The three significant force couples that occur in normal overhead shoulder movement are the scapular force couple (acting on the scapula), the transverse force couple, and the coronal force couple.^4,5,6 The latter two force couples act at the glenohumeral joint.

The scapular force couple consists of cocontraction of the serratus anterior and the lower trapezius, resulting in elevation of the acromion, thereby increasing the relative distance between the coracoacromial arch and the humeral head.⁷

The transverse plane force couple occurs between the anteriorly oriented subscapularis muscle and the posteriorly positioned infraspinatus and teres minor. A cocontraction of all three of these muscles downwardly translates and compresses the humeral head during overhead movements. The muscles offer protection of the structures inferior to the coracoacromial arch and also act as dynamic stabilizers.

The coronal plane force couple occurs between the deltoid muscle and the inferiorly placed rotator cuff muscles during overhead elevation of the shoulder as follows: Contraction of the deltoid and supraspinatus begin to pull the humeral head superiorly, while cocontraction of the transverse force couple muscles (subscapularis, infraspinatus, and teres minor) exert a caudal force on the humeral head, simultaneously compressing the humeral head into the glenoid fossa in conjunction with the supraspinatus. The resultant motion is smooth overhead motion of the humerus.

Scapulohumeral rhythm occurs as the transverse and coronal force couples perform their work during the movement of overhead elevation. The scapula begins to abduct and upwardly rotate a moment later via the scapular force couple. The sequence of events in normal scapulohumeral rhythm in regard to overhead elevation is:⁴

• 0–30 degrees: Scapula creates a stable platform, all significant motion occurs at the glenohumeral joint
• 30–90 degrees: Scapula abducts and upwardly rotates 1 degree of motion for every 2 degrees of humeral elevation
• 90 degrees to full abduction: Scapula abducts and upwardly rotates 1 degree for each 1 degree of humeral elevation.

The smaller (and weaker) muscles of the rotator cuff work with larger glenohumeral muscles to protect the critical zone from trauma. The scapular force couple aids the glenohumeral force couples by promoting scapulohumeral rhythm. The cuff tendons lying in the critical area of the shoulder in the adult are relatively avascular. The glenohumeral joint itself is weak by bony and ligamentous arrangement with stability being sacrificed for mobility. The articular surfaces of the glenoid fossa and humeral head are lined with hyaline cartilage and joint congruency is increased by the fibrocartilage rim around the glenoid, known as the glenoid labrum. Thus the shoulder articular surfaces are subject to degenerative changes, as are the tendons, most notably those of the rotator cuff.