Mechanical power flow changes during multijoint movement acquisition.

We investigated the differences in mechanical power flow in early and late practice stages during a cyclic movement consisting of upper arm circumduction to clarify the change in mechanical energy use with skill acquisition. Seven participants practiced the task every other day until their joint angular movements conformed to those of an expert. During the practice sessions, participants' motions were digitally recorded once a week using four high-speed infrared cameras, and the joint kinematics and joint powers of the right arm were calculated. With practice, the inflow power derived from the net joint force increased at the hand, forearm, and upper arm segments by 143.1 +/- 17.2%, 57.1 +/- 7.3%, and 198.1 +/- 35.4%, respectively. In contrast, the power caused by the muscle joint moments was not significantly increased. These results suggested that participants acquired a motor pattern promoting transfer of the joint reaction forces. Results may provide some support for Bernstein's (1967) ideas that skill acquisition involves improving movement efficiency by greater use of nonmuscular forces.

Practice changes the usage of moment components in executing a multijoint task.

We examined changes in the usage of muscular and motion-dependent moments during the long-term practice of a complex, multijoint movement. Seven participants practiced a cyclic movement of the upper limbs until their joint angular movements conformed to those of an expert. The motions of the participants were digitally recorded using four high-speed infrared cameras, and the joint kinematics and kinetics of the right arm were calculated. Practice brought about changes in the patterns of the net joint moment and in the contributions of the muscular and motion-dependent moments to the net moments. Practice also brought about a growing opposition between the directions of the two moments. These changes seemed to be important for improving the dynamic equilibrium of the movements.