Peculiarities of Activation of the Upper Limb Muscles in Humans during Realization of Two-Joint Movements

Coordination of the activity of flexors and extensors of the shoulder and elbow joints during realization of slow cyclic two-joint movements by the upper limb of the tested subjects was examined; the above movements were performed within the horizontal plane. The subject tracked the trajectory of the command signal shown on a monitor. Test movements corresponded to circular shifts of a handle; there were initiated in either clockwise (L_cw) or counterclockwise (L_ccw) directions. An external loading could be also applied in the above directions (M_cw and M_ccw, respectively). Surface (integral) EEGs were recorded from eight muscles of the upper limb and shoulder belt. As was found, the EMG amplitudes at opposite directions of tracking the target trajectory were dissimilar. At the clockwise direction of the external loading, the EMG amplitudes of most examined muscles during the counterclockwise movement were higher than those shown during the clockwise movement. Parameters of EMGs depended on the mechanical parameters of the movement. Under M_ccw conditions, shorter loading waves act upon flexors of both joints, while longer waves act upon extensors. The situation is opposite under M_cw conditions (longer waves influence the flexors, while shorter waves act on the extensors).