1. A method of target tracking and modeling, the method comprising: representing a human target with a machine-readable model configured for adjustment into a plurality of different legal poses constrained by one or more rules; receiving an observed depth image of the human target from a source; applying forces to force-receiving locations of the machine-readable model based, at least in part, on differences between the observed depth image and the machine-readable model; determining proposed positions for the force-receiving locations in accordance with the forces applied to the force-receiving locations; and adjusting the proposed positions of the force-receiving locations to comply with the one or more rules if one or more of the proposed positions violates any of the one or more rules.