Optimized movement trajectories and joint stiffness in unperturbed, inertially loaded movements

An attempt is made to integrate theoretically the mechanical, electromyographic, and psychophysical lines of inquiry into the control of movement by investigating the significance of joint stiffness in the reduction of effort. Attention is focused on single-joint, unperturbed movements of specified duration performed from one specified position to another in the presence of an inertial load. A theoretical measure of the sense of effort is formulated in the light of psychophysical observations and mechanical considerations. This measure is such that it is increased by reciprocal changes in the central drives to opposing sets of muscles, as well as by enhancement of joint stiffness. Mathematical analysis of the interplay of these factors reveals that, in any given condition, the minimization of this measure of effort necessitates a particular value of joint stiffness and a particular trajectory of movement. The predicted stiffness and trajectory are shown to be in quantitative agreement with available observations. In addition, the conditions in which a higher value of stiffness is predicted to be advantageous for reducing the effort are shown to be the conditions that are known to promote greater coactivation of the agonist and antagonist muscles. It is concluded that the seemingly wasteful coactivation may serve to optimize the stiffness. The stiffness, therefore, need not be viewed simply as a means of resisting imposed perturbations, but as a means of reducing the alterations in the central drives necessary for the performance of movement, thereby reducing the effort.