HORRIGAN, J. M., F. G. SHELLOCK, J. H. MINK, and A. L. DEUTSCH. Magnetic resonance imaging evaluation of muscle usage associated with three exercises for rotator cuff rehabilitation.Med. Sci. Sports Exerc.,Vol. 31, No. 10, pp. 1361–1366, 1999.Purpose:Methods of determining muscle usage for exercises involving rotator cuff muscles are limited. Therefore, this investigation used magnetic resonance imaging (MRI) to evaluate the effect of three different exercises used for rehabilitation of the rotator cuff.Methods:Five normal volunteer subjects (3 men, 2 women, mean age 31.4 yr) were studied. The exercises were scaption with internal rotation (SIR), military press (MP), and side-lying 45° abduction (SLA). MR imaging was performed immediately before and after exercise using a “fast” spin echo STIR sequence and oblique coronal plane imaging. Changes in signal intensity pre- and post-exercise were measured at comparable section locations for the MR images of the supraspinatus, infraspinatus, teres minor, subscapularis, deltoid, and trapezius.Results:The SLA showed the greatest increase in signal intensity in all the muscles (percent change,P< 0.01) except for the trapezius, which was used more by the MP and SIR. None of the exercises activated the teres minor (percent change,P= not significant).Conclusion:These findings have important implications in efficacy of physical rehabilitation of the rotator cuff and avoidance of subacromial impingement exercise motions.