Experimental quantification of balance using whole-body stability regions from postural sway exercises

Background Comprehensive balance assessment is essential for evaluating balance deficiencies, particularly in individuals with limited mobility and increased fall risk. However, clinical balance assessments often rely on subjective scoring and simplified models that may not capture the full dynamics of human postural control. Additionally, center of pressure (COP)-based metrics and inverted pendulum models offer limited insight into whole-body balance ability. Balance regions (BRs) offer a more holistic approach by quantifying balanced center of mass (COM) states. Previously, BRs were used to assess the balancing capabilities of bipedal systems and humans in simulation; however, there is a lack of real-world implementations of whole-body BR analyses in human balance. Methods This study presents a novel experimental framework to quantify human balance using COM-based BRs derived from large postural sway tasks. The subjects comprised 22 healthy young adults who performed voluntary, supported, and perturbed anterior–posterior (AP) sway exercises while standing on force plates, with a full-body motion capture setup. COM trajectories were obtained from individualized musculoskeletal models in OpenSim to construct BRs, which were compared with linear inverted pendulum (LIP) limits. Key COM-based metrics included maximum AP margin of stability (MoS), maximum AP velocity, extrapolated COM (XcoM) range, and BR areas. COP measures (root mean square (RMS), range, 95% confidence ellipse area, mean AP velocity) and joint kinematics were also analyzed. Results COM trajectories largely stayed within the LIP-based analytical boundaries, with BRs capturing individualized balance envelopes. Significant sex differences were observed in the maximum posterior MoS ( p  = 0.01974), XcoM range metrics ( p  < 0.03924), and BR areas ( p  < 0.045). COP metrics varied significantly across tasks in the RMS, 95% confidence ellipse area, and mean velocity ( p  < 0.02046), and inverse kinematics revealed distinct joint coordination patterns during sway; there was a significant sex difference in the COP range during perturbed backward sway ( p  = 0.03053). Conclusions This study presents a novel, COM-based experimental framework for balance assessment that captures whole-body stability regions from dynamic postural tasks. BRs provide a quantitative and individualized measure of balance capacity beyond traditional metrics. This framework has potential for fall risk evaluation, balance training, and integration with wearable or markerless motion capture in clinical and neurorehabilitation settings.