You can tell that a quarter horse and a thoroughbred are two distinct breeds merely by looking at them. What is it that really catches your eye? Muscles! What we're talking about here are "skeletal muscles," muscles that surround the limbs. These are also called voluntary muscles because they are consciously and actively used to move.

Whether in horses or in humans, muscles are really just a giant bundle of stringy fibers gathered together at their ends and attached by tendons to bone. But it's not that simple because there are more than one "type" of fibers in each bundle. Animals have actually evolved to have different amounts of each fiber type in their bundles, depending on what they need to do. In this respect, horses differ from goats and kangaroos, and quarter horses differ from thoroughbreds.

Some muscles are made to contract quickly and explosively, resulting in power, strength, and immediate speed. These are called the "fast twitch" or Type IIb fibers.

Other muscles are meant to work steadily, without fatigue, for many hours, resulting in great endurance. These are classified as Type I, or slow-twitch

A third type of muscle fiber in the body is an intermediate, able to generate both rapid power and endurance ("Type IIa). Humans have the same mix of fiber types.

Horses that are meant for quick power and speed, such as quarter horses, will possess a greater proportion of Type IIb fast-twitch muscle fibers. Horses that are meant for endurance, such as Arabians, possess a greater proportion of Type I muscle fibers. Horses that are meant for both great speed and (relative) endurance, such as thoroughbreds, possess a greater proportion of Type I and IIa fibers, the slow and intermediate types.

The cellular makeup of the muscle fibers themselves, and the energy source that the muscle cell uses, that determine the type of contraction that they produce. The type of energy source that the muscle uses, in turn, determines the amount of blood supply to the muscle. Type I muscles are absolutely reliant on the presence of oxygen in order to work. They are necessary for aerobic exercise. Type II muscles can function without the presence of oxygen, and are necessary for anaerobic exercise

Muscle fiber types do not change with the type of training that the horse receives – it is actually the nerve supply during development of the muscle that determines its fiber type. However, work will enlarge or "hypertophy" certain types of muscles. For example, Type II fibers will respond to power work, like pumping iron in the gym, and Type I by long slow work analogous to running.