Early corrective reactions of the leg to perturbations at the torso duringwalking in humans

The contribution of afferent feedback to the regulation of locomotion in hu mans is not well understood. Animal experiments have suggested that loading of the leg during the stance phase may enhance the magnitude of extensor b urst activity and delay the onset of swing phase. The aim of the present st udy was to determine whether transient loading of the leg at the end of sta nce would enhance extensor-muscle activity and delay the onset of swing in walking humans. To test this hypothesis, we applied loads to the hips of su bjects so that the load was applied along the long axis of the leg at the e nd of stance (down-back unsupported, DBU). This resulted in an unexpectedly complex reaction characterised by rapid co-contraction of antagonist pairs of muscles around the ankle and knee and a prolongation of the stance phas e. We speculated that the complexity of the reaction was, in part, due to a disturbance in equilibrium. To address this possibility, two additional pe rturbation paradigms were tested: (1) subjects held a rail during the loadi ng paradigm (down-back supported, DBS), or (2) subjects received only a pos teriorly directed perturbation of the hips, which added no additional load to the leg (backward unsupported, BU). As predicted, the DBS perturbation r esulted in an enhancement of the ongoing soleus-muscle activity, and the un expected tibialis anterior burst that was observed during the DBU paradigm was absent. Allowing the subjects to hold a rail substantially reduced the change in the timing of the step cycle observed in the DBU paradigm. The BU perturbation prolonged the stance phase duration and, as expected, resulte d in a burst of activity in tibialis activity. This was usually accompanied by a reduction in the ongoing soleus activity. Two important conclusions a re drawn from the present study. First, loading of the leg at the end of st ance phase enhances the ongoing extensor-muscle activity. We suggest that a fferent feedback responding to the increase load supported by the leg leads to rapid enhancement of the active extensor muscles to compensate for the increased load and prevent collapse of the leg. Interestingly, the duration of the stance phase was only marginally increased when loading was applied without a postural disturbance (DBS). Second, posterior perturbation of th e centre of mass at the end of stance phase evokes an "automatic postural r esponse" in tibialis anterior. Of particular interest, this evoked postural response can occur simultaneously with an enhanced activation of soleus. T his indicates that the DBU perturbation employed in this study elicited two responses, one to prevent the collapse of the leg and the other to stabili se the centre of mass.