The center of mass is an important concept in all the martial arts. In Chinese, it is known as the Dan Tian. The outer Dan Tian refers to an acupuncture point a few inches below the naval. The inner Dan Tian is visualized as a ball inside the abdomen and roughly corresponds to the fascial layer surrounding the abdominal cavity. This "ball" lies just under the diaphragm and breathing greatly influences its shape and position. Many energy skill exercises focus on contracting, expanding, and rolling this "ball" in conjunction with breathing. In Eastern medical theory, energy is flowing around and through this "ball." In terms of Western physics, the center of mass is changing as a result of abdominal rotation and angular motion. When coupled with proper stability this angular motion allows great force to be expelled. If there is no stability, then the amount of force generated will be limited by the relationship between the mass and the velocity of the rotation. With stability, the transfer of force from the ground up through the joints of the body is coupled with rotation of the center of mass to generate even more power. With stability, when contact is made with a target, the reacting force is directed down the legs, where it hits the ground and rebounds up and out to the target. If you can align your body properly so the forces are transferred smoothly, then the earth is added to your mass. This is good news for older, slower, smaller people who cannot rely on body mass and speed to generate power.
Some principles of stability include:
- Stability is inversely proportional to the vertical distance of the center of mass above its base. The deeper the legs bend, the greater the stability.
- Stability is inversely proportional to the horizontal distance of the center of mass from the center of its base. The more the body leans, the less the stability.
- Stability is directly proportional to the area of the base. The greater the distance between the feet, the greater the stability.
- Stability is directly proportional to the mass. The heavier the person, the greater the stability.
- Stability in a given direction is directly proportional to the horizontal distance of the center of mass from that edge of the base. The closer the center of mass is to an edge of the base (while remaining within the base) the weaker stability is in that direction and the stronger it is in the opposite direction.