Our understanding of the postural control responses in the event of external perturbation has focused almost exclusively on the early automatic adjustments. The present study addresses another postural reaction that is functionally important: compensatory stepping. The purpose was to identify the relative importance by comparing the prevalence of compensatory stepping with and without instructions constraining the subjects' responses. Subjects stood on two force plates which were mounted on a ''moveable'' platform. Their posture was perturbed by the translation of the platform either forward or backward at various accelerations. Following a practice period, seven subjects each performed under two different tasks: ''constrained'' (keep feet in place) and ''unconstrained'' (no specific instructions given). The primary focus of the analysis was on responses to forward platform translations. Analysis revealed that the frequency of stepping tended to be higher in ''unconstrained'', as opposed to ''constrained'', tasks. The frequency of stepping was also related to the interaction between the tasks and the order in which they were given. Specifically, subjects stepped most frequently when they received the ''unconstrained'' task first. The frequency of stepping also increased as the magnitude of the platform acceleration increased. Time of onset of stepping, as defined from the force plate measures, began as early as 160 ms in one subject and averaged 250 ms across all subjects. These relatively fast response times suggest that step initiation often occurs well before the limits of stability are reached. A novel and unexpected finding was the identification of a third response type, intermediate to stepping and (bilaterally symmetrical) non-stepping responses. This intermediate response was characterized by a lateral weight shift and appeared to be the initiation of a stepping response which was aborted before the foot was lifted. In summary, it is concluded that stepping may be more prevalent in unconstrained behaviour. Paradigms that implicity or explicitly constrain foot movement may well lead to misleading conclusions regarding ecologic validity of specific postural strategies. The occurrence of stepping is also strongly influenced by the magnitude of the perturbation, implying that some ''decision'' process occurs before initiation of stepping. Moreover, initiation of compensatory stepping (lateral weight shift) and continuation (foot lift and swing) may involve separate and sequential ''decisions'', suggesting a more complex contribution from sensory cues than might be predicted for a completely preplanned reaction.
