Identification of human balance control in standing

The goal was to identify the contribution of intrinsic mechanical properties of the muscular-skeletal system and the reflex gains in controlling balance during standing. The combination of balance perturbations experiments and closed loop identification schemes made it possible to identify the reflex loop gains and intrinsic mechanical properties in various environmental conditions. Human balance responses were studied by placing subjects on a movable support base while keeping their eyes open or closed. EMG, body motion and the ground reaction forces were recorded. From the platform perturbation data the frequency response functions of the controller dynamics and the admittance of the balance control system were estimated with a non-parametric closed loop identification technique. Using a parametric model of balance control and a fitting procedure the sum of the intrinsic stiffness and neural position feedback gain, the neural time delay, the neural velocity feedback gain and the intrinsic damping were uniquely identified. The results show that subjects balancing on a randomly moving platform in the forward-backward direction applied a minimal stiffness strategy. The required damping to avoid oscillations was mainly due to neural velocity feedback rather than to intrinsic damping.