Abstract

Postural sway provides important insight into balance control. Although center of pressure (CoP) measured by force plates is the standard metric, its reliance on specialized equipment limits broader use beyond laboratory settings. This limitation has motivated the search for alternative, more accessible measures of postural sway. Head and trunk kinematics have therefore been proposed as practical surrogates, but their relative validity within controlled perturbation settings has not been systematically established. Here, we evaluated head and trunk motion against CoP trajectories during quiet standing under six optic-flow perturbations. Trunk motion showed stronger coupling with CoP than head motion, particularly along the anterior-posterior axis, across both time- and frequency-domain analyses. Feature-level comparisons further demonstrated closer agreement between trunk-derived and CoP-derived sway metrics. We further evaluated a minimal markerless approach using a single overhead CCTV camera and a Lucas-Kanade optical flow algorithm. Video-derived anterior-posterior trunk trajectories closely matched those obtained from motion capture, whereas medio-lateral estimates were less stable. Together, these findings establish trunk motion as a robust surrogate for CoP and demonstrate that clinically relevant postural sway can be captured using low-cost, non-contact video sensing compatible with existing infrastructure.