<P> Thoroughbreds are often crossed with horses of other breeds to create new breeds or to enhance or introduce specific qualities into existing ones . They have been influential on many modern breeds, including the American Quarter Horse, the Standardbred, and possibly the Morgan, a breed that went on to influence many of the gaited breeds in North America . Other common crosses with the Thoroughbred include crossbreeding with Arabian bloodlines to produce the Anglo - Arabian as well as with the Irish Draught to produce the Irish Sport Horse . Thoroughbreds are often crossed with various Warmblood breeds due to their refinement and performance capabilities . </P> <P> Although Thoroughbreds are seen in the hunter - jumper world and in other disciplines, modern Thoroughbreds are primarily bred for speed, and racehorses have a very high rate of accidents as well as other health problems . </P> <P> One tenth of all Thoroughbreds suffer orthopedic problems, including fractures . Current estimates indicate that there are 1.5 career - ending breakdowns for every 1,000 horses starting a race in the United States, an average of two horses per day . The State of California reported a particularly high rate of injury, 3.5 per 1000 starts . Other countries report lower rates of injury, with the United Kingdom having 0.9 injuries / 1,000 starts (1990--1999) and the courses in Victoria, Australia producing a rate of 0.44 injuries / 1,000 starts (1989--2004). Thoroughbreds also have other health concerns, including a majority of animals who are prone to bleeding from the lungs (exercise induced pulmonary hemorrhage), 10% with low fertility, and 5% with abnormally small hearts . Thoroughbreds also tend to have smaller hooves relative to their body mass than other breeds, with thin soles and walls and a lack of cartilage mass, which contributes to foot soreness, the most common source of lameness in racehorses . </P> <P> One argument for the health issues involving Thoroughbreds suggests that inbreeding is the culprit . It has also been suggested that capability for speed is enhanced in an already swift animal by raising muscle mass, a form of selective breeding that has created animals designed to win horse races . Thus, according to one postulation, the modern Thoroughbred travels faster than its skeletal structure can support . Veterinarian Robert Miller states that "We have selectively bred for speeds that the anatomy of the horse cannot always cope with ." </P>

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