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 Selective breeding and discipline specific training has led to equine breeds adept at various athletic disciplines. Breed-specific skeletal muscle adaptations have been studied in many breeds but not Warmbloods (WB). We evaluated gluteal muscle contractile muscle fiber types and citrate synthase activity (CS), a marker for mitochondrial volume density, in WB trained for dressage (second level-Grand Prix) contrasted with Quarter Horses (QH). Gluteus medius muscle biopsies from 14 unfit/18 fit dressage-trained WB and 20 unfit/16 fit reining/working cow QH were analyzed fluorometrically and fiber types determined by ATPase activity. Comparisons were made by one-way ANOVA. Unfit and fit WB had significantly higher % type 1 and lower % type 2X fibers than QH. Unfit WB had significantly higher CS than unfit QH but CS did not differ between fit WB and fit QH. CS was only significantly higher in fit versus unfit QH, not fit versus unfit WB. In conclusion, WB gluteal muscle has an inherently high % type 1/low % type 2X fibers and high mitochondrial content whether unfit or trained for dressage, contrasting QH with an inherently low % type 1/high % type 2X and low mitochondrial content, that was enhanced in fit QH. Similar CS activity in fit WB versus QH despite a two-fold difference in % type 2X fibers indicates that mitochondrial volume density cannot accurately be predicted from contractile fiber type composition.