We examined the peculiarities of central coordination of motor commands coming to the muscles of the shoulder belt and shoulder in the course of generation of targeted isometric efforts by the arm. The dependence of these commands on changes in the effort direction and position of the forearm within the working space were analyzed. The intensity of the central commands was estimated according to the amplitudes of rectified and averaged EMGs recorded from the corresponding muscles. Sector diagrams of EMG activity of the above muscles depending on the direction of the effort vector, EV, were plotted [1]. Preferential sectors of activity where the efforts were formed due to activation of definite functional muscle groups were identified. As was found, the direction of these sectors depends significantly on the EV orientation. Differences between the patterns of coactivation of the examined muscles were demonstrated. Organization of the motor commands under conditions of creation of extensor efforts is distinguished by a more complex pattern than that related to flexor efforts. In the former case, the activity of extensor muscles is accompanied by more significant activation of the flexors.

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In tests with four volunteers, we examined manifestations of hysteresis in EMG activity recorded from eight shoulder belt and shoulder muscles; the subjects should develop, by their arm, targeted isometric efforts of eight different directions within the horizontal plane. Totally, 250 realizations were analyzed; we characterized interrelations between the levels of rectified and integrated EMGs and amplitudes of the effort under conditions where the effort was developed and then returned to a zero level; force trajectories corresponded to double trapezes. Significant manifestations of hysteresis were found in 118 cases (46%). Hysteresis with the loops for interrelations between the EMG and force levels having a clockwise direction was observed in 107 cases (or 91%); the counterclockwise-directed loops were found in 11 cases (9%). It is hypothesized that manifestations of hysteresis under conditions of our tests are related to differences between the processes of recruiting/derecruiting of motor units in the course of natural voluntary activation of the muscles. The CNS demonstrates high flexibility in the selection of the type of nonlinear activation of the muscles in the course of generation of "two-joint" isometric efforts produced by the arm.