Do anticipatory postural adjustments precede compensatory stepping reactions evoked by perturbation?

Many recent studies have revealed the consistency of anticipatory postural adjustments preceding single leg movements, such as stepping, in standing humans. These adjustments appear to involve an active lateral transfer of weight to the stance leg. The present study demonstrates that these anticipatory adjustments often do not appear in compensatory stepping responses evoked by postural perturbation. Compensatory stepping without anticipatory active weight transfer seemed to be more likely to occur when stepping was not pre-planned or when the perturbation characteristics were novel. The substantial variability seen in the anticipatory weight transfer, when it did occur, contrasts the consistency of the anticipatory responses seen in gait initiation and other non-compensatory leg movements. The results suggest that there may be fundamental differences in the control of compensatory versus non-compensatory stepping, and may provide insight into the sequencing of anticipatory postural reactions and subsequent leg movements.