A biomechanical analysis of scapular rotation during arm abduction in the scapular plane.

The dynamic pattern of scapulohumeral rhythm and the scapular instantaneous center of rotation were studied in 20 healthy young male subjects. The most common pattern of scapulohumeral rhythm was characterized by three separate phases, with the greatest relative amount of scapular rotation occurring between 80 and 140 degrees of arm abduction. The scapular instantaneous center of rotation was observed to be located initially at, or near, the medial root of the scapular spine, and gradually migrated toward the region of the acromioclavicular joint as arm abduction progressed. A biomechanical model of the shoulder girdle was proposed by correlating the results from the present study with the electrical activity of the scapular rotators. This model is useful in demonstrating the true dynamic function of the muscles acting on the scapula as well as the mechanical efficiency of upward scapular rotation during arm abduction.