Activation of the Shoulder-Belt and Shoulder Muscles in Two-Joint Arm Movements Performed in Humans with the Action of Opposite Loadings

In tests on four volunteers, we examined coordination of central motor commands (CMCs) controlling slow two-joint movements of the arm within the horizontal plane. Current amplitudes of EMGs recorded from six muscles of the shoulder belt and shoulder and subjected to full-wave rectifying and low-frequency filtration were considered correlates of these commands. In particular, we studied the dependence of coordination of CMCs on the direction of an external force applied to the distal forearm part. As was found, coordination of CMCs significantly depends on the direction of the force flexing the elbow joint. According to our observations, EMGs of definite muscles in the case of performance of a two-joint movement can, in a first approximation, be presented as linear combinations of the EMGs recorded in the course of separate sequential single-joint movements under conditions of shifting the reference point of the hand toward the same point of the operational space as that in the two-joint movement. These data can be interpreted as confirmation of the principle of superposition of elementary CMCs in the performance of complex movements of the extremity.