Adaptation of Equine Locomotor Muscle Fiber Types to Endurance and Intensive High Speed Training

Skeletal muscle constitutes the largest organ system in the mammalian body and is essential for movement and force generation. Muscle tissue has the unique ability to adapt and remodel with regular exercise. Adaptation of equine contractile apparatus to exercise training with a different character occurs at the structural to the cellular and molecular levels and depends on age, breed, and sex. In Andalusian and Arabian horses from 3 months to 24 years of age, it was found that the mean cross-sectional area occupied by IIA and IIX fibers was greater in stallions than in mares. In muscle of Dutch Warmblood foals from birth to 1 year of age, a significant number of fibers coexpress either developmental and type IIa or cardiac–alpha and type I myosin heavy chain (MyHC) isoforms. Endurance training results in increased mitochondrial density, capillary supply, changes in key metabolic enzymes, and increased maximal oxygen uptake and promotes a transition from type II to type I muscle fiber. In horses, prolonged aerobic exercise training has been shown to induce a further decline in the percentage of type IIx MyHC isoform expression and an increase of type I and IIa MyHC isoform expression. Short-duration, high-intensity exercise training stimulates type IIA and hybrid (IIA/IIX) fibers. Therefore, intensive high-speed trotting facilitates muscle fiber hypertrophy and increases the oxidative capacity of type IIX fibers.