There is a relationship between fiber types and breed with respect to performance. Horses bred for slower, but longer duration events, such as the Arabian horse, have a higher percentage of slow oxidative fibers. Different breeds will generally fall out where one would expect. In establishing these fiber type distributions, care must be taken in sampling from the same muscle and the same relative depth. The reliance on fiber types for picking a performance animal can quickly lead to spurious results.
Fibers can change their characterization with training.
There are instances in which fibers may change from type I to II or vice versa, but in general, there is little conversion on the basis of speed. However, muscle fibers show great plasticity in their oxidative capacity. If muscle are conditioned to long, slow workouts, mitochondrial enzymes, such as citrate synthase and HOAD, are increased without a concomitant change in the fiber size. In contrast, high intensity workouts stress the anaerobic machinery and will not produce increases in the mitochondria. In these fibers, the stimulus is to increase the amount of myosin and actin.