DAMPING OF THE WRIST JOINT DURING VOLUNTARY MOVEMENT

Damping characteristics of the musculoskeletal system were investigate d during rapid voluntary wrist flexion movements. Oscillations about t he final position were induced by introducing a load with the characte ristics of negative damping, which artificially reduced the damping of the wrist. Subjects responded to increases in the negatively damped l oad by stronger cocontraction of wrist flexor and extensor muscles dur ing the stabilization phase of the movement. However, their ability to counteract the effects of the negatively damped load diminished as th e negative damping increased. Consequently, the number and frequency o f oscillations increased. The oscillations were accompanied by phase-l ocked muscle activity superimposed on underlying tonic muscle activati on. The wrist stiffness and damping coefficient increased with the inc reased cocontraction that accompanied more negatively damped loads, al though changes in the damping coefficient were less systematic than th e stiffness. Analysis of successive half-cycles of the oscillation rev ealed that the wrist stiffness and damping coefficient increased, desp ite decreasing muscle activation, as oscillation amplitude and velocit y declined. This indicates that the inverse dependence of the damping coefficient on oscillation velocity contributes significantly to dampi ng of joint motion. It is suggested that this property helps to offset a negative contribution to damping from the stretch reflex.