Quantitative Electromyographic Analysis of Deltoid and Supraspinatus Muscles during Shoulder Abduction

ABSTRACT

Measurements of local shoulder muscle function during shoulder abduction are of a great interest in biomechanics research and in ergonomic applications. There have been so many opinions that the supraspinatus muscle acts in synergy with the deltoid muscle as a single unit throughout the shoulder abduction. However the specific actions of deltoid and supraspinatus muscles have been subjects of controversy. Electromyography is an established evaluation method of biomechanical study. It reflects the electrical activity at the muscle membrane level and indirectly the mechanical output of the muscle. The purpose of this study was to evaluate the role of deltoid and supraspinatus muscles during shoulder abduction by the comparison of motor unit action potentials using a quantitative electromyographic analysis method, to provide a good insight into the biomechanics of shoulder abduction. Motor unit action potentials of deltoid muscle were seen earlier than those of supraspinatus muscle at the time of initiation of shoulder abduction. The Root Mean Square (RMS) voltage of deltoid muscle was increased gradually from 0o to 90o of shoulder abduction, and then decreased gradually above 90o to 180o of shoulder abduction. The RMS voltages of deltoid muscle were significantly higher than those of supraspinatus muscle at each degree of shoulder abduction wholly. There was no differences in the RMS voltages of deltoid muscle, during shoulder abduction between the loading of 1 kg and without loading. However, the RMS voltages of supraspinatus muscle were significantly higher in the loading state than without loading. The Mean Rectified Voltages (MRV) were similar to the RMS voltages of deltoid and supraspinatus muscles during shoulder abduction. Based on these results, we concluded that the deltoid muscle was not only an initiator but also a major contributor in shoulder abduction, where as the supraspinatus muscle acts as a secondary muscle for the initiation of shoulder abduction and a supporting muscle when there is a resistance against shoulder abduction.