In Chinese medicine, human body is considered as a microcosm of the universe. Humans stand on the earth with their feet and there is the heaven above their head.
Chinese medicine conceptualizes head of a human as heaven and legs as the earth. The earth, which sustains everything on it, is heavy and stable. The heaven is light and encompasses everything. Just as the earth is heavy and stable, in ideal physical condition of a human body, legs must be strong and sustain its body. Just as heaven is light, head of a human body must be light and feel no pressure.
The 70% of body strength must reside in legs and the 30% of body strength must be in the upper body. By strengthening your legs and emptying your mind, you can be healthy. In Chinese martial arts, it is important that 70%of your strength is in your legs and 30% of your strength is in your upper body. By having 70% of your strength in your legs, you can stand stably and move agilely.
The heaven being light and the earth being heavy is the natural state of the universe. By applying this natural state on your body, which is the microcosm of the universe, you can be healthy and do self defense well.
This post is based on Yellow Emperor's Classic of Medicine「皇帝内経」参照