Racehorse injuries and fatalities are a side effect of training and competition. The problem with equine injuries is that they so often result in death.[1][2]  A 2005 study by the United States Department of Agriculture found that injuries are the second leading cause of death in horses, second only to old age.[3] Nureyev's recovery from a broken leg while retired at stud in 1987 typifies the struggle horses have after being injured.[4]

Skeletal fractures account for 87% of fatal injuries. When a horse's leg hits the ground at racing speed on a straightaway, it bears a load that is three times its weight (with the exception of harness racing). When negotiating a turn, centrifugal force increases the load to between 5 and 10 times body weight. When a horse hits the ground, the repetitive impact produces microscopic cracks and crevices inside bone so tiny they are undetectable by standard X-rays. If the horse isn't given enough time for healthy bone tissue to repair the damage with a process called remodeling, the cumulative stress can progress silently to the point where overload causes bones to break.[34]  The American Association of Equine Practitioners (AAEP) recommends a period of rest of at least 10 days between races for all horses to provide an opportunity to refresh and diminish the volume of persistent cyclic loading.[35]