Fiber type composition of the architecturally distinct regions of human supraspinatus muscle: a cadaveric study.

The human supraspinatus muscle is clinically important as it is frequently injured in older adults and the elderly. We have previously shown that the supraspinatus has a complex architecture with two distinct regions each consisting of three parts. Further we have found dynamic changes in architectural parameters such as fiber bundle length markedly vary between these regions. Fiber types of the supraspinatus have not been thoroughly investigated throughout its volume and are of interest to clinicians treating supraspinatus pathologies. In this study we investigated the distribution of fiber types within the distinct regions and parts of supraspinatus. Samples of supraspinatus were excised from six distinct parts of each muscle from five formalin embalmed specimens (one male, four female; mean age 77±11.1 years) free of tendon pathology. Samples were frozen in liquid nitrogen and then cryosectioned. Serial sections were labeled using immunohistochemical techniques and antibodies against fast or slow myosin heavy chain isoforms. The mean percentage of Type I (slow) fibers ranged from 56.73% to 63.97%. Results demonstrated significant variations in fiber type distribution. The middle part of the anterior region has a significantly greater percentage of Type I fibers compared to that of the posterior. The superficial part of the anterior region has a greater percentage of Type II (fast) fibers compared to the middle and deep parts. Findings aid in highlighting the distinct functions of the anterior and posterior regions, and prompt the need to re-evaluate assessment and treatment techniques established on a limited understanding of the fiber type distribution.