Intramuscular wire electromyography of the subscapularis

The action of the subscapularis muscle is an important component in maintaining shoulder stability. Because of its relative inaccessibility, there have been few electromyographic (EMG) studies of its normal patterns of activity. The subscapularis is innervated by two or more distinct nerves, and therefore the upper and lower parts of the muscle may have different functional roles depending on the position of the humerus. The purpose of this study was to develop safe, reproducible insertion paths to the upper and lower parts of the subscapularis. Six subjects with no previous history of shoulder injury were evaluated. The paths of insertion were designed based on previous anatomical studies as well as dissections. Two pairs of intramuscular wire electrodes were inserted: one directed toward the upper subscapularis and one toward the lower subscapularis. Electrode locations were confirmed using posteroanterior and lateral radiographs and through electrical stimulation. EMG data were recorded during isometric internal rotation exercises with the humerus in 0 or 90 degrees abduction. Significant differences were observed in the EMG activity recorded from the two pairs of electrodes. The EMG activity of the upper subscapularis either remained the same or decreased in going from 0 to 90 degrees abduction, while that of the lower subscapularis increased. The observed differential response confirmed that the electrodes were in different parts of the subscapularis. These preliminary results suggest that in future EMG studies, the subscapularis should be considered as at least two independent muscle units.