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) were crossed with three amplitudes of target motion. The relative phase φ_rel between angular motion of ankles and hips was analyzed. Two preferred patterns emerged; close to in-phase (φ_rel ≈ 0°), and close to anti-phase (φ_rel ≈ 180°). On the solid surface increasing target amplitude produced a change from in-phase to anti-phase coordination. There were no amplitude-related changes in hip-ankle relative phase on the rollers where only in-phase coordination was observed, or the foam (only anti-phase coordination). We conclude that (1) hip-ankle relative phase is useful for describing postural coordination, (2) 0° and 180° are two spontaneous coordination modes in the hip-ankle postural space, and (3) these modes emerge differentially under the mutual pressures of task and support surface properties.