Trunk motion reflects center-of-pressure dynamics and supports markerless single-camera postural sway assessment

Abstract

Postural sway provides important insight into balance control. Although center of pressure (CoP) is widely used to quantify postural sway, reliance on dedicated force-measurement systems can limit use beyond specialized settings. This has motivated the search for accessible kinematic measures of CoP-related sway. However, the relative correspondence of head and trunk motion with CoP remains unclear. We compared head and trunk motion with Wii Balance Board-derived CoP trajectories during quiet standing under six optic-flow conditions. In the anteroposterior direction, trunk motion showed stronger linear correspondence with CoP than head motion in both time- and frequency-domain analyses, whereas correspondence was comparable in the mediolateral direction. Trunk-derived positional sway measures showed consistently greater concordance with CoP-derived measures than the corresponding head-derived measures. We then evaluated trunk tracking using a single elevated rear-oblique camera and Lucas–Kanade optical flow. Video-derived anteroposterior trunk trajectories closely corresponded with motion-capture-derived trajectories, whereas mediolateral correspondence was weaker and more variable. A separate three-participant proof-of-concept further demonstrated simultaneous tracking within a shared camera view. Together, these findings identify trunk motion as a representative upper-body kinematic measure of CoP-related sway, particularly in the anteroposterior direction, and support the feasibility of infrastructure-light, markerless, single-camera tracking.