plan. A horse's fitness plan must be coordinated properly in order to prevent injury or lameness. If these are to occur, they may negatively affect a horse's willingness to learn. Sprinting exercises are appropriate for training two-year-old racehorses, but the number is limited by psychological factors as well as physical. A horse's skeletal system adapts to the exercise it receives. Because the skeletal system does not reach full maturity until the horse is at least six years of age, young racehorses often suffer injuries. Horse racing by continent North America United States In the United States, Thoroughbred flat races are run on surfaces of either dirt, synthetic or turf. Other tracks offer Quarter Horse racing and Standardbred racing, on combinations of these three types of racing surfaces. Racing of other breeds, such as Arabian horse racing, is found on a limited basis. American Thoroughbred races are run at a wide variety of distances, most commonly from ; with this in mind, breeders of Thoroughbred race horses attempt to breed horses that excel at a particular distance (see dosage index). Horse racing in the United States and on the North American continent dates back to 1665, which saw the establishment of the Newmarket course in Salisbury, New York, a section of what is now known as the Hempstead Plains of Long Island, New York. This first racing meet in North America was supervised by New York's colonial governor, Richard Nicolls. The area is now occupied by the present Nassau County, New York, a region of Greater Westbury and East Garden City. The South Westbury section is still known as Salisbury. The first record of quarter-mile length races dated back to 1674 in Henrico County, Virginia. Each race consisted of only two horses, and they raced down the village streets and lanes. The Quarter Horse received its name from the length of the race. The American Stud Book was started in 1868, prompting the beginning of organized horse racing in the United States. There were 314 tracks operating in the United States by 1890; and in 1894, the American Jockey Club was formed. The Pleasanton Fairgrounds Racetrack at the Alameda County Fairgrounds is the oldest remaining horse racing track in America, dating from 1858, when it was founded by the sons of the Spaniard Don Agustín Bernal. Belmont Park is located at the western edge of the Hempstead Plains. Its mile-and-a-half main