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 direction and selectivity of an appropriately modified version of shoulder flexion is dependent upon initial angles of shoulder abduction. Analysis indicated that initial small angles of shoulder abduction were associated with longer electromyographic (EMG) durations of the agonist (anterior deltoid) muscle. Moreover, as initial angles became smaller, EMG onsets of the antagonist (posterior deltoid muscle) occurred nearer to those of the agonist. Modulations of duration of the agonist EMG activity can be explained by changes 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 on the load of the upper limb, dependent on different angles of shoulder abduction. The second concerns the changing role of the antagonist to a synergist, 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-antagonist muscle activity of the deltoid during flexion of the shoulder.