For nearly a century, physiologists erroneously believed that a buildup of lactic acid in muscle tissue was responsible for the soreness that many people experience after strenuous exercise. The acid, they claimed, is the waste produced by metabolic activity in the muscle and reaches "threshold" levels, causing soreness, when the muscle has depleted its oxygen supply. Researchers have recently discovered, however, that lactic acid is actually the fuel that powers muscular activity. Therefore, the cause of muscle soreness remains unknown.