Upright stance is controlled close to critical stability across different postures

Feedback stabilization of upright standing postures 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 if we could identify the feedback parameters. Next, we tested this model in 14 young adult volunteers standing in different postures.

Our model effectively reconstructed F _corr in both simulations (R ² : 0.50∼0.99) and human experimental data (R ² : 0.76∼0.99). Strikingly, estimated position and velocity gains were similar across postures even between bipedal and unipedal standing, with the position gain slightly above critical stiffness. The estimated delay in normal standing (246±21ms) was significantly larger than that in unipedal standing and step posture (204±24ms and 193±29ms, respectively).

In conclusion, our results support that an estimate of the COM state is used to generate corrective ground reaction forces to stabilize upright posture, and our feedback model can be used to characterize this control mechanism. The proposed model requires only easily measurable inputs which may yield value in assessment of balance disorders. Notably, our results showed that stability of upright stance is achieved by maintaining feedback gains at levels just above critical stability.