In advanced traditional Chinese kung fu (martial arts), Neijin (Traditional Chinese: 內勁; pinyin: nèijìn) refers to the conscious control of the practitioner's qi, or "life energy" via strengthening self physical posture, to gain power delivery capability and efficiency advantages in combat.To put in Master Wu 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 Nèijìn 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. Neijin 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. Neijin (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.).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 The stronger one's muscles and bones become, the more powerful and skillful the level of kung fu is.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
On the other hand, the level of the Neijin 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 Neijin.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
The Li force is observable when it is employed. Unlike the Li force, Neijin is said to be invisible. Both Li and Neijin need the "pivot point", but Neijin is intelligent enough to deliver contact force near muscle force when able to minimize the resistance arm length of self body's mechanical disadvantage lever. At the point of attack, one must ‘song’ (loosen) himself to change the contact force angle and position for proper lever control, to generate all Neijin energy one possesses and to direct this energy stream through one's contact point with an opponent.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 The contact point only represents the gateway to conduct Neijin energy at the point of attack.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
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 Neijin is said to continually grow as long as one lives and always practices in lever control.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
Net force and lever control Self controllable physical elements in martial art include: mass, muscle strength, and lever
Neijin needs net force control to maximize the force constraint based on one's muscle strength training and lever control to maximize the muscle force delivery efficiency. The following joint safety is also important for Neijin:
1. For extension, reserve flexion angle and prevent overextension. All extended strikes, whether they successfully hit the opponent or are unexpectedly redirected by them, must be quickly retracted to continue maintaining defense, evasion, and readiness for the next strike.
2. For active strike or push-pull contact actions, keep parallel relation between shoulder/hip torque and elbow/knee pivot, and avoid perpendicular relation(only able for defense posture or slipping direction adjustment) between them for forearm/palm and lower leg/sole not to be controlled.
Net force control Whatever one is going to strike, block or dodge(away from opponent or perpendicular to the distance between one and opponent), the following force sources for temporary self body mass or momentum increase can resist, stop the effect of contact force applied by opponent and even control self body's net force actively:
self weight
ground friction
normal reaction forces from standing ground and holding wall, handrail, and opponent body
Since the coefficient of friction changes with the environment, frictional force becomes unstable. Direct normal force is more reliable and allows muscular strength to be exerted more effectively. If one can only rely on ground friction, the following short‑term conditions must be met in order to maximize impact force close to the limit of ground friction:
Assuming the ground is not wet or slippery, the preferred method of pushing off is first with the heel and then naturally with the forefoot. The secondary option is to use the forefoot directly. This reduces the chance of ankle joint misalignment or tendon strain.
The rear foot serves as the driving foot relying on friction, while the front foot acts as the braking foot. First use the iliopsoas muscle or the forefoot to push against the ground and lift the braking foot, allowing the driving foot and the body’s center of gravity to lean freely. This prevents excessive pressure on the ankle, excessive pulling of the hip and thigh tendons, or unexpected calf cramps, thereby safely accelerating the center of gravity.
At the moment when imbalance causes the center of gravity to nearly topple, the ground‑contact position of the limb and the tilt of the body’s center of gravity directly affect the speed and direction of the center of gravity’s propulsion when that limb pushes off.
While the center of gravity is accelerating and the braking foot has not yet landed, use the hand or foot to strike in a straight line close to the direction of forward movement. This raises the impact force to approach muscular strength, or even approach the limit of ground friction. Regardless of whether the strike lands, the striking limb must be quickly retracted. Then, using the iliopsoas and gluteal muscles, control the braking foot so that the whole sole or ankle lands first (reducing the risk of sprains from improper landing on uneven ground). Continue maintaining defense and prepare for the next strike.
Additionally, in any state where one’s own free fall or projectile motion is intentionally or unintentionally triggered, the following points must be noted:
In such a state, the greatest or only net force source is gravity.
The predetermined trajectory of the projectile motion cannot be changed until landing, touching, or colliding with an external object, whether expected or unexpected.
Lever control Since effort arm length formed by muscle attachment and relevant leverage joint is short and fixed, when one is able to maintain the best mechanical disadvantage lever with the following two conditions by minimizing resistance arm in terms of pivot joints (e.g, center of gravity, shoulder and hip, elbow and knee), the contact force can be near the muscle force:
1. Minimize the direction difference between net force control, contact force and pivot joints movements
2. Minimize the perpendicular distance between the paths of net force control, contact force and pivot joints movements
The worst mechanical disadvantage lever, which violates the two conditions and lengthens the resistance arm to make contact force significantly lower than the muscle force, should be avoided in self body. With proper limb strike/catch angle and timing, keeping the best mechanical disadvantage lever to control opponent's worst mechanical disadvantage lever by maximizing their resistance arm will take the best mechanical advantage lever. Therefore, one's longer arm span (wingspan or reach) or leg length allows longer striking distance, but also allows opponent to take longer effort arm for destabilizing in striking or controlling in grappling if the one's corresponding muscles are not strong enough to resist. Besides, self body creating mechanical advantage lever involves combining own limbs, allowing the direction of combined force to be changed at any time (direct normal force is preferable). This maintains one limb with a shorter resistance arm to push or pull another limb with a longer resistance arm, a method commonly seen in grappling or throwing techniques as auxiliary locking or suppression maneuvers.
Through these control methods, nutritional intake can be transferred efficiently to Neijin.
Sports example The Neijin physical body structures with best self body mechanical disadvantage lever can be seen in following sports examples:
Hands and legs in series(net force control and contact force are near parallel to spine)
Hands in series(net force control and contact force are near parallel to shoulders distance)
Hands and legs in parallel
Hands in parallel
Legs in parallel
The above exercise examples not only allow one to experience the concepts of joint safety in internal force and the optimization of leverage to reduce wasted muscular effort, but they can also be extended to the level of mechanical operation to appreciate the convenience of using transport tools to assist human labor. For instance, a handcart can replace manual carrying, a forklift can further use energy to drive transportation, and drones can go even further with remote control operation. However, because the leverage effects of tools or machines directly challenge material endurance and balance control, accidents and injuries may occur. Therefore, it is essential to establish and follow safety operating guidelines, strengthen awareness of mechanical safety, and enforce licensing systems to reduce the risk of accidents.
Lever in striking techniques (without capture) Striking techniques generally employ evasion and blocking. These not only reduce the opponent’s chance of striking one when one is in a weaker leverage state, but also increase one own chance of striking the opponent when they are in a weaker leverage state, potentially causing them to lose balance. Among these, the flexion and extension angles of hinge joints such as the elbow and knee, and their relationship to internal force leverage distance, have different tactical functions:
1. Elbow/knee joint at or greater than a right angle
Commonly used in straight punch, pushing palm, ulna near elbow strike, pushing kick/side kick, tibia near knee strike, or positional evasion.
The distance between the contact force point and the shoulder/hip becomes nearly parallel to the direction of applied force (short resistance arm), placing the body in a stronger leverage state.