Antiphase Trunk Motion Reduces Sway Velocity and Ankle Torque During Quiet Stance

The purpose of this study was to determine the role of antiphase trunk motion during quiet stance while maintaining constant visual and support surface conditions. Eyes-open quiet stance trials were performed on a firm support surface while wearing a rigid hip-knee orthotic brace that reduced antiphase trunk motion. Amplitude spectral density, coherence, and cophase were compared for hip-locked, hip-unlocked, and no-brace conditions. Amplitude spectral density calculations showed that trunk and leg sway velocities, and ankle torque (AT) decreased when antiphase trunk sway was allowed. Coherence and cophase estimates identified in-phase trunk-legs sway below 1 Hz and antiphase at higher frequencies. Legs-AT cophase calculations showed that the legs lagged the application of AT at all frequencies, while trunk-AT cophase showed the trunk lagged AT below 1 Hz and led AT at higher frequencies. The results demonstrate that antiphase trunk sway helps reduce sway velocity and AT. Furthermore, the trunk-leading cophase relationship with AT showed that antiphase trunk motion occurred before AT was applied. This implies that antiphase trunk motion facilitates changes in sway direction and helps regulate sway velocity. The results have significant implications for predicting postural control deficiencies due to injury, disease, and aging.