STUDY DESIGN: Descritive cadaveric laboratory study. OBJECTIVE: To identify the fiber type composition of the rotator cuff and teres major muscles in human subjects.  BACKGROUND: The rotator cuff is commonly injured in athletics and is a major focus of sports medicine.  Although the anatomy and architecture of each muscle have been described in great detail, these muscles have never been fiber typed using immunohistochemistry or gel electrophoresis.  Fiber typing is important in modeling function, exercise training, and rehabilitation. METHODS AND MEASURES: We harvested tissue samples for all 4 rotator cuff muscles, as well as the teres major muscle from cadavers. Tissues were frozen in liquid nitrogen and sectioned. Cryosections were labeled with commercially available antibodies against fast and slow isoforms of myosin heavy chain (MHC). We also harvested fresh (unembalmed) tissue from deceased subjects and labeled tissue sections with antibodies against fast or slow MHC and wheat germ agglutinin. Gel electrophoresis followed by silver staining was also used to identify and quantify MHC isoforms in fresh tissue samples. RESULTS: All of the muscles were of mixed fiber type composition. As a whole, 44% of rotator cuff fibers labeled positively for slow MHC, with slow MHC content of 54% in supraspinatus, 41% in infraspinatus, 49% in teres minor, 38% in subscapularis, and 40% in teres major. Mixed MHC isoform distribution was confirmed by SDS-PAGE, which also indicated that the IIa and IIx isoforms were roughly equally present across the muscles. CONCLUSIONS: Human rotator cuff muscles, at least in older subjects, have a mixed fiber type. Because we only examined older subjects, we must limit our interpretation to this population.

J Orthop Sports Phys Ther. 2008; 38(11):674-680, Epub 18 July 2008. doi:10.2519/jospt.2008.2878 

KEY WORDS: immunohistochemistry, myosin heavy chain, shoulder, supraspinatus