The Nei Jin reference article from the English Wikipedia on 24-Apr-2004
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Nei Jin

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Pinyin: nèi jìn; an internal power or coordination said to be built through Chinese Wu Shu (martial arts). Proponents say that it is characterized by elasticity, flexibilty, natural coordinated leverage through the joints, and has nothing to do with brute force. Chinese "soft style" martial artists claim that as one ages the power of muscles expanding and contracting in tension decreases but nei jin and its resulting leverage will increase if it is cultivated assiduously.

It is said to be a special feature of Chinese soft styles, alleged to be rarely, if at all, found in other martial arts, although different schools have differing definitions of the term.

In general, the training used to cultivate nei jin is called "nèi gōng," in distinction to external training that is known as "wài gōng."

See also: Chinese martial arts, Kung Fu, Taijiquan, Hsing Yi, Bagua zhang, Nei chia, Qi, Qigong.