A principal cause of subacromial impingement (SAI) is failure of the rotator cuff to center the humeral head in the glenoid during shoulder motion, counteracting the effect of the deltoid. As rehabilitation of the rotator cuff endeavors to restore balance between these muscle groups, the purpose of this companion study was to evaluate, in the symptomatic shoulders of patients with SAI, (1) the conditions of resisted isometric external rotation (ER) that optimized the contribution of the infraspinatus and (2) the load of ER at which adduction was most effective at reducing the deltoid contribution and then to compare this with the relative contribution of the infraspinatus and the posterior and middle deltoid in asymptomatic shoulders. In 14 subjects (18 shoulders) with SAI, surface electromyographic activity of the infraspinatus and the posterior and middle deltoid and pectoralis major was recorded at low, medium, and high loads of resisted isometric ER, with and without adduction. These data were normalized to find each muscle's relative contribution to the task and were compared with normalized data from subjects with healthy shoulders. In subjects with SAI, low loads of isometric ER (10%-40% maximum voluntary isometric contraction) optimized the relative contribution of the infraspinatus. Adduction with isometric ER at 10% maximum voluntary isometric contraction reduced the middle deltoid involvement. Higher loads preferentially activated the middle deltoid over the infraspinatus and may have effected unwanted humeral head superior translation, counteracting the presumed benefits of rotator cuff ER exercises. An individualized loading regimen and the use of surface electromyography may have significant implications during rotator cuff rehabilitation.