MUSCLE-FIBER TYPE COMPOSITION AND FIBER SIZE IN SUCCESSFULLY AND UNSUCCESSFULLY ENDURANCE-RACED HORSES

Triplicate biopsies from three different depths of the gluteus medius muscle were obtained in 36 endurance-raced horses, aged 8.42 +/- 2.85 yr. Twenty of the horses were considered excellent endurance performer s according to the mean speed of their three fastest records in endura nce events for the past 2 or 3 years, whereas 16 were moderate perform ers, with a mean racing speed <12.5 km/h (in 120- to 180-km endurance rides), <14 km/h (in 80- to 120-km endurance rides), or <13.5 km/h (in 40- to 60-km endurance rides). Significant differences in muscle fibe r type composition and fiber size were recorded; excellent performers had a higher percentage and a larger size of type I and type IIa fiber s (high and low myosin adenosinetriphosphatase activity at pH 4.5, res pectively) and a lower percentage of type IIb fibers (moderate myosin adenosinetriphosphatase activity at pH 4.5), including both type IIb o xidative (moderate to high NADH-tetrazolium reductase activity) and II b nonoxidative (low NADH-tetrazolium reductase activity). The differen ces in distribution of myofiber types and in fiber sizes were more mar ked in the deeper parts compared with the superficial regions of muscl e. Our results also imply a greater homogeneity among the fiber type s izes across the muscle in horses with a superior endurance performance than in horses that had been poorly or moderately endurance raced. Th us the results show that fiber type proportions and fiber size in equi ne skeletal muscle are directly related to the athletic ability of the horse for endurance events.