EFFECT OF TRAINING AND DETRAINING ON MUSCLE-FIBER SIZE IN SEVERAL HORSE BREEDS

To determine the effect of training on muscle fiber size, stallions of different breeds (8 Andalusians, 7 Arabians and 5 Anglo-arabians) whose ages ranged between 5 and 14 years, were subject to a conventional 3 month long endurance training programme. Muscle biopsy samples were taken from the right gluteus medius muscle at depths of 2 (superficial region) and 6 (deep region) cm below the fascia, before training, after training and 3 months after detraining. They were histochemically and morphometrically analyzed, and the mean cross-sectional area and the least fiber diameter of the various fiber types (I, IIA, IIB oxidative and IIB non-oxidative) were calculated for each muscle biopsy specimen. A significant hypertrophy of myofibers of types I and IIA was observed after training in Andalusian and Arabian horses, particularly in the deep region of the muscle. In general, significant adaptations to training were not recorded in Anglo-arabian horses with respect to fiber size. But, a clear tendency towards a decreased type II fiber size was found in the superficial region of the muscle. Some of these muscle adaptations to training were maintained after 3 months of detraining. These results suggest that the size of the most oxidative fibers could be increased by means of a conventional endurance-training programme.