The activities of the scapular muscles during isometric contraction of internal and external shoulder rotation

Abstract

[Purpose] The activities of the scapular muscles during isometric contraction were measured to examine the mechanism of scapular stability. [Method] The activities of the scapular muscles were measured during isometric contraction of internal and external shoulder rotation at 0 degrees and 90 degrees abduction with three kinds of resistance loads. Scapular muscles were classified into agonists and antagonists of each motion, and the changes in each muscle activity were analyzed. [Results] The integrated electromyograms of both agonists and antagonists increased significantly with increasing resistance load in internal and external rotation at 0 degrees and 90 degrees abduction. In the case of the strongest resistance load at 0 degrees abduction, the integrated electromyograms of the agonists showed from 14 to 24 times as much as the start position and those of antagonist showed from 3 to 5 times that of the start position. At 90 degrees abduction, the integrated electromyograms of agonists showed from 2 to 12 times and those of antagonists showed from 2 to 7 times. [Conclusions] All scapular muscles needed to be active during concentric contraction of shoulder rotation. We consider that the agonists might work against external force and the antagonists might contribute to the stability of the scapula and trunk, in light of a previous study.