In sports and certain occupational activities, persons use their bodies and large muscles to generate momentum and energy which are then guided through their shoulder to achieve the task. Injuries occur when the muscle and soft tissue of the shoulder are requested to generate or dissipate such load and energy beyond their capacities, either as a single event or on a repetitive basis. It has been recognized in principle, based on clinical experiences and limited laboratory studies, that the rotator cuff muscle complex is functionally important to provide the power and maintain the stability of the glenohumeral joint of the shoulder. Consequently, the rotator cuff tendon will encounter relatively high stress, subjecting it to potential injury. The three hypotheses regarding the rotator cuff are: a) the integrated action of the rotator cuff muscle complex and deltoid muscle exceeds the sum of their isolated contribution to shoulder strength; b) rotator cuff muscles are either primary or secondary contributors to glenohumeral constraint according to direction of displacement; and c) the rotator cuff tendon is inadequate to withstand the load placed on it by some activities; that is, the tendon encounters relatively high load in reference to its failure strength. To test these three hypotheses, we propose to accomplish the following six specific aims: 1) to measure the moment arms of cuff muscles and the muscles crossing the glenohumeral joint under selected functions and joint positions in vitro; 2) to measure the shoulder strength of normal subjects and patients with defined cuff pathology in selected functions; 3) to measure the displacement of the humeral head under selected loadings: a) with and without cuff muscle force, b) with and without capsuloligamentous constraints; 4) to develop and verify an analytic model to predict the displacement tendencies of the humeral head that must be constrained during joint and muscle loading; 5) to calculate, analytically, the rotator cuff force under selected loading and especially during overhead activities; 6) to measure the mechanical properties of the cuff tendon and document the mode of failure.