Figure 1.

The trip was induced by an obstacle device consisting of a hinged metal plate 8-cm high, 27-cm wide, and 0.5-cm thick. During regular walking, the plate was locked in a flat position by a pair of electromagnets. During the perturbation trials, the springs returned the plate to its upright position to induce a trip by obstructing the left leg. The trip plate was triggered by the ground reaction force detected by force plates beneath the walkway. Participants were protected during all trials by a safety harness connected through a load cell mounted to the ceiling along the walking path.