Dependence of deltoid muscle activity upon initial angles of shoulder abduction prior to flexion

The present study was undertaken to investigate the hypothesis that the dir ection and selectivity of an appropriately modified version of shoulder fle xion is dependent upon initial angles of shoulder abduction. Analysis indic ated that initial small angles of shoulder abduction were associated with l onger electromyographic (EMG) durations of the agonist (anterior deltoid) m uscle. Moreover: as initial angles became smaller, EMG onsets of the antago nist (posterior deltoid muscle) occurred nearer to those of the agonist. Mo dulations of duration of the agonist EMG activity can be explained by chang es in amplitude of movement. Two possible interpretations of the changes of EMG onset of the antagonist in accordance with different initial angles of shoulder abduction are considered. One concerns the effect of the change o n the load of the upper limb, dependent on different angles of shoulder abd uction. The second concerns the changing role of the antagonist to a synerg ist, dependent upon decreased initial angles of shoulder abduction prior to the shoulder flexion. Based on the present findings, it is suggested that angles of shoulder abduction are an important determinant of agonist-antago nist muscle activity of the deltoid during flexion of the shoulder.