Effect of show jumping training on the development of locomotory muscle in young horses.

Objective—To investigate whether training for show jumping that is commenced early after birth affects the characteristics of equine locomotory muscle. Animals—19 Dutch Warmblood horses. Procedures—Horses were assigned to a trained or not trained (control) group. After weaning, training (free jumping [2 d/wk] that was alternated with a 20-minute period of exercise in a mechanical rotating walker [3 d/wk]) was started and continued until horses were 3 years old. Fiber type composition (determined from myosin heavy chain [MyHC] content), fiber area, diffusion index (area supplied by 1 capillary), citrate synthase activity, and Na+,K+-ATPase content were assessed in gluteus medius muscle specimens collected at 0.5, 1, 2, and 3 years. Results—Developmental changes included an increase in MyHC fiber type IIa and a decrease in type IIad; increases in fiber area, diffusion index, and citrate synthase activity; and a decrease in Na+,K+-ATPase content. The MyHC fiber type I and type IId were detected in high and l...