Restricting arm use enhances compensatory reactions of leg muscles during walking

A number of recent studies have indicated that whole-body coordinated reactions are employed to regain balance following disturbances during walking. However, it is not always the case that all body segments are available to contribute to balance corrective strategies. We hypothesize that balance corrective strategies will adapt to task and environment constraints such that greater responses are generated in the available body segments when other body segments are unable to participate. In this study, we tested the hypothesis that voluntarily restricting the arms during walking would result in an increase in the amplitude of the electromyographic responses evoked in leg muscles when subjects are perturbed at the torso during walking. To do so, subjects were asked to walk on a motorized treadmill while either crossing their arms across their front or back, or with their arms swinging normally. Periodic perturbations, forwards and backwards, were applied at the pelvis randomly throughout the step cycle. This resulted in short latency responses in leg muscles. The amplitude of these responses was increased when subjects walked with their arms crossed, as compared with normal, unrestricted walking. Facilitation of these evoked responses was restricted to the early part of the stance phase, particularly at heel-strike. The pattern of muscle activation and the latency of the responses were not affected by restricting the arms. We suggest that this finding indicates that whole-body balance corrective strategies employed during walking are selected based upon the demands of the general features of the task, but that components of the strategy are scaled according to the specific context-dependent needs of the task.