Electromyographic analysis of posterior deltoid, posterior rotator cuff and trapezius musculature in different shoulder exercises

Background: The shoulder external rotator muscles and the different portions of the trapezius muscle have never been studied in exclusivity. However, the literature has provided several exercises which have been used in this study. Purpose: To quantify electromyographic activity of the shoulder external rotator muscles and the upper, middle and lower trapezius in seven exercises. Methods: 20 healthy males performed 7 exercises in random order. Surface electromyography was recorded for the posterior deltoid, infraspinatus, teres minor, upper, middle and lower trapezius. Results: The four prone exercises presented the highest levels of EMG activation in the External Rotators Synergy (the average activation of arm external rotator muscles group) and in the Trapezius Synergy (the average activation of the three portions of trapezius). The infraspinatus muscle obtained the highest activation values in exercises 1 (prone horizontal abduction at 90° with full external rotation, thumb up), 4 (prone external rotation at 90^o abduction and elbow at 90^o), and 5 (side-lying ER with elbow on the trunk). The highest activation level of the teres minor muscle was found in Exercise 1. Conclusions: The four prone exercises demonstrated the highest EMG activity in the shoulder, considering both the external rotator muscles and the trapezius. However, if the focus of the strength training process is mainly to strengthen the two external rotator muscles of the rotator cuff, with an adequate intramuscular coordination pattern for the trapezius, the side lying ER with the elbow resting on the trunk seems to be the most appropriate exercise.