Hysteresis in EMG Activity of Muscles of the Human Upper Limb at Rotations of the Isometric Effort Vector

We studied the dynamics of EMG activity of muscles of the human upper limb under conditions of changes in the direction of a vector of the isometric effort created by the hand. Averaged EMG activities of seven muscles of the shoulder and shoulder girdle at nine combinations of the angles in the shoulder and elbow joints were evaluated. Mean values of the angles of the maximum activity (AMAs) of the muscle during slow counterclockwise and clockwise rotations of the above-mentioned vector were compared. A significant dependence of the AMA in a considerable part of the muscles on the direction of the target trajectory hodograph by the vector of the generated effort was observed; this dependence is, most likely, due to the manifestations of hysteresis in the activity of the examined muscles. In the case where the analysis was performed in more detail, AMA values for the mm. triceps brachii, pectoralis major, and deltoideus pars scapularis were significantly greater at clockwise rotation, while those for the m. brachioradialis demonstrated an opposite trend.

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