Management of irreparable massive rotator cuff tears remains a challenging and controversial problem. Defining glenohumeral force relations may allow for the development of treatment strategies based on biomechanical principles. Five fresh-frozen adult human cadaveric shoulder specimens were dissected to determine fiber length, mass, and lever arm of (a) the 3 bellies of the deltoid and (b) the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis). From these data, physiologic cross-sectional areas and moment relations were calculated. These relations provide evidence for a balanced axial force couple between the anterior and posterior rotator cuff. Demonstration of an axial force couple across the glenohumeral joint may have clinical significance for treatment of irreparable massive rotator cuff tears and may explain why many patients with full-thickness rotator cuff tears can regain acceptable shoulder function.