Nei Jia 內家 literally means “inner family or inner school.” It is a term used to refer to styles of martial arts that in English we call “internal,” particularly if we add the character for “fist” resulting in the term 內家拳 Nei Jia Quan. Today these styles are considered to include Tai Ji Quan, Xing Yi Quan, Ba Gua Zhang, Tong Bei Quan, Yi Quan (Da Cheng Quan) and Liu Ho Ba Fa. Other styles are sometimes called internal including the Japanese art of Aikido.
At first glance, the primary difference between internal and external martial arts seems to be one of method. Speaking generally, the focus of internal arts is on principles rather than specific techniques. Internal arts have techniques, but from the very beginning it is understood that techniques are merely expression of the principles and that the ultimate goal is to create techniques in the moment out of the interaction of one’s energy and intention with the opponent’s energy and intention. Secondly, while generally speaking the external arts focus their training methods on developing muscular strength, speed and athletic prowess, internal arts stress relaxation, mind-intention, stillness and natural movement. Lastly the internal arts use alignment, breath and structural dynamics to actualize the movement of the vital force through the channels and collaterals (Jing Luo) or meridians. This is said to cultivate “whole body power” which does not rely on muscular strength, speed and athleticism. This idea has considerable overlap with the idea of body mechanics – bio-mechanical principles of movement that increase efficiency. However, the two concepts are not identical.
See the article and video series entitled: What is an Internal Martial Art?