Mechanically induced stumbling during human treadmill walking.

A new method to study the reactions to unexpected mechanical perturbations during human walking on a treadmill is presented. Perturbations consisted of an obstruction of the forward swinging foot during the early swing phase. These were caused by obstacles which were dropped on the treadmill in front of the subject. The timing of the perturbation was controlled by an electromagnet which released the obstacle at a preprogrammed delay after left or right heel strike. This kind of perturbation evoked stumbling reactions. The electromyographic (EMG) responses during these stumbling reactions had mean latencies of 76 ms in both the ipsilateral biceps femoris and rectus femoris when perturbations were applied in early swing. During the perturbed swing, increased flexion in the knee occurred to lift the foot over the obstacle. Both the EMG and kinesiologic responses were reproducible when perturbations were presented in the same part of the swing phase of different step cycles.