The functional anatomy of the shoulder is reviewed and the biomechanics presented. Deltoid force was maximal with the arm abducted 90°, and was calculated to be 8.2 times the weight of the extremity. At 60° of abduction, the downward (short rotator) force was maximal at 9.6 times limb weight. The mechanism whereby the deltoid and the downward-acting short rotators combine to effect abduction is known as a force-couple. The biceps mechanism provides weak abduction in deltoid paralysis. Scapular rotation normally occurs with glenohumeral abduction and is known as scapulohumeral rhythm. Scapular rotation permits the deltoid to maintain an optimum length-tension ratio during most of the abduction range. The acromioclavicular articulations and pathomechanics are also discussed.
Lucas DB. Biomechanics of the Shoulder Joint. Arch Surg. 1973;107(3):425-432. doi:10.1001/archsurg.1973.01350210061018