Some Aspects of Sensory-Motor Control and Adaptation in Man

Abstract

The control of human movement and spatial orientation is notably complex. The body is multiply articulate and as the trunk and limbs change their orientation with respect to the gravitational-force vector, the forces necessary to move a limb through a given angle also change. Moving the forearm back and forth through the same angular distances in a vertical and then in a horizontal plane represents a simple example of this. Even though the motor commands necessary to bring about the “same movement” in the two cases differ because of the load and orientation changes, one nevertheless, unless fatigued, experiences little or no difference in the effort required to bring about the movements. This means that the skeletomuscular system is “calibrated” such that body movements of a given extent are perceived as equivalent in terms of apparent force despite often radical differences in the actual forces involved. Such a calibration is possible only through a continual monitoring of the relative configuration and orientation of the body in relation to the substrate of support and the gravitoinertial-force vector. Actually, as Mach (1897) pointed out long ago, the eyes are really the only movable parts of the body that can be controlled by means of innervation sequences that bring about the same movements regardless of body orientation.¹