Feedback control stabilizing the center of mass can be identified in unperturbed, upright standing

Feedback stabilization of upright standing should be reflected by a time-lagged relationship between the ground reaction force (GRF) and the center of mass (COM) state. In this study, we propose a model relating corrective ground reaction forces (F_corr) to preceding COM position (PCOM) and velocity (VCOM). We first checked the model's validity by simulating an inverted pendulum model with known intrinsic and feedback parameters, to see whether and to what extend we could effectively identify the feedback parameters. Next, we tested the model in 15 young adult volunteers in standing. Our model effectively reconstructed F_corr in both simulations (R²: 0.77-0.99) and human experimental data (R²: 0.92-0.98). The effective delay in the mediolateral direction (239 ± 20 ms) was significantly shorter than in the anteroposterior direction (271 ± 28 ms). Additionally, position gains were significantly larger in the mediolateral compared to anteroposterior direction, with values of -1.09 ± 0.04 and - 1.03 ± 0.02 times critical stiffness, respectively. No significant differences between the directions were found in velocity gains. Our model can be used to identify feedback control in human standing without applying external perturbations or measuring physiology activity. Although the estimated parameters represent the lumped effects of both intrinsic and feedback contributions, they remain informative of the feedback mechanism because the intrinsic impact is negligible under normal standing. Our results show that stability of upright stance in healthy young adults is achieved by position feedback gains at levels just above critical stability. The proposed model requires easily measurable inputs which may yield value in assessment of balance disorders.