Biomechanics of the shoulder

Man's shoulder girdle is of the general pattern of his tree-swinging ancestors. With assumption of the upright posture, man's thorax has flattened from anterior-posterior. This results in a rotation of the scapula to a position of 45 degrees with the sagittal plane. In addition to this, man's forearm is habitually used in a position of approximately 45 degrees of internal rotation. This places the biceps tendon "off its trolley" and leads to biceps tinosynovitis. Motions of the glenohumeral joints result from the force couple of the deltoid muscle plus the rotator cuff muscles. The rotator cuff alone can abduct the arm with 50% force throughout the full range of its motion. In the absence of the supraspinatus muscle, however, the force couple is disrupted. Although initiation of abduction is with full force, the force rapidly falls off to 90 degrees. Above 90 degrees the arm can barely support its own weight. Although the shoulder undergoes progressive degenerative changes with age, the necessity for a joint implant, either partial or total--although successful--is rare.