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In advanced traditional Chinese kung fu (martial arts), Neijing (Traditional Chinese: 內勁; pinyin: nèijìng) refers to the conscious control of the practitioner's qi, or "life energy", to gain advantages in combat. Nèijìng is developed by using "Neigong" (Traditional Chinese: 內功; pinyin: nèigōng) (內功), or "internal exercises," as opposed to "wàigōng" (外功), "external exercises."
Li vs. Neijing
Practitioners of kung fu refer to two separate forms of personal force: Li (Traditional Chinese: 力) refers to the more elementary use of tangible physical (or "external") force, such as that produced by muscles. Neijing (Traditional Chinese:內勁) or Neigong (Traditional Chinese: 內功), in contrast, refer to "internal" forces produced via advanced mental control over psychic energy (the qi).
The degree of Li force one can employ in kung fu depends on several variables such as resilience of muscles, strength of bones, speed and timing of attack and so on. An effective way to enhance the Li force is to exercise one’s muscles and bones by applying increasing pressure on them (weight training, gym exercises, etc.). The stronger one’s muscles and bones become, the more powerful and skillful the level of kung fu is.
On the other hand, the level of the Neijing force depends on the extent one can exercise over one’s will power to release an inner qi energy. Within the framework of Chinese martial arts, every person is believed to possess the inborn energy of qi. Martial artists can harness the force of qi so that it is strong enough to be applied in combat. When qi is being directed by one’s will, it is called Neijing.
The Li force is observable when it is employed. Unlike the Li force, Neijing is said to be invisible. The "pivot point" essential to Li combat is not necessary in Neijing. At the point of attack, one must ‘song’ (loosen) himself to generate all Neijing energy one possesses and direct this energy stream through one’s contact point with an opponent. The contact point only represents the gateway to conduct Neijing energy at the point of attack.
The kung fu component of Li force is limited by one’s physical condition. When a person passes his/her prime age, one’s kung fu ability will pass the optimum level, too. The degree of kung fu will decline when muscles and bones are not as strong as they used to be. On the other hand, the kung fu aspect of Neijing is said to continually grow as long as one lives. 
The key to unlock and nurture Neijing is said to be the practice of ‘song’ (Traditional Chinese: 鬆 ). The term ‘song’ can function as a verb which means to keep one’s mind and body loose resilient and expanding like the consistency of cotton or clouds or relaxed yet concentrated like the sharp alertness of cats immediately before attack. The term can also be used as an adjective which has the same meaning as described above. The greater the extent one can achieve ‘song’ and minimize the use of Li, the greater the release of Neijing force.
Neijing trainees are often reminded to refrain from using the Li force, because the energy of Neijing will be locked and blocked whenever the Li force is applied. So, Neijing and Li are said to be mutually exclusive.
The Taijiquan master Yang Chengfu used the concept of ‘song’ as a benchmark in his daily teaching. It was his daily routine to keep reminding his disciples to ‘song’ thoroughly more than 10 times when he inspected them.
- To put in Master Ng Tunan’s words in Chinese,「識得內功休再問，貫徹拳經千萬篇」。吳圖南。《太極拳之研究：吳圖南太極功/ 吳圖南講授 ; 馬有清編著》。香港：商務印書館，2004。頁11 Wu Tunan, Tai Ji Quan Zhi Yan Jiu: Wu Tunan Tai Ji Gong/ Wu Tunan Jiang Shou; Ma Youqing Bian Zhu, Hong Kong: The Commercial Press (H.K.) Ltd, 2004, page 11
- 陳靖。《拳道中等學 : 太極拳鍊與悟》。香港：陳靖，第三版：2004年。頁89。 Chen Jing, Quan dao zhong deng xue: Tai ji quan lian yu wu (Insights in Practice on TaiJi), Hong Kong: Chen Jing, 2004, page 89
- 陳靖。《拳道要略》。香港：陳靖，修訂版：2005年12月。頁21至24。 Chen Jing,Quan dao yao lue (Principles of Quan Dao), Hong Kong: Chen Jing, 2004, pp. 21-24
- 陳靖。《拳道中等學 : 太極拳鍊與悟》。香港：陳靖，第三版：2004年。頁38至41。 Chen Jing, Quan dao zhong deng xue: Tai ji quan lian yu wu (Insights in Practice on TaiJi), Hong Kong: Chen Jing, 2004, pp 38-41
- 陳靖。《鑿琢順軌太極拳論評說. 1, 王宗岳太極拳論》。香港：陳靖，2002。頁38，79至82。 Chen Jing, Zuo zhuo shun gui tai ji quan lun ping shuo zhi yi.1, Wang Zong Yue Tai ji quan lun (Commentary on Wang Zong Yue¡¦s Tai Chi Theory), Hong Kong: Chen Jing, 2002, page 38 and pp. 79-82
- 陳靖。《拳道要略》。香港：陳靖，修訂版：2005年12月。頁25至38。 Chen Jing,Quan dao yao lue (Principles of Quan Dao), Hong Kong: Chen Jing, 2005, pp. 25-38
- 陳靖。《拳道要略》。香港：陳靖，修訂版：2005年12月。頁22，33至34。 Chen Jing,Quan dao yao lue (Principles of Quan Dao), Hong Kong: Chen Jing, 2005, page 22 and pp. 33-34
- To put in Master Ng Tunan’s words in Chinese,「鬆者，蓬鬆也；寛而不緊也；輕鬆也；放開也；輕鬆暢快也；不堅凝也。含有小孔以容其他物質之特性也」。吳圖南。《太極拳之研究：吳圖南太極功/ 吳圖南講授 ; 馬有清編著》。香港：商務印書館，2004。頁12。 Wu Tunan, Tai Ji Quan Zhi Yan Jiu: Wu Tunan Tai Ji Gong/ Wu Tunan Jiang Shou; Ma Youqing Bian Zhu, Hong Kong: The Commercial Press (H.K.) Ltd, 2004, page 12
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- 陳靖。《拳道中等學 : 太極拳鍊與悟》。香港：陳靖，第三版：2004年。頁43至54，頁89。 Chen Jing, Quan dao zhong deng xue: Tai ji quan lian yu wu (Insights in Practice on TaiJi), Hong Kong: Chen Jing, 2004, pp. 43-54 and page 89
- In Chinese, 「要鬆，要鬆，要鬆凈。要全身鬆開。」「不鬆，不鬆，不鬆就是挨打的架子。」, 鄭曼青。《鄭子太極拳十三篇》。台北：時中拳社，1950年。頁45。 Zheng, Manqing. Zheng zi tai ji quan shi san pian, [Taibei] : Shi zhong quan she, Min 39 , page 45
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