Muscle activity for initiation of planar, two-joint arm movements in different directions.

We studied planar, point-to-point arm movements to various target positions, starting from a fixed initial position. The movements involved coordinated rotations about the shoulder and elbow joints. One of our aims was to determine for each joint whether flexion was always initiated by flexor muscle activity and, similarly, whether extension was always initiated by extensor activity. Based on electromyographic records we found for all subjects that this was not the case for movements to certain areas of the workspace. An alternative hypothesis, that the initial muscle activity should be appropriate for exerting an isometric force in the direction of the final position, was also contradicted by our observations. Nevertheless, the data for each joint revealed that flexor-initiated movements occurred for final positions in a certain region of the workspace that was distinct from the region of final positions for extensor-initiated movements.