Homogeneity of muscle fiber composition in the M gluteus medius of the horse

Muscle biopsy samples (80 mg) were removed from 5 sites at the same depths (5 cm) within the gluteus medius of 5 Quarter Horse geldings. Muscle samples were rolled in glycerol, frozen in isopentane for 2 min, pre-cooled in liquid nitrogen (liq. N₂) and sectioned to 10 μ on a microtome cryostat. Sections were stained for myosin ATPase (pH 9.4) and succinic dehydrogenase (SDH) activities and fiber types identified. Serial sections from each slide were counted 4 different times on different days to more accurately account for any operator differences. Coefficient of variation between counts on the same site was 3.52, 2.74 and 5.36 for fast twitch high oxidative (FTH) fibers, fast twitch low oxidative (FT) fibers, and slow twitch high oxidative (ST) fibers, respectively. Muscle fiber composition was not different (P>.10) between sites within the area sampled on the same horse. However, there was a difference (P<.05) in muscle fiber composition between horses.