Balance-Keeping Control of a Human Being Upon a Sudden External Perturbation.

This study aims to estimate the control law employed by the CNS (Central Nervous System) 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 conjecture that the CNS commands the muscular actuators of the human joints according to a PD control law but it updates the control gains and the set points continuously. This conjecture is accompanied with a major assumption that the CNS is able to acquire perfect and instantaneous position and velocity feedback by means of an appropriate fusion of the signals coming from the proprioceptive, somatosensory, vestibular, and visual sensory 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, it is concluded that the CNS indeed uses an adaptive PD control law as conjectured here. 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, the decisions of the CNS on the variations of the control parameters are hardly predictable.