[Study in predictive posture control during the performance of actions]

In order to maintain an ordinary upright posture, the body's center of gravity must be predicted and controlled on the supporting surface based on synthesis of multiple-sensory inputs. In order to evaluate the accuracy of predictive (feedforward) posture control during the performance of actions, we measured maximal shifts in the body's center of gravity using a force platform in 6 normal adults under two visual conditions (with eyes open and closed) and four standing conditions (one-legged; heel to toe posture; and both legs together or apart). Subjects were asked to perform five kinds of voluntary actions (static standing; head tilting forward, backward and laterally; head turning; bending forward; and maximal inclinations to the anterior, posterior and lateral directions). The smaller the supporting surface, the more markedly displacement of the body's center of gravity was increased upon closing the eyes. This finding suggests that the accuracy of predictive posture control decreases as a result of the reduction in the number of multisensory inputs which determine spatial orientation. Assessing purposive shifts in center of gravity during voluntary actions, we can conclude that the body's center of gravity is accurately controlled even in the presence of head movements under ordinary standing conditions. To evaluate balance during the performance of routine actions, we must devise a better method of discriminating purposive shifts in center of gravity from control disorder errors as well as a method of quantifying the accuracy of feedforward regulation.