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Selective breeding

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 M. Miller states that "We have selectively bred for speeds that the anatomy of the horse cannot always cope with."

Poor breeding may be encouraged by the fact that many horses are sent to the breeding shed following an injury. If the injury is linked to a conformational fault, the fault is likely to be passed to the next generation. Additionally, some breeders will have a veterinarian perform straightening procedures on a horse with crooked legs. This can help increase the horse's price at a sale and perhaps help the horse have a sounder racing career, but the genes for poor legs will still be passed on.