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Estimation of the Balance-Keeping Control Law Applied by a Human Being Upon a Sudden Sagittal Tilt Perturbation

This study aims to estimate the control law employed by the central nervous system (CNS) to keep a person in balance after a sudden disturbance. For this aim, several experiments were carried out, in which the subjects were perturbed sagittally by using a single-axis tilt-platform and their motions were recorded with appropriate sensors. The analysis of the experimental results leads to the contribution of this paper as a conjecture that the CNS commands the muscular actuators of the joints according to an adaptive proportional-derivative (PD) control law such that its gains and set points are updated continuously. This conjecture is accompanied with an assumption that the CNS is able to acquire perfect and almost instantaneous position and velocity feedback by means of a fusion of the signals coming from the proprioceptive, somatosensory, and vestibular systems. In order to verify the conjectured control law, an approximate biomechanical model was developed and several simulations were carried out to imitate the experimentally observed motions. The time variations of the set points and the control gains were estimated out of the experimental data. The simulated motions were observed to be considerably close to the experimental motions. Thus, the conjectured control law is validated. However, the experiments also indicate that the mentioned adaptation scheme is quite variable even for the same subject tested repeatedly with the same perturbation. In other words, this experimental study also leads to the implication that the way the CNS updates the control parameters is not quite predictable.