The mechanisms involved in rapidly terminating human gait were studied. Subjects were asked to walk at a comfortable speed and to stop walking as soon as they felt an electrical stimulus to the superficial peroneal nerve. This simulated hitting an obstacle with the top of the foot. Stimuli were presented repeatedly at random during a 20-min period of walking. Electromyograms and joint angular movements of the right leg and forces under both feet were recorded. The step cycle was divided into 16 parts, and the responses to stimuli in each part were analyzed separately. Subjects generally stopped with the right foot in front of the left or vice-versa, depending on when the stimulus was applied in the step cycle. There was also a transition region in which subjects would rise up on their toes and either back down or take one more quick, short forward step. Three different mechanisms were used to produce a stop. 1) An extension synergy in the swing leg was initiated just before this leg hit the ground to brake the forward momentum of the body. 2) The push-off phase of the stance leg was inhibited to reduce the forward thrust and maintain the stance leg on the ground behind the body. 3) If these mechanisms were insufficient, the body rose up onto the toes of the extended forward leg and thereby converted more kinetic energy to potential energy. A decision to take an additional step depends on whether the momentum of the body is sufficient to carry the center of mass in front of its support on the forward leg. If so, an additional step is taken. Despite the complexity of the decisions that must be made, changes in electromyographic activity are seen throughout the legs and trunk in 150-200 ms.
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