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The studies of myofibrillar composition in athletic horses make it possible to best direct them according to their specific fitness and certainly saves time and resources in the preparation of these athletic animals. Through the technique of percutaneous muscle biopsy and different enzyme-histochemical techniques have been possible to classify these fibers in three different types, type I, IIA and IIX. This information can be applied in the selection of those horses and to the possibility the proper conduct of training protocols. In order to evaluate the myofibrillar composition of Gluteus Medius to the selected mode of Eventing by Army Riding School -RJ, thirteen horses, crossbred breeds of Hanoverian, Thoroughbred and Brazilian Equestrian, in the age range 6-12 years, were used. All animals were submitted to a percutaneous muscle biopsy procedure and the fragments subjected to enzyme-histochemical techniques for distinguishing the three different types of muscle fibers by assessing the reactivity of the enzyme adenosine triphosphate miosínica ( mATPase ) and succinate dehydrogenase ( SDH). The animals used had an average relative distribution of different types of muscle fibers apparently quite homogeneous, with 25.9 %, 35.7 % and 38.4 % of fibers of type I, IIA and IIX, respectively, but lying statistically significant difference , with a higher proportion of type II fibers compared to type I , suggesting that it was a direct genetic influence of the training squad races of the Brazilian Army .