Training and growth induced changes in the middle gluteal muscle of young Standardbred trotters

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Abstract
The middle gluteal muscle of five, 2-yr-old untrained trotters was investigated by repeated needle biopsy sampling over a training period of 6 mo. A 2nd group of five, 3-yr-old untrained horses was included to examine the effect of growth. After the training period increases were found in the relative distribution of slow twitch (ST) fibers from 18-25% and fast twitch (FTa) fibers from 36-45%, and a decrease in FTb fibers from 46-30%. A proportionally equal reduction (.apprx. 18%) in the cross sectional area of all fiber types was observed after the first 2 mo. of training succeeded by an increase to approximately pretraining levels at the end of the period. The number of capillaries per fiber was enhanced from 1.7-2.4. Proliferation of capillaries occurred around fibers of all types. The metabolic adaptations showed increases in the activities of 3-hydroxy-acyl-CoA dehydrogenase (HAD) (50%) and citrate synthase (31%). Growth had no effect on the relative fiber type distribution nor the capillary per fiber ratio, but as the mean fiber area increased 36% (primarily because of increases in FT fibers) the number of capillaries/mm2 was lower in the older untrained horses (350 capillaries/mm2) compared with the younger untrained ones (460 capillaries/mm2). Increase with growth was found in the activity of phosphorylase and HAD and a decrease was seen in the activity of hexokinase. The training program exclusively induced alterations which improved the aerobic capacity of the muscle.