What is the difference between internal and external martial arts?

The external martial arts, such as kung fu, boxing, karate and tae kwon do, generate powerful movements from tense, muscles and use muscular strength, bursts of physical tension, growls and sometimes overt aggression that is often catalyzed by the release of adrenaline. The internal martial arts, for example tai chi, hsing-i, ba guaⁱ, and Aikidoⁱ, generate movements from chi energy. Practitioners’ bodies move in a relaxed, soft, and fluid manner. The internal martial arts do not encourage overt emotional aggression and anger as a primary power source. People who have difficulty comprehending how tai chi’s slow movements can translate into fast fighting will have even more difficulty understanding how fighting that does not engage emotional aggression and anger can be as or more effective than fighting techniques that do.