This paper compares the muscle firing patterns of 12 shoulder girdle muscles in competitive butterfly swimmers with painful and normal shoulders. Seven of the 12 muscles revealed statistically significant differences between the two populations. The posterior deltoid demonstrated more activity in the painful shoulders during hand entry while the upper trapezius and serratus anterior exhibited less activity. This alteration in muscle firing patterns allowed for the humerus to be positioned for a wider hand entry, which decreased the pain of impingement of the supraspinatus on the coracoacromial arch. Correspondingly, there was significantly less activity in the supraspinatus. The teres minor and serratus anterior revealed significantly less muscle action throughout pulling as they respectively failed to balance the humeral rotation and did not reverse their origins and insertions to pull the body over the arm. Also, the subscapularis and infraspinatus displayed increased activity in the painful shoulders as they depressed the humeral head to avoid impingement. There were no significant differences between the two groups in the rhomboids, pectoralis major, latissimus dorsi, or the anterior and middle deltoids. From this information, accurate preventative and rehabilitative exercise programs for the competitive butterfly swimmer can be developed.