Phase-dependent compensatory responses to perturbation applied during walking in humans.

The interaction between the peripheral and the central regulation of locomotion was studied by examining the dependency of the response to unexpected perturbation on the phase of the step cycle. The changes in the latency and magnitude of various muscle responses to electrical stimulation of the toe and applied unexpectedly at different phases of the locomotor cycle in humans are described. The results show that response to perturbation is gated and modulated in both ipsi- and contralateral limb muscles. These muscle responses, when present, were always excitatory in nature. They were not correlated with the normal locomotor activity, thus suggesting a more complex organization of the response. Except for one muscle in the contralateral limb, the latency of the other muscle responses did not vary across the step cycle. in response to the perturbation, the appropriate phase of the step cycle was shortened. The results from this study suggest that the perturbation applied elicits a phase-independent, normal ipsilateral flexor response in the tibialis anterior muscle, while the gating and modulation of other ipsi- and contralateral muscles provide appropriate phase-dependent adaptive response to maintain postural stability and continue with the ongoing task of locomotion.