Influence of lateral destabilization on compensatory stepping responses.

Previous studies of compensatory stepping, in response to postural perturbation, have focussed on forward or backward stepping; however, the ability to step in other directions is of equal functional importance, since the perturbations encountered in daily life may often include a lateral component. The primary objective of this study was to determine how lateral destabilization affects the compensatory stepping response, in terms of: (1) swing-leg selection, (2) preparatory unloading of the swing leg, and (3) spatial and temporal characteristics of the swing trajectory. A novel multi-directional moving platform was used to apply transient perturbations in eight horizontal directions, in 10 healthy young adults. Perturbation magnitude was varied unpredictably over a wide range and subjects were instructed to try not to step, so as to discourage preplanned 'volitional' foot movement. The predominant strategy, seen in 96% of stepping responses to lateral destabilization, was to swing the leg that was unloaded by the perturbation. This strategy allowed a much more rapid foot-lift but required a longer and more complex swing trajectory, compared to responses where the perturbation-loaded leg was swung. When compared to forward and backward steps, the addition of a lateral component to the perturbation led to a 20% (90 ms) reduction in time to foot-off, a 20% (7 cm) increase in step length and a 70% (110 ms) increase in swing duration, on average. The results clearly demonstrate that compensatory stepping responses to non-sagittal perturbations are strongly influenced by biomechanical constraints and affordances that do not affect the forward and backward stepping behaviour that has been studied traditionally. These findings underscore the need to assess postural responses in multiple directions, in order to understand more fully how balance is maintained in the exigencies of everyday life.