Early activation of arm and leg muscles following pulls to the waist during walking

Many studies have investigated the compensatory reactions in humans elicited during walking when the support surface is perturbed. This has led to the description of characteristic responses generated in the muscles of the legs and torso, and recently the arms. The present study aimed to investigate the compensatory reactions elicited when balance was challenged by a perturbation applied to the waist, to determine to what extent balance corrective responses are generalized across perturbation modalities. A second aim was to characterize the arm responses elicited by the perturbations applied to the waist. We measured muscle activity of the left arm and leg following application of backward pulls of the waist while the subjects walked on a motorized treadmill. This resulted in robust activation of tibialis anterior and vastus lateralis, with co-activation of soleus and biceps femoris also evident when perturbations were applied at heel strike. These early responses occurred with a distal to proximal temporal organization. The responses in the leg muscles displayed a phase-dependent modulation in amplitude, decreasing in amplitude later in the stance phase. Leg muscle responses were not evident during the swing phase, except for the end of swing, just prior to heel strike. Arm muscle responses were observed in all subjects; however, the pattern of the arm responses varied considerably between subjects. Generally, shoulder muscles were more likely to respond than elbow muscles, at latencies consistent with the leg responses. Two important conclusions are drawn from the present study. First, the responses evoked in the legs with a pull to the waist are very similar to what has been reported for perturbations of the support surface, despite the very different locus of the perturbation. This suggests that balance control during walking may be achieved by preprogrammed reactions or synergies, which are triggered by multiple sensory cues. Second, rapid arm actions are integrated with these leg responses. However, the arm responses are more flexible, likely reflecting the fewer constraints imposed upon the actions of the arms, compared to the legs, during normal locomotion.