Résumé
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 between angular motion of ankles and hips was analyzed. Two preferred patterns emerged; close to in-phase (phase relative ≈ 0°), and close to anti-phase (phase relative ≈ 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 1) that hip-ankle relative phase is useful for describing postural coordination, 2) that 0° and 180° are two spontaneous coordination modes in the hip-ankle postural space, and 3) that these modes emerge deferentially under the mutual pressures of task and support surface properties