A study of 68 clinically healthy stallions (52 Andalusians and 16 Arab
ians) was undertaken to evaluate the effects of endurance training on
percentage and size of skeletal muscle-fiber types in adults of these
two breeds. The findings suggest that the ratio of slow- to fast-twitc
h fibers is very stable in adult horses within a given breed, and the
fact that it does not change as a result of training points to a stron
g genetic influence. With respect to fiber size, the main findings in
this study were an increase in fiber size (especially of mean lesser f
iber diameter) recorded in Type 1 and Type 2B fibers in endurance-trai
ned Andalusians, the lack of variation in Type 2A fibers between untra
ined and endurance-trained Andalusians, and the size of all types of f
ibers between untrained and endurance-trained Arabians.