Identification of Standing Balance System Considering Center of Mass Control for Support Surface Sway

One approach for developing simulation models of human standing or for evaluating sensory functions and the central nervous system is to identify mathematical models by applying external perturbations to standing subjects and measuring their responses. However, a standardized approach has not yet been established. This requires a simplified model that captures the dominant dynamics. This study aimed to identify individual balance systems by focusing on the control of the center of mass (COM) in the low-frequency range below 0.7 Hz, under horizontal perturbations applied to the support surface. We modeled the human body as a single inverted pendulum and proposed a delayed-state feedback control system that accounts for shifts of the COM equilibrium position depending on the support surface velocity. Furthermore, we introduced a practical COM estimation method using measurements of ground reaction forces and support surface movement without optical motion capture systems. Twenty healthy young adults participated in the experiment over three consecutive days, and stable models were successfully identified for all subjects. The intraclass correlation coefficient for the identified models exceeded 0.5 across two consecutive days, indicating moderate reproducibility. These findings suggest that the proposed method has the potential to be a practical tool for evaluating balance function.