PARADOXICAL EFFECTS OF PRACTICE OF FAST SINGLE-JOINT MOVEMENTS

We studied the effects of extensive practice of fast, unidirectional, single-joint elbow flexions against a small extending torque bias upon the kinematic and electromyographic (EMG) characteristics of the move ments as well as upon the reconstructed hypothetical control patterns (equilibrium trajectories). The subjects were tested at different dist ances, both with and without the bias torque prior to and after the pr actice sessions. The basic finding was paradoxical: The subjects did n ot improve their performance at the practiced task (against the bias) and at other distances in the same condition; however, they showed an increase in movement speed and a decrease in movement time al all dist ances in unpracticed conditions (without the bias). Changes in the EMG patterns were similar in both conditions. We hypothesize that the pri nciple of learning the dynamics of interaction with the experimental s etup in combination with a very steep learning curve form the basis fo r the observed paradoxical effects of practice. The equilibrium-point hypothesis of movement control provides the least controversial descri ption of these effects as compared to the force-control and EMG-contro l approaches.