Biomechanical analysis of human arm stabilization during force production.

The activity of muscles of the human arm can produce force against an external system which may be interpreted as a directional force and a stabilizing force. This paper investigated the relationship between muscle activity in the human arm during both maximal and submaximal external force development against a handle. Three different handle arrangements were studied; fixed, free to move in the horizontal direction and free to move in the vertical direction. Ten subjects were utilized. Analyses of angle, force and electromyographic data revealed that the increased muscle activity at the wrist when compared with action against the movable external system was a function of the stabilizing force and not of directional force production. It is suggested that the extensor muscles of the wrist are the initiators of the stabilizing force as well as the major stabilizers of the wrist joint during the resultant force production against the handle.