Fiber Type Composition of Cadaveric Human Rotator Cuff Muscles

Abstract

STUDY DESIGN Descriptive 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 has 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.

Expression of Myosin Heavy Chain Isoforms in the Human Supraspinatus Muscle: Variations Related to Age and Sex

“…The higher values observed by the former studies could possibly be affected by the presence of hybrid fibers expressing both MHC-I and MHC-IIa [Smerdu et al, 1994]. The higher expression of the fast MHC-II isoforms, including the fastest (MHC-IIx), which was observed in the present study and also by others [Lovering and Russ, 2008], may reflect the functionally powerful nature of the supraspinatus muscle, which is capable of fast contractions but has low resistance to fatigue [Larson and Moss, 1993;Harridge et al, 1998]. This interpretation of the supraspinatus muscle highlights its function in the elevation of the upper extremity, where, together with the deltoid muscle, it forms part of the upper unit [Inman et al, 1944].…”

“…Both SDS-PAGE and RT-PCR are indicative of the higher expression of the fast MHC-II isoforms compared with the slow MHC-I isoforms in the human supraspinatus muscle, although MHC-I shows the highest expression when the three isoforms are considered separately, followed by MHC-IIa and finally by MHC-IIx. The proportion of type-I fibers observed by ATPase staining [Srinivasan et al, 2007] and immunohistochemistry [Lovering and Russ, 2008], 50 and 54%, respectively, is higher than the 36.72% observed by us for MHC-I expression using RT-PCR. The higher values observed by the former studies could possibly be affected by the presence of hybrid fibers expressing both MHC-I and MHC-IIa [Smerdu et al, 1994].…”

“…A marked difference in the percentages of fiber types exist between those identified by ATPase staining or im- munohistochemistry and the percent of MHC isoforms obtained in the present study using RT-PCR. However, when Lovering and Russ [2008] quantified the protein levels of MHC isoforms by SDS-PAGE in 3 supraspinatus muscles, they found that the proportion of MHC-I expression was 33-47%, that of MHC-IIa was 19-42%, and that of MHC-IIx was 17-35%. These results are along the lines of our findings: 33-42% MHC-I, 30-36% MHC-IIa and 25-35% MHC-IIx.…”

“…Using immunohistochemistry, Lovering and Russ [2008] found 54% ( 8 6%) of type-I fibers in 9 supraspinatus muscles. A marked difference in the percentages of fiber types exist between those identified by ATPase staining or im- munohistochemistry and the percent of MHC isoforms obtained in the present study using RT-PCR.…”

“…Together with the subscapularis, infraspinatus and teres minor muscles, the supraspinatus muscle forms part of the rotator cuff, which is the principal stabilizer of the glenohumeral joint [Michener et al, 2003;Ward et al, 2006;Lovering and Russ, 2008]. In addition to this stabilizing function, the supraspinatus muscle, along with the deltoid, with which it forms the upper unit described by Inman et al [1944], elevates the upper extremity in the scapular plane [Inman et al, 1944;Basmajian and de Lucca, 1985;McMahon et al, 1995;Alpert et al, 2000;Halder et al, 2001].…”