Skeletal muscle fiber size in untrained and endurance-trained horses

Abstract

Summary The mean area and minimal diameter of 3 histochemically determined myofiber types (1, 2A, and 2B; myosin ATPase in acid buffer) were calculated in middle gluteal muscle biopsy specimens from 62 stallions, 47 Andalusians and 15 Arabians, ranging in age from 6 to 12 years. Fourteen Andalusians and 7 Arabians were untrained, and the remainder were actively endurance-trained. The 6-month training schedules involved walking, slow trotting, and cantering. Fourteen Andalusians were moder-ately endurance-trained, whereas the other 19 Andalusians and 8 Arabians were strongly endurance-trained. Significant differences were not recorded between untrained and endurance-trained Arabians with respect to the area (type 1, 3,194 ± 869 μm2 and 3,150 ± 370 μm2; type 2A, 3,819 ± 890 μm2 and 3,380 ± 356 μm2; and type 2B, 4,872 ± 962 μm2 and 4,417 ± 646 μm2) or minimal diameter (type 1, 52.2 ± 7.4 μm and 52.8 ±3.1 μm; type 2A, 58.1 ± 6.7 μm and 55.0 ± 2.8 μm; and type 2B, 65.3 ± 6.4μm and 63.4 ± 4.3 μm) of the 3 fiber types, nor between untrained and endurance-trained Andalusians with respect to the area (untrained, 3,990 ± 690 μm2; moderately endurance-trained, 3,882 ± 347 μm2; and strongly endurance-trained, 3,758 ± 510 μm2) and minimal diameter (untrained, 58.1 ± 4.7 μm; moderately endurance-trained, 59.7 ± 2.7 μm; and strongly endurance-trained, 58.7 ± 4.5 μm) of 2A fibers. Moderately endurance-trained Andalusians had larger 2B fibers (area 18.6%; minimal diameter 14.3%) than untrained Andalusians, and the minimal diameter of type-1 and type-2B fibers was greater in strongly endurance-trained Andalusians than in untrained Andalusians (9.5% for type 1; 10.0% for type 2B). These results suggest minimal effects of endurance training on the myofiber size of adult Andalusians and Arabians, but a tendency toward increased type-1 and -2B fiber size was found for Andalusians.