Velocity-dependent activation of postural muscles in a simple two-joint synergy

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A simple two-joint synergy was studied over a range of movement velocities. We hypothesized that focal and postural components of the synergy are consequences of a single control process and, as such, will demonstrate similar scaling with movement velocity. Healthy subjects performed discrete elbow or wrist flexion or extension movements in a sagittal plane under the instruction to move one of the joints at different speed in different trials of a series. Joint angles and electromyographic (EMG) signals from two flexor and two extensor muscles were recorded and analyzed. Irrespective whether the focal movement took place in the elbow or in the wrist joint, and irrespective of the movement direction 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 flexion (extension) movements in either joint, the latencies of both elbow and wrist extensors (flexors) decreased with velocity of the focal movement. Integrals of the EMG bursts in all the muscles increased with movement speed in all the series. Typically, there was a close to linear relation between the integral EMG indices for the elbow and wrist flexors as well as for the elbow and wrist extensors. We conclude that there exists a simple, scalable synergy which is used by the central nervous system in a wide range of movement velocities to simplify control of the postural component of a motor task.