Multiple-step strategies to recover from stumbling perturbations.

This study has analysed the recovery from an induced stumble whilst walking on a treadmill. Four stumbling conditions were tested; at early swing with short and long durations and at mid and late swing with short duration. The experiment set-up, including the possibility of being stumbled, did not alter the normal gait patterns and the recovery strategies depended on the perturbation conditions. For the early swing perturbation, delayed lowering and elevating strategies were performed using the perturbed leg. A lowering strategy was seen for mid and late swing perturbations. An elevating strategy consisted of an elevation of the swing limb while a lowering one consisted of bringing the foot quickly to the ground. There were two groups of reactions to the experimental perturbation of gait. In the first, there was an effort to complete the disturbed step as normally as possible, so the following steps were less constrained to maintain treadmill speed. In the second group of reactions, the perturbed step was aborted and the recovery effort transferred to the contralateral limb. In many cases, several steps were needed to regain normal gait pattern. The study of recovery reactions from gait perturbations should include at least three steps after the perturbed one.