THE EFFECT OF MUSCLE MECHANICS ON HUMAN MOVEMENT OUTCOMES AS REVEALED BY COMPUTER-SIMULATION

Computer simulations were performed using a model of the human elbow joint as controlled by a single equivalent flexor muscle. The mechanical output of the muscle was determined from a user-specified neural drive according to the force-time, force-length, and force-velocity relations. The model allowed no storage of mechanical energy and no potentiation via the stretch reflex. The effects of different activation patterns as well as initial kinematic conditions on the maximum final velocity were examined. The results revealed a very nonlinear dependence of final velocity on both initial joint angle and angular velocity. Contrary to the principles of particle physics, it could be shown that an activation pattern that reached a maximum early in the movement achieved shorter movement times and, quite often, higher final velocities than an activation pattern that reached a maximum later in the movement. It was also found that by taking advantage of the nonlinear force-time, force-length and force-velocity relations, higher final velocities could be achieved if the muscle contracted from a previously stretch state with the absence of stored elastic energy. It was also found that with the same neural drive, the final velocities that could be achieved when the muscle was first required to absorb large amounts of energy in an eccentric contraction were similar to the final velocities that could be achieved when no negative work needed to be performed.