Wrestling, one of the world's oldest sports, is offered at various levels of competition, including the Olympics, the American Athletic Union, the U.S. Wrestling Federation, and high school and college-sponsored tournaments. It's a sport for all sizes of people, and both male and female participants compete, even at the Olympic level. Competition rules require that athletes be paired against each other according to their weight class. Some competitions require that contestants be matched by age, experience, and/or gender. This not only allows more people to participate, but also decreases the risk for injury. Nevertheless, injuries do occur, particularly in the knee, shoulder, skin, and head.