Five subjects made rapid, discrete elbow flexion movements over different distances, against different inertial loads, as well as under distance and load combinations that kept movement time constant. The results demonstrated that an increase in peak movement velocity was associated with an increase in the temporal symmetry ratio of the movement (acceleration time divided by deceleration time), as well as with an increase in both agonist electromyographic (EMG) burst duration and antagonist EMG latency. Since an increase in peak movement velocity is associated with faster agonist muscle shortening, as well as with faster stretching of the antagonist muscle, we hypothesize that the velocity-related changes in movement symmetry can be viewed as, at least partially, a consequence of muscle viscosity. Viscosity increasingly resists the shortening agonist and assists the lengthening antagonist when movement 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 hypothesis that viscosity is responsible for the velocity-associated changes in the symmetry ratio, we performed a second experiment with distance 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. We conclude that changes in the symmetry associated with changes in movement velocity may be due to the effects of either muscle viscosity or changes in how muscles are activated to account for differences in viscous force.
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