COMPENSATION FOR MECHANICALLY UNSTABLE LOADING IN VOLUNTARY WRIST MOVEMENT

In order to study the roles of muscle mechanics and reflex feedback in stabilizing movement, experiments were conducted in which healthy hum an subjects performed targeted wrist movements under conditions where the damping of the wrist was reduced with a load having the property o f negative viscosity. If the movement speed and negative viscosity wer e sufficiently high, the wrist oscillated for several hundred millisec onds about the final target position. Subjects increased the activatio n of both wrist flexor and extensor muscles to increase joint stiffnes s to damp the oscillations. With practice, both the tendency to oscill ate and the level of muscle activation were reduced. A small bias torq ue in either direction, added to the negative viscosity, enhanced the oscillations as well as the amount of flexor and extensor muscle activ ation during the stabilization phase of fast movements. The tendency f or the wrist to oscillate was also seen during slow movements where th e oscillations were superimposed upon the voluntary movement. We sugge st that this reduction in mechanical stability is primarily of reflex origin. As wrist stiffness increases, the natural frequency of the wri st also increases, which in turn produces an increase in the phase lag of the torque generated by the myotatic reflex with respect to wrist angular velocity, effectively reducing damping. The oscillation freque ncy was often close to a critical frequency for stability at which tor que, due to the myotatic reflex, lagged angular velocity by 180-degree s (6-7.5 Hz). Nevertheless, subjects were able to damp these oscillati ons, probably because the torque due to intrinsic muscle stiffness (in phase with position and hence lagging velocity by only 90-degrees) do minated the torque contribution of the myotatic reflex. Increasing sti ffness with declining oscillation amplitude may also have contributed significantly to damping.