Sensory integration for human balance control.

The upright stance position is inherently unstable since the smallest deviation from a perfect upright orientation produces forces due to gravity that accelerate the body toward the ground. Stability is achieved by generating appropriate joint torques that correct for deviations from a desired orientation with orientation changes detected by sensory systems (primarily somatosensory/proprioceptive, visual, and vestibular systems). Functionally, balance control can be viewed as a closed-loop feedback control system with the integration of different sources of sensory orientation information being one component of the overall system, but with the system's feedback nature placing constraints on the sensory integration process. Analysis of body sway evoked by balance perturbations allows for the measurement of "sensory weights" that represent the relative contributions of different sensory systems to an internal estimate of orientation that, in turn, is used to generate corrective actions. Experiments reveal that sensory weights are not fixed quantities, but vary as a function of environmental and experimental conditions as well as neurologic disorders that affect the quality of sensory information available from different sensory systems. Because environmental conditions can change rapidly, sensory reweighting must also occur rapidly enough to prevent instability due to an under- or overproduction of corrective action.