Controlling balance during quiet standing: proportional and derivative controller generates preceding motor command to body sway position observed in experiments.

To compensate for significant time delays in the control of human bipedal stance, it was suggested that a feed-forward control mechanism is needed to generate a preceding motor command to the body sway position observed in quiet standing. In this article, we present evidence that a feedback proportional-derivative (PD) controller can effectively generate a desired preceding motor command. We also discuss the following characteristics of the proposed PD controller: (1) the level of robustness of the controller with respect to neurological time delays and (2) how well the controller replicates the system's dynamics observed in experiments with able bodied subjects, i.e. how well the controller generates the observed preceding motor command. Human quiet stance was simulated using an inverted pendulum model regulated by a PD controller. The simulations were used to calculate the center of mass (COM) position and velocity data, and the motor command (ankle joint torque) data as a function of time. These data and the data obtained in the experiments were compared using cross-correlation functions (CCFs). The results presented herein imply that a PD feedback controller is capable of ensuring balance during human bipedal quiet stance, even if the neurological time delays are considerable. The proposed feedback controller can generate the preceding motor command that was observed in the experiments. Therefore, we conclude that a feed-forward mechanism is not necessary to compensate for the long closed-loop time delay in human bipedal stance as suggested in recent literature, and that the PD controller is a good approximation of the control strategy applied by able bodied subjects during quiet stance.