Analysis of Stance Posture in Humans

The generation of a purposeful movement requires that the sensorimotor system regulate and control three kinds of activities: (1) the basic patterns of the movement must be generated in each limb and the patterns of activities among the limbs must be coordinated, (2) the muscular activities generating the basic movement pattern must be adaptively modified to suit the external loading and sensory conditions of the particular task, and (3) the movement patterns must be adjusted to maintain the equilibrium of the body as a whole. The control of upright stance by normal human subjects is one specific example of such a sensorimotor process. Understanding upright posture is made difficult because most of what is known about sensorimotor control has been learned from studies of acute and chronically prepared experimental animals. Apart from the immediate interest in understanding more about ourselves, one might be initially inclined to question how a study in which the experimental paradigms and measurement techniques are severely limited can contribute significantly to the detailed physiological information about the generation and coordination of movements. One aim of this chapter is to show that studies of the performance of awake human subjects are answering important questions about the adaptive and equilibrium controls of the sensorimotor system that have not been addressed in experiments using animal preparations.