EFFECT OF TRAINING AND DETRAINING ON MUSC LE-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) who se 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, aft er training and 3 months after detraining. They were histochemically a nd morphometrically analyzed, and the mean cross-sectional area and th e least fiber diameter of the various fiber types (I, IIA, IIB oxidati ve and IIB non-oxidative) were calculated for each muscle biopsy speci men. A significant hypertrophy of myofibers of types I and IIA was obs erved 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 fi ber size. But, a clear tendency towards a decreased type II fiber size was found in the superficial region of the muscle. Some of these musc le adaptations to training were maintained after 3 months of detrainin g. These results suggest that the size of the most oxidative fibers co uld be increased by means of a conventional endurance-training program me.