A study of bilateral shoulder muscle activity during the golf swing was undertaken using electromyography and high-speed photography. Understanding of the muscle firing patterns could lead to injury prevention and development of appropriate training and conditioning regimens. The swings of seven adult male right-handed professional golfers without shoulder problems were examined. Indwelling electrodes were inserted into the supraspinatus, subscapularis, infraspinatus, latissimus dorsi, pectoralis major, anterior deltoid, middle deltoid, and posterior deltoid on the right side. Each subject was allowed to warm up until he felt comfortable. Films of each subject were taken at 450 frames per second. The swing was broken into four segments to which electromyographic signals were synchronized electronically. The EMG tracings were subjected to analog-to-digital conversion and a relative measure of quantity obtained. All tests were repeated on the left side for each subject. Results indicate that all portions of the deltoid were inactive on the right side throughout the swing. The deltoid was likewise inactive on the left except for a brief spurt from the anterior portion during the milliseconds immediately preceding ball contact. Of the rotator cuff muscles, on the left the supraspinatus fired at a low level throughout the swing, as did the infraspinatus. The latter had a slightly larger burst of activity immediately after ball contact. The subscapularis was more active than any other muscle throughout the swing. The cuff muscles on the right side showed as much activity overall as those on the left. In addition, the latissimus dorsi and pectoralis major seemed to provide power bilaterally, with marked activity during the acceleration phase.(ABSTRACT TRUNCATED AT 250 WORDS)