Slow dynamics of human balance control.

When standing on a tilting surface, humans' sway behavior at frequencies below 0.1 Hz indicates the contribution of a slow feedback component. We suggest this may reflect a self-calibration mechanism of the balance control system, constantly referencing orientation estimates based on kinematic sensory cues to a reference based on force cues. However, attempts to identify this mechanism have been limited by insufficient experimental trial durations and small sample sizes. This study aimed to assess the properties of the mechanism that reduces body sway at very low frequencies in upright standing. Anteroposterior body sway responses to short- and long-duration surface tilts were measured and interpreted using balance control models. Four feedback control model variants, with different mechanisms to account for the slow dynamics, were fit to experimental data. Furthermore, we tested how estimates of the slow component are affected by stimulus period duration. We hypothesized that the model variants containing force cues would provide the best fit to experimental sway responses, particularly in response to long-duration surface tilts. Our results confirm this hypothesis and suggest that humans use integrated force afferents from the feet and legs in a slow, positive feedback mechanism during standing to remain upright. Despite stimulus period durations of ~ 180 s, some properties of this mechanism were difficult to estimate. The positive torque feedback mechanism aligns with the notion of self-calibration.