Skeletal muscle fiber size in untrained and endurance-trained horses

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 moderately 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 micrometers(2) and 3,150 +/- 370 micrometers(2); type 2A, 3,819 +/- 890 micrometers(2) and 3,380 +/- 356 micrometers(2); and type 2B, 4,872 +/- 962 micrometers(2) and 4,417 +/- 646 micrometers(2) or minimal diameter (type 1, 52.2 +/- 7.4 micrometers and 52.8 +/- 3.1 micrometers; type 2A, 58.1 +/- 6.7 micrometers and 55.0 +/- 2.8 micrometers; and type 2B, 65.3 +/- 6.4 micrometers and 63.4 +/- 4.3 micrometers) of the 3 fiber types, nor between untrained and endurance-trained Andalusians with respect to the area (untrained, 3,990 +/- 690 micrometers(2) moderately endurance-trained, 3,882 +/- 347 micrometers(2); and strongly endurance-trained, 3,758 +/- 510 micrometers(2)) and minimal diameter (untrained, 58.1 +/- 4.7 micrometers; moderately endurance-trained, 59.7 +/- 2.7 micrometers; and strongly endurance-trained, 58.7 +/- 4.5 micrometers) 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.