Kettlebells, one of the oldest pieces of gym equipment around, have become one of the hottest workouts today. Weighing up to 200 pounds, kettlebells originally were used by soldiers and body builders to build muscles. . "It's probably the only workout in which each exercise works nearly every muscle set in the body," says Ray Anderson of Personal Fitness Ltd. In Allentown, Pa., in an interview with McClatchy Newspapers.