Activation of the Shoulder Belt and Shoulder Muscles in Humans Providing Generation of “Two-Joint” Isometric Efforts

Abstract

This review deals with the processes of coordination of central motor commands (CMCs) that arrive at human arm muscles in the course of realization of "two-joint" voluntary isometric efforts. The dependence of formation of such motor events on changes in the direction of the generated effort is described. Differences between patterns of co-activation of the studied arm muscles under conditions of isometric contractions are presented. Based on the obtained data, directions of the developed efforts are classified as coinciding and, correspondingly, easier for realization (flexion-flexion and extensionextension in both joints), and as discordant "inconvenient" (extension in the shoulder joint-flexion in the elbow joint and flexion in the shoulder joint-extension in the elbow joint). Programming of coactivation of flexors and extensors of the shoulder and elbow joints seems to be possible at the level of CMCs. Predominant sectors of the activity of separate functional muscle groups during realization of the studied efforts are estimated. We found that the orientation of such sectors depends, in most cases, on the orientation of the effort vector but not on the position of the limb within the horizontal plane.

Hysteresis Properties of EMG Activity of the Shoulder Belt and Shoulder Muscles at the Development of Isometric Efforts by the Human Arm

“…As was mentioned above, the level of EMG activity of all muscles significantly depended on the direction of the developed effort; this statement completely agrees with the results of our earlier studies [10][11][12][13]. Findings made in the above-described tests supported, in general, conclusions formulated in our earlier studies on animals.…”