1. A method, comprising: accessing, by a processor, image data that tracks a user; forming, by the processor, a body model for the user that includes a plurality of body parts based on the image data; computing, processor, a center of mass for different ones of the body parts; determining, by the processor, a center of mass state vector for each of the plurality of body parts, the center of mass state vector for each of the body parts comprises position of the center of mass of the body part, velocity of the center of mass of the body part, orientation of the body part, and angular velocity of the body part; and determining, by the processor, a center of mass for the user as a whole based on the center of mass of the different body parts.