The control of lateral stability during rapid stepping reactions evoked byantero-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 foo l. The present study examined whether this type of anticipatory control pla ys a functional role in maintaining lateral stability during the rapid comp ensatory stepping reactions that are evoked when whole-body stability is ch allenged by unpredictable perturbation. Forward and backward stepping react ions were evaluated in five healthy young adults (ages 22-28) under three t ask conditions: (1) unconstrained compensatory stepping evoked by platform translation (no specific instructions), (2) constrained compensatory steppi ng cued by platform translation (prior instruction to step rapidly), and (3 ) rapid voluntary stepping to a light cue. Mt APAs occurred during 70% of p erturbation reactions but were too small and brief to have a substantive in fluence on the lateral movement of the COM occurring during leg lift or swi ng phase. In contrast, during the light-cued stepping, the Mt 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 presenc e of an Mt APA during compensatory stepping may represent an attempt to pre plan a stereotypical stepping response, but that the ability to fully expre ss 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, unpredictabl e antero-posterior perturbation. (C) 1999 Elsevier Science B.V. All rights reserved.