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. 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."

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.). 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 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.

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. The contact point only represents the gateway to conduct Neijin 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 Neijin is said to continually grow as long as one lives and always practices in 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: