Japanese form of wrestling.A contestant loses if he is forced out of the ring (a 15-ft circle) or if any part of his body except the soles of his feet touches the ground. In sumo, a wrestler's weight, size, and strength are of the greatest importance, though speed and suddenness of attack are also useful. The wrestlers, who are fed a special protein diet and may weigh over 300 lbs (136 kg), wear only loincloths and grip each other by the belt. Sumo wrestling is an ancient sport with a complex system of ranking; at the top of the hierarchy is the yokozuna (grand champion). Lengthy rituals and elaborate posturings accompany the bouts, which are extremely brief, often lasting only a few seconds.