In Kung Fu there is a definite distinction between Internal and External forms/power.
External (In Mandarin: Wai) refers to the use of muscular force or mechanical energy in the physical body. Thus movements are hard, being pushed by the muscles behind that movement. Hard punches and kicks can be seen as being generated from External Strength.
Internal (In Mandarin: Nei) refers to intrinsic power generated by the Qi or life force energy contained in the body. In the internal Martial Arts (Neijia), the use of Internal Strength is of utmost importance. Internal Strength is not generated through muscular action, but rather is a product of the inherent binding quality of body tissue. It is utilized in combat through the relaxed coordination of the legs and waist to bear on objects through contact made by the hands, arms, or other points on the upper body.