Spatial distribution of fiber types within skeletal muscle fascicles from Standardbred horses

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Abstract
Skeletal muscle fascicles from superficial and deep portions of semitendinosus (ST) and gluteus medius (GM) muscles from Standardbred trotters were analyzed with regard to muscle fiber type proportion (types I, IIA, and IIB) and spatial distribution. Muscle fibers within a fascicle were divided into four layers (L1–4) from the fascicle periphery toward the center. The observed proportions of fiber types among layers were found to be statistically significantly different from a random distribution of fiber types. Type IIB fibers predominated in the peripheral layer, type I fibers prevailed in the layer underneath, and proportions close to the mean of the whole fascicles were observed in the central layer. This pattern of spatial distribution of fiber types within the layers of the fascicles was observed at all four muscle sampling sites. The functional significance of the common pattern is unknown, but possible functional roles are discussed. Anat Rec 268:131–136, 2002.