A punching bag is a sturdy bag designed to be repeatedly punched. It's used for physical exercise or stress relief. Athletically, it's used to improve one of three areas: physical strength, aerobic fitness, or punching technique.
Punching bags have been used in martial arts and swordplay for the entire written history of military training. Similar apparatus in Asian martial arts include the Okinawan makiwara and the Chinese mook jong, which may have padded striking surfaces attached to them. In martial arts and combat sports such as Karate, Taekwondo, and Muay Thai; heavy bags, standing bags, and similar apparatuses have been adapted for practicing kicking and other striking maneuvers in addition to developing punching technique.