Techniques described herein determine a center of mass state vector based on a body model. The body model may be formed by analyzing a depth image of a user who is performing some motion. The center of mass state vector may include, for example, center-of-mass position, center-of-mass velocity, center-of-mass acceleration, orientation, angular velocity, angular acceleration, inertia tensor, and angular momentum. A center of mass state vector may be determined for an individual body part or for the body as a whole. The center of mass state vector(s) may be used to analyze the user's motion.