The Effect of Scapular Retraction Motion according to the Abduction Angle of the Shoulder Joint on the Muscle Thickness and Muscle Length of the Middle Trapezius and Rhomboid Muscles

Shoulder impingement syndrome, the main cause of shoulder pain, is often caused by weakness of the scapular retractor muscles. The major scapular retractor muscles are the middle trapezius and rhomboid muscles, however, related research is still lacking. In this study, we used an ultrasound imaging device and a tape measure to quantitatively measure the muscle thickness and muscle length of the middle trapezius and rhomboid muscle, which are activated according to the shoulder abduction angle during the retraction motion in 24 healthy adults. Three motions were performed with shoulder abduction angles of 30°, 60°, and 90°. The mean and standard error of measurement of the muscle thickness and length were calculated, and repeated measures analysis of variance and post hoc tests were performed. Muscle thickness was significantly increased at abduction angle of 90° for the middle trapezius muscle (p<.05) and abduction angle of 60° for the rhomboid muscles (p<.05), respectively. Muscle length was significantly decreased at abduction angle of 90° at both middle trapezius muscle (p<.05) and rhomboid muscle (p<.05), separately. This study suggests that it can be effectively applied by distinguishing the shoulder abduction angles of the two muscles, middle trapezius and rhomboid, during scapular retraction.