The control of lateral stability during rapid stepping reactions evoked by antero-posterior perturbation: does anticipatory control play a role?

Volitional step initiation invariably includes a medio-lateral anticipatory postural adjustment (ML APA), which causes the center of mass (COM) to be propelled toward the stance-limb side prior to the lifting of the swing foot. The present study examined whether this type of anticipatory control plays a functional role in maintaining lateral stability during the rapid compensatory stepping reactions that are evoked when whole-body stability is challenged by unpredictable perturbation. Forward and backward stepping reactions were evaluated in five healthy young adults (ages 22-28) under three task conditions: (1) unconstrained compensatory stepping evoked by platform translation (no specific instructions), (2) constrained compensatory stepping cued by platform translation (prior instruction to step rapidly), and (3) rapid voluntary stepping to a light cue. ML APAs occurred during 70% of perturbation reactions but were too small and brief to have a substantive influence on the lateral movement of the COM occurring during leg lift or swing phase. In contrast, during the light-cued stepping, the ML APA propelled the COM toward the stance-limb side prior to the lifting of the swing limb, and effectively reduced the tendency of the COM to fall toward the swing-limb side during the execution of the step. It is proposed that the presence of an ML APA during compensatory stepping may represent an attempt to preplan a stereotypical stepping response, but that the ability to fully express the anticipatory phase is disrupted by the need to react rapidly to the unpredictable antero-posterior instability imposed by the perturbation. The results suggest that anticipatory control is not the primary mechanism by which the central nervous system deals with the lateral instability arising during rapid compensatory stepping reactions evoked by large, unpredictable antero-posterior perturbation.