Interaction between task demands and surface properties in the control of goal-oriented stance

Standing subjects were asked to track the fore-aft motion of a target with their heads. Three support surface conditions (standard, foam, rollers) wer e crossed with three amplitudes of target motion. The relative phase phi(re l) between angular motion of ankles and hips was analyzed. Two preferred pa tterns emerged; dose to in-phase (phi(rel) approximate to 0 degrees), and c lose to anti-phase (phi(rel) approximate to 180 degrees). On the solid surf ace increasing target amplitude produced a change from in-phase to anti-pha se coordination. There were no amplitude-related changes in hip-ankle relat ive phase on the rollers where only in-phase coordination was observed, or the foam (only anti-phase coordination). We conclude that (1) hip-ankle rel ative phase is useful for describing postural coordination, (2) 0 degrees a nd 180 degrees are two spontaneous coordination modes in the hip-ankle post ural space, and (3) these modes emerge differentially under the mutual pres sures of task and support surface properties. (C) 1999 Elsevier Science B.V . All rights reserved.