The hypothesis that strength of active muscles affects the symmetry of the velocity profiles of voluntary movements was tested. In particular, it was assumed that the duration of acceleration and deceleration phases reflects the ability of the antagonistic muscles to exert torque in such a way that stronger muscle requires less time for action. Twelve subjects performed consecutive 50° flexions and extensions in blocks of either discrete or oscillatory movements. They were tested under high and moderate speed conditions, as well as within different ranges of elbow joint angles. The symmetry ratio (SR; acceleration lime divided by deceleration time) was calculated in order to assess movement symmetry. The results demonstrated SR > 1 under most of the discrete and, particularly, oscillatory movement conditions. A velocity-associated increase in SR was recorded, while different ranges of elbow movements, assumed to provide different torques of the agonist and antagonist muscles, also provided different SR. The findings were generally in line with the predicted effects of movement conditions on muscle strength, particularly those related to elbow angle and elbow angular velocity. Deviations from me ideal movement symmetricity have usually been interpreted as either weakness of various motor control models and hypotheses, or as a sub-optimal control of movements in certain subject populations; the present study suggests an alternative interpretation based upon the ability of active muscles to exert torque.
Dragan M. Mirkov, Sladjan Milanovic, Dusko B. Ilie, and Slobodan Jaric
Dusko B. Ilic, Dragan M. Mirkov, and Slobodan Jaric
Nine subjects (experimental group) were tested on rapid elbow flexion and extension movements performed in the same final position, before and after extensive practice of the movements. Nine additional subjects (control group) were also tested, but without any practice between the tests. Comparison of the pretest and posttest results suggested that the experimental group decreased their variable error (i.e., standard deviation of the final movement position) in both practiced (elbow flexion) and nonpracticed (elbow extension) movements. The control group, however, did not improve in either of tested movements. The experimental group demonstrated lower variable error in the nonpracticed elbow extensions than the control group, while the same difference for practiced elbow flexion movements was slightly below the level of significance. The results support the importance of the final position in programming of rapid, self-terminated movements; however, they do not rule out the role of other kinetic and kinematic variables (such as movement distance).