Asymmetrical trajectory formation in cyclic forearm movements in man.

Predictions of the minimum-jerk model for a human cyclic motion were given in terms of asymmetry in movement trajectories. A detailed kinematic analysis of cyclic forearm motion, i.e., extension/flexion movements around the elbow joint in a horizontal plane ranging in frequency from 2-5.5 Hz, was made to examine the validity of the predictions. The kinematics of the trajectories were described in terms of deviation from symmetry in velocity and acceleration profiles, and jerk cost. The asymmetry could be accounted for by the solution of the minimum-jerk model using the boundary condition differences between extension and flexion during a movement cycle. The trajectory was asymmetrical at relatively low frequencies, and symmetrical at higher frequencies; the frequency boundary from asymmetrical to symmetrical trajectories differed among subjects with a range of 3-4.3 Hz. It was suggested for the asymmetrical trajectory formation that consecutive extension and flexion in a cycle could be processed as a unit in which speed and acceleration in each direction were differentiated. The shift from asymmetrical to symmetrical trajectories with increasing frequency was accompanied by a reduction in jerk cost and mechanical energy. The oscillators underpinning the high frequency movements were mainly non-linear. The results suggested a shift of control from the "rhythmic" sequencing of extension and flexion which resulted in trajectory asymmetry, to non-linear oscillation with no directional difference.