Lehedza O. Activation of the muscles of the human hand in the conditions of visual tracking of the basic parameters of two-joint movements

The work is devoted to determining the features of the patterns of EMG activity and the central coordination of the muscles of the shoulder girdle and shoulder of a human in conditions of visual tracking of the efforts of the hand and its position in the operating space. The current amplitudes of the rectified and averaged EMGs, which were recorded from the muscles of the shoulder girdle and shoulder, were considered as correlates of the CMC intensity. It was found that when forming an isometric force, the same in all directions relative to the point of its application, the activity of extensors occurred mainly during extension in the shoulder and elbow joints, while in flexors it was observed in all directions, and its maximum amplitude was directed towards bending. It is assumed that the activity of the muscles of the hand during the task in terms of tracking the target signal in a circle is organized on the principle of synergies, which may be related to the characteristics of the task. Differences in EMG amplitudes in opposite directions of the force vector and coactivation of flexor muscles were shown, while extensors showed their activity only within their functional sectors. The ratio of the levels of the rectified and averaged EMG was analyzed under conditions of the progressive development of the effort along linear trajectories for eight different directions. Possible manifestations of hysteresis were recorded in 118 cases (46%). Hysteresis, the loop of which for the ratio of the EMG force level was oriented clockwise, was observed in 107 cases (or 91%); in 11 cases (9%), a loop oriented counterclockwise was noted. It was suggested that the manifestations of hysteresis of EMG activity under the conditions of the described tests are associated with disagreements in the processes of recruiting / de-recruiting motor units during natural muscle activation. The central nervous system, obviously, shows high flexibility in determining the type of nonlinear muscle activation during the generation of two joint isometric efforts created by the hand. EMG patterns in the course of slow movements along parafrontal trajectories are described. EMGs in both flexor and extensor muscles have been shown to show strong hysteresis (loops counterclockwise and counterclockwise, respectively). The flexors of the elbow and shoulder joints showed synergistic unification, mainly activating when creating a hand force in the direction of flexion of the joints, and similar synergy was manifested by extensors of both joints in response to extensor forces. It is shown that for more distal positions the synergy effects changed in different directions, increasing in flexors and decreasing in extensors. The obtained results allow us to conclude that the hysteresis properties of muscle contraction and related signal transmission characteristics in the motion control system lead to strong, hysteresis-related modifications of the central commands coming to the muscles during different motor tasks.