with the background EMG activity and may not be evoked in the swing phase when the plantarflexors are not active. In contrast, stretch of the ankle dorsiflexor muscles evoke relatively small responses in the swing phase when these muscles are active, but very large responses in the stance phase when the muscles are silent. Part of these responses may have a transcortical nature. These findings illustrate the complexity with which sensory input may contribute to the ongoing muscle activity during walking and may also mediate adequate responses to sudden external perturbations.