Sumo is a Japanese-style of wrestling and Japan's national sport. It originated in ancient times as a performance to entertain the Shinto deities. Many rituals with religious background, such as the symbolic purification of the ring with salt, are still followed today. In line with tradition, only men practice the sport professionally in Japan.

The rules are simple: the wrestler who first exits the ring or touches the ground with any part of his body besides the soles of his feet loses. Matches take place on an elevated ring (dohyo), which is made of clay and covered in a layer of sand. A contest usually lasts only a few seconds, but in rare cases can take a minute or more. There are no weight restrictions or classes in sumo, meaning that wrestlers can easily find themselves matched off against someone many times their size. As a result, weight gain is an essential part of sumo training.