Calisthenics is the oldest form of resistance training using only body weight. It was practiced by ancient Spartan armies and Greeks who valued physical strength, beauty, and proportion. Calisthenics experienced a golden age in the 1800s and 1950s but then declined as bodybuilding focused solely on muscle size using steroids and weights instead of natural whole-body development. It has endured for millennia due to its simplicity and effectiveness without equipment.