SKELETAL MUSCLE FIBER DISTRIBUTION AND AREA IN TRAINED AND STALLED STANDARDBRED HORSES

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Abstract
Four Standardbred horses followed a daily interval training program (5 × 700 m and 6 × 1000 m with 5-min rest periods) for 8 wk. Four more randomly selected animals served as stalled controls. Biopsy samples were taken from six limb muscles before and after the training period from all eight animals. No changes were observed in the percentage of fiber types for either group. However, significant (P < 0.05) increases in fiber area were noted for all three fiber types: slow twitch, high oxidative (ST) 13–17%; fast twitch, high oxidative (FTH) 13–17%; and fast twitch, low oxidative (FT) 21–29% for the six limb muscles in the trained animals. The results suggest that fiber area can be changed rather quickly with enforced exercise, and that FT fibers adapt more readily to high intensity interval exercise.