racing and football. Horseback riding has a higher hospital admittance rate per hours of riding than motorcycle racing, at 0.49 per thousand hours of riding and 0.14 accidents per thousand hours, respectively. Head injuries are especially traumatic in horseback riding. About two-thirds of all riders requiring hospitalization after a fall have sustained a traumatic brain injury. Falling from a horse without wearing a helmet is comparable to being struck by a car. Most falling deaths are caused by head injury. The use of riding helmets substantially decreases the likelihood and severity of head injuries. When a rider falls with a helmet, he or she is five times less likely to experience a traumatic brain injury than a rider who falls without a helmet. Helmets work by crushing on impact and extending the length of time it takes the head to stop moving. Despite this, helmet usage rates in North America are estimated to be between eight and twenty percent. Once a helmet has sustained an impact from falling, that part of the helmet is structurally weakened, even if no visible damage is present. Helmet manufacturers recommend that a helmet that has undergone impact from a fall be replaced immediately. In addition, helmets should be replaced every three to five years; specific recommendations vary by manufacturer. Rules on helmet use in competition Many organizations mandate helmet use in competition or on show grounds, and rules have continually moved in the direction of requiring helmet use. In 2011, the United States Equestrian Federation passed a rule making helmet use mandatory while mounted on competition grounds at U.S. nationally rated eventing competitions. Also in 2011, the United States Dressage Federation made helmet use in competition mandatory for all riders under 18 and all riders who are riding any test at Fourth Level and below. If a rider competing at Prix St. Georges and above is also riding a test at Fourth Level or below, he or she must also wear a helmet at all times while mounted. Riding astride The idea that riding a horse astride could injure a woman's sex organs is a historic, but sometimes popular even today, misunderstanding or misconception, particularly that riding astride can damage the hymen. Evidence of injury to any female sex organs is scant. In female high-level athletes, trauma to the perineum is rare and is associated with certain sports (see Pelvic floor#Clinical significance). The type