Shoulder Muscle Architecture, Physiology, and Plasticity

Abstract

Muscles generate movement of the shoulder complex (glenohumeral, scapulothoracic, and acromioclavicular joints) and provide the stability needed to protect vital anatomic structures, in coordination with the rest of the neuromusculoskeletal system (i.e., tendons, ligaments, and the nervous system). One subset of muscles, the rotator cuff (supraspinatus, infraspinatus, subscapularis, and teres minor), is assembled in a way that optimizes them for generating dynamic stability—in this case of the glenohumeral joint. Of course, injury and degeneration are common in the tendons of the rotator cuff, which cause pain, instability, and impaired upper extremity function. Although the tendons of the rotator cuff, and their associated bony footprint, have been the focus of a tremendous amount of research related to pathophysiology and surgical reconstruction techniques, more recent data suggests that atrophic changes in these muscles complicate reconstruction and impair function and are persistent even after successful tendon reconstruction. And beyond tendinopathy and frank tendon tears, rotator cuff dysfunction has also been implicated in a variety of pathologic conditions, such as glenohumeral instability, internal derangement, and non-cuff tendinopathy. However, the mechanisms (neural or otherwise) that relate muscle function (or dysfunction) to pathologic processes are unclear.