Ten human cadaveric shoulders were tested with a dynamic shoulder model simulating physiologic rotator cuff, deltoid, and biceps muscle forces. The combined effect of the muscle forces and acromial structure on subacromial impingement was measured with minimally invasive, miniature pressure transducers. Shoulders with large acromial spurs had significantly greater impingement pressures at the anterolateral acromion in neutral, internal, and external rotation compared with those with flatter acromia. Application of a biceps muscle force reduced anterolateral acromial pressures by 10%. Failure to simulate a supraspinatus force decreased acromial pressure 52% in shoulders with type III acromia in neutral rotation. Without rotator cuff forces applied, the maximum deltoid muscle force required to elevate the arm increased by 17%. Acromial pressures were increased when no rotator cuff forces were applied, but the increases were not significant. After an anterior acromioplasty, pressures decreased by 99% anteriorly. However, failure to achieve a flat surface posteriorly increased pressures in this location, especially with the shoulder in external rotation. Modeling the rotator cuff and deltoid muscle forces demonstrated the importance of the muscular force couple to center the humeral head during elevation of the arm. The inferior forces of the infraspinatus, teres minor, and subscapularis muscles were necessary to neutralize the superior shear force produced by the deltoid and supraspinatus muscles.