CHANGES IN THE SYMMETRY OF RAPID MOVEMENTS - EFFECTS OF VELOCITY AND VISCOSITY

Five subjects made rapid, discrete elbow flexion movements over differ ent distances, against different inertial loads, as well as under dist ance and load combinations that kept movement time constant. The resul ts demonstrated that an increase in peak movement velocity was associa ted with an increase in the temporal symmetry ratio of the movement (a cceleration time divided by deceleration time), as well as with an inc rease in both agonist electromyographic (EMG) burst duration and antag onist EMG latency. Since an increase in peak movement velocity is asso ciated with faster agonist muscle shortening, as well as with faster s tretching of the antagonist muscle, we hypothesize that the velocity-r elated changes in movement symmetry can be viewed as, at least partial ly, a consequence of muscle viscosity. Viscosity increasingly resists the shortening agonist and assists the lengthening antagonist when mov ement velocity increases. Therefore, the agonist muscles require more time to produce the required impulse, while the antagonist muscle can brake the movement in a shorter period of time. In order to test the h ypothesis that viscosity is responsible for the velocity associated ch anges in the symmetry ratio, we performed a second experiment with dis tance and load combinations identical to those of the first experiment , but with different external viscous loads, which resisted the slower and assisted the faster movements. The results demonstrated that the movements became more symmetrical in the presence of the viscous load. There were also changes in agonist duration and antagonist latency. W e conclude that changes in the symmetry associated with changes in mov ement velocity may be due to the effects of either muscle viscosity or changes in how muscles are activated to account for differences in vi scous force.