The martial arts are perhaps best known as means of self-defense, but they also are used to improve physical fitness and promote mental and spiritual development. The highly disciplined movements and forms are thought to unite body and mind and to bring balance to the individual's life. External methods (such as karate and judo) stress endurance and muscular strength, whereas internal methods (such as tai chi and aikido) stress relaxation and control. Tai chi has been used as part of treatment for back problems, ulcers, and stress.