CHANGES IN EARLY AUTOMATIC POSTURAL RESPONSES ASSOCIATED WITH THE PRIOR-PLANNING AND EXECUTION OF A COMPENSATORY STEP

There is growing interest in the perturbation-evoked stepping response as an important element in the movement repertoire to maintain upright stance. It is possible that there is an important relationship between the well-documented, early 'automatic' postural responses and subsequent stepping responses. In this study, we characterize changes in early 'automatic' postural responses associated with the prior-planning and execution of a compensatory (i.e. stabilizing) stepping response. Seven subjects were tested on a 'moveable' platform which could translate in the anterior-posterior direction. The subjects' responses were analyzed for two different tasks: (1) 'constrained' (keep feet in place), and (2) perturbation-cued 'reaction time' stepping (step when the platform moves). Only responses to forward platform translations were analyzed in this study. Responses for each of these tasks were evoked using two different perturbation magnitudes. The higher magnitude was sufficiently large to evoke stepping responses in a large proportion of the 'constrained' trials despite instructions not to step. We compared the magnitude of the early evoked postural reactions between trials when subjects had pre-planned a stepping response against trials characterized by either (1) no stepping or (2) unplanned stepping responses. The results revealed that the early 'automatic' responses in tibialis anterior were always present in all subjects and all tasks. However, the evoked 'automatic' responses were approximately 36% smaller (over the first 50 ms) when followed by a pre-planned stepping response ('reaction-time' task). This task-related difference was similar for both perturbation magnitudes i.e. it occurred regardless of whether the 'constrained' tasks involved feet-in-place responses (small perturbations) or stepping responses (large perturbations). The changes seen in muscle activation were paralleled by change in centre-of-pressure displacement. The results highlight the automatic nature of the early postural responses, demonstrating that these early postural responses persist even when another compensatory postural response (stepping) is pre-planned. The results do, however, show that the early responses can be modulated, and the attenuation of these responses that occurred when stepping was pre-planned may reflect centrally mediated changes in gain occurring prior to the onset of perturbation.