Founded in 1983, 24 Hour Fitness USA Inc. is one of the largest fitness club chains in the United States. Based in Chino Hills, Calif., the center provides customizable personal training services, group exercise classes and cardio/functional training equipment. 24 Hour Fitness USA Inc. also conducts classes for kick boxing, water exercise, strength training and dance. The center serves four million members in more than 420 clubs. In 2005, Forstmann Little & Co., an equity firm acquired 24 Hour Fitness USA Inc. It has partnerships with the U.S. Olympic team, San Francisco 49ers and New York Yankees, among others.