Obstruction of the swing phase during gait: phase-dependent bilateral leg muscle coordination.

We have investigated bilateral leg muscle activation following an obstruction of the forward swinging leg during gait. When the holding impulse was released at the beginning of the swing phase, weak gastrocnemius (GM) and biceps femoris (BF) responses appeared in the contralateral, standing leg. When the holding impulse was released at the end of the swing phase, strong tibialis anterior (TA) and rectus femoris (RF) responses appeared in the swinging leg, and GM and BF responses in the standing leg. In the latter condition the TA response was followed by a premature impact associated with a GM activation. The latency between onset of impulse and appearance of the responses was 65-70 ms in both legs. While in the first condition the duration of GM and BF response corresponded to the duration of the impulse, it was independent for the duration of GM and BF response in the latter. It is concluded that different strategies for compensation are at work. In the first condition, body stability is maintained during the obstruction by the standing leg. In the second condition, body support is provided by a premature touchdown of the swinging leg. It is suggested that the appropriate pattern is, in part, released by a spinal generator.