VELOCITY-DEPENDENT ACTIVATION OF POSTURAL MUSCLES IN A SIMPLE 2-JOINTSYNERGY

A simple two-joint synergy was studied over a range of movement veloci ties. We hypothesized that focal and postural components of the synerg y are consequences of a single control process and, as such, will demo nstrate similar scaling with movement velocity. Healthy subjects perfo rmed discrete elbow or wrist flexion or extension movements in a sagit tal plane under the instruction to move one of the joints at different speed in different trials of a series. Joint angles and electromyogra phic (EMG) signals from two flexor and two extensor muscles were recor ded and analyzed. Irrespective whether the focal movement took place i n the elbow or in the wrist joint, and irrespective of the movement di rection and velocity, the elbow flexor and the wrist flexor tended to demonstrate simultaneous EMG bursts, while the elbow extensor and the wrist extensor also showed similar patterns of activation. During flex ion (extension) movements in either joint, the latencies of both elbow and wrist extensors (flexors) decreased with velocity of the focal mo vement. Integrals of the EMG bursts in all the muscles increased with movement speed in all the series. Typically, there was a close to line ar relation between the integral EMG indices for the elbow and wrist f lexors as well as for the elbow and wrist extensors. We conclude that there exists a simple, scalable synergy which is used by the central n ervous system in a wide range of movement velocities to simplify contr ol of the postural component of a motor task.