larger hind limb muscles than the Arabian, which make it less suitable for endurance racing. It also has more type II-b muscle fibers, which allow the Quarter Horse to accelerate rapidly. When Quarter Horse racing began, it was very expensive to lay a full mile of track so it was agreed that a straight track of four hundred meters, or one-quarter of a mile, would be laid instead. It became the standard racing distance for Quarter Horses and inspired their name. With the exception of the longer, distance contests, Quarter Horse races are run flat out, with the horses running at top speed for the duration. There is less jockeying for position, as turns are rare, and many races end with several contestants grouped together at the wire. The track surface is similar to that of Thoroughbred racing and usually consists of dirt. In addition to the three main racing breeds above and their crosses, horse racing may be conducted using various other breeds: Appaloosa, American Paint Horse, mules, Selle Français, AQPS and Korean Jeju. Horse breeds and muscle structure Muscles are bundles of contractile fibers that are attached to bones by tendons. These bundles have different types of fibers within them, and horses have adapted over the years to produce different amounts of these fibers. Type 1 Type I muscle fibers are adapted for aerobic exercise and rely on the presence of oxygen. They are slow-twitch fibers. They allow muscles to work for longer periods of time resulting in greater endurance. Type 2 Type II muscles are adapted for anaerobic exercise because they can function in the absence of oxygen. Type II-a fibers are intermediate, representing a balance between the fast-twitch fibers and the slow-twitch fibers. They allow the muscles to generate both speed and endurance. Thoroughbreds possess more Type II-a muscle fibers than Quarter Horses or Arabians. This type of fiber allows them to propel themselves forward at great speeds and maintain it for an extended distance. Type II-b fibers are fast-twitch fibers. These fibers allow muscles to contract quickly, resulting in a great deal of power and speed. Training The conditioning program for the horses varies depending on the race length. Genetics, training, age, and skeletal soundness are all factors that contribute to a horse's performance. The muscle structure and fiber type of horses depends on the breed; therefore, genetics must be considered when constructing a conditioning