Can stabilizing features of rapid triggered stepping reactions be modulated to meet environmental constraints?

This study examined whether the stabilizing features of rapid triggered compensatory stepping reactions can be modulated to accommodate an environmental constraint, in the form of an obstacle placed in front of the subject. The compensatory stepping reactions were evoked, in 11 healthy young adults, by unpredictable multidirectional platform translation; the forward-step reactions evoked by large backward translations were analyzed. Clearance of the obstacle required a doubling of the usual (no obstacle) swing duration and therein presented significant challenges to both anteroposterior (a-p) and lateral stability, yet the central nervous system (CNS) was able to decelerate and stabilize the body's center of mass (COM) without taking additional steps. Findings that the response was modulated successfully without prior practice or exposure to the perturbation suggest that the CNS automatically incorporates exteroceptive information into the control of compensatory stepping. Control of a-p stability appeared primarily to involve an increase in forward step distance. Surprisingly, the control of lateral stability involved both an anticipatory postural adjustment (APA) and lateral step placement. By including a much larger APA prior to foot-lift (137% increase in amplitude and 50% increase in duration versus no-obstacle trials), the COM was propelled to a more stable position prior to swing phase. This was achieved with minimal (32 ms) delay in foot-lift, due, in part, to more rapid initiation and execution of limb unloading. In contrast to previous findings that APAs are commonly absent or severely reduced during compensatory stepping, the present results demonstrate that large APAs can be incorporated into these reactions when demanded by task conditions. However, these large APAs were still insufficient to counter the increased tendency of the COM to fall during the prolonged swing phase. The fact that the APA was included at all may indicate a hybrid control, in which predictive control (via APA) is used to reduce the anticipated lateral instability, and reactive control of the final foot placement, governed by sensory discharge related to the actual COM motion, provides any additional stabilization required.