DISCUSSION: The COM trajectories showed that subjects attacked with COM moving
forward. A subject needed to move backward to ward off another’s attack. The movement of
COM did not rely on moving steps and flexing the waist. In additional, the rotation of the
waist was more important. The small flexion-extension range revealed a principle of Tia Chi
Chuan, that is, keep an upright posture of the trunk. And the small displacement of COM in
vertical indicated the same purpose, that is, keep balance. The GRF varied with COM motion.
It was always greater than half body weight on one foot at the maximal COM displacement.
These results were responded to Tai Chi Chuan theories. The flexion-extension motion of
elbow was not small and it conflicted to the principles of Tai Chi Chuan. It was considered
about the level of subjects.