Journal of Shoulder & Elbow Surgery

Patrick J. McMahon, M.D., Richard E. Debski, B.S., William O. Thompson, M.D., Jon J.P. Warner, M.D., Freddie H. Fu, and Savio L-Y. Woo, Ph.D.


Muscle force values and tendon excursions across the glenohumeral joint during unconstrained glenohumeral abduction (GHA) in the scapular plane were evaluated using a dynamic shoulder testing apparatus. This was achieved by simulating rotator cuff and middle deltoid activity applied in four plausible muscle force ratios: (1) Equal force to each tendon; (2) 2:3 ratio of force applied to the middle deltoid:supraspinatus tendons; (3) 3:2 ratio of force applied to the middle deltoid:supraspinatus tendons; (4) zero force applied to the supraspinatus tendon to simulate supraspinatus paralysis. The glenohumeral joint was then moved to 5°, 15°, 30°, 45°, 60°, and maximum GHA while muscle forces, tendon excursions, and glenohumeral joint kinematics were monitored. Full GHA was achieved in all four test conditions. When the muscle force combination favored the middle deltoid, the smallest supraspinatus force was required from 30° to maximum GHA; however, when the supraspinatus was favored, the largest supraspinatus force was necessary to achieve maximum GHA. With simulated supraspinatus paralysis, the middle deltoid required the greatest increase in force from 15° through 45° of GHA. These results indicate that muscle efficiency during GHA is highly dependent on the ratio of applied force between the middle deltoid and supraspinatus. A larger contribution of force from the supraspinatus was required near the beginning of motion while the middle deltoid was more important near the end of SPA. Tendon excursion for the middle deltoid (6.4±0.2 cm) and supraspinatus (3.8±0.2 cm) were proportionately larger than those for the subscapularis and infraspinatus. Humeral head translations on the glenoid were less than 2 mm in all four conditions evaluated; therefore, the glenohumeral joint behaves kinematically as a "ball-and-socket" articulation during GHA. Simulated supraspinatus paralysis does not change normal joint kinematics and does not prevent full GHA.