The human shoulder has adapted to enhance the use of another unique and specialized structure — a hand with an opposable thumb. However, our species has sacrificed a great deal of shoulder stability to produce what is arguably the most mobile articulation in the human body. To allow multiplanar movement, shoulder biomechanics have evolved via the development of at least three finely tuned force couples that must occur at the scapula and at the shoulder in both transverse and coronal motions.

Four muscles of the rotator cuff interact with the three heads of the deltoid to produce one of these force couples. Collectively, the four muscles — supraspinatus, infraspinatus, teres minor, and subscapularis — form the rotator cuff. The serratus anterior, rhomboids, levator scapula and the trapezius group provide motion at the scapula, and the pectoralis major, teres major, and latissimus dorsi provide large gross movements.¹

Because of the complexity and instability of the shoulder anatomy, the shoulder is at risk for the development of numerous pathologies, many of which are amenable to evidence-based physical therapy interventions. Moreover, because the rotator cuff acts in concert with larger, more powerful muscles of the shoulder girdle to create a total shoulder complex motion known as scapulohumeral rhythm, the rotator cuff itself is often incriminated in shoulder pathologies.

Joints of the Shoulder Complex

Three synovial joints and one “muscle joint” make up the articulations of the shoulder and shoulder girdle. The synovial joints include the sternoclavicular, acromioclavicular (AC), and the glenohumeral joints. The muscle joint is the scapulothoracic joint. For normal scapulohumeral rhythm to occur, normal motion must take place in all four of these joints. Particularly important is the motion that occurs at the scapulothoracic joint; that is, the movement of the scapula on the chest wall, and the glenohumeral joint, a freely moveable synovial joint of a convex humeral head moving in the shallow concave surface of the glenoid fossa. The shallow glenoid cavity is deepened by the glenoid labrum, a fibrocartilage ring surrounding the glenoid rim.

Muscles of the Shoulder Complex

One method particularly useful to physical therapists and biokinesiologists for classifying muscles that attach to and move the shoulder girdle and shoulder is according to their kinetic functions:^1,2

Pivoters include both scapulovertebral muscles (trapezius, levator scapula, and rhomboids) and scapulothoracic muscles (serratus anterior and pectoralis minor).

Positioners are muscles that position the humerus during or following action of the pivoters and include the anterior, middle, and posterior heads of the deltoid muscle.

Propellers are muscles that offer a wide array of strong, gross humeral movements, depending on concomitant or prior scapular and humeral positioning by the pivoters and positioners. Propellers include latissimus dorsi, pectoralis major, and teres major. The first two of these muscles originate at the vertebral column and anterior chest wall respectively, while the teres major originates on the dorsal aspect of the inferior angle of the scapula. The three muscles insert respectively on the floor, lateral lip, and medial lip of the bicipital groove of the humerus. In this latter structure, the long head of the biceps brachii tendon courses its way to insertion on the supraglenoid tubercle; the biceps muscle acts as an important humeral decelerator.

Protectors act as a force couple with the positioners (deltoid group) to keep structures, in what has been referred to as the critical zone (between the coracromial arch and the humeral head), out of harm’s way.³ The protectors also act as decelerators to prevent subluxation and/or dislocation of the humerus during powerful, explosive, or repetitive movements, such as throwing or swimming. These muscles include the supraspinatus, infraspinatus, teres minor, and subscapularis. The first three of these muscles originate on the scapula and insert in close proximity to the others on the greater tubercle of the humerus. The subscapularis originates anteriorly on the scapula and inserts on the anteriorly positioned lesser tubercle of the humerus. The supraspinatus is a shoulder abductor (although it cannot be isolated from the middle deltoid except as a joint compressor), the infraspinatus and teres minor are external rotators, and the subscapularis is an internal rotator.