Skeletal Muscle Fiber Type Composition and Citrate Synthase Activity in Fit and Unfit Warmbloods and Quarter Horses

•Warmblood (WB) gluteal muscle has an inherently high percentage of slow twitch type 1 fibers and low % percentage of fast contracting type 2X fibers•Quarter Horse (QH) gluteal muscle has an inherently low percentage of type 1 fibers and high percentage of type 2X fibers•The mitochondrial volume density measured as citrate synthase activity in WB gluteal muscle is inherently high in unfit horses and is not different from WB trained for dressage•The mitochondrial volume density in QH gluteal muscle is inherently low in unfit horses but increases in fit reining/working cow QH•Similar CS activity in fit WB versus QH despite a 2-fold difference in % type 2X fibers indicates that mitochondrial volume density cannot accurately be predicted from contractile fiber type composition