1. A computer readable storage device having computer readable software embodied thereon for programming at least one processor to perform a method for modeling a pose of articulated body portions of an object, the method comprising: obtaining depth pixels of the articulated body portions in one or more frames; processing the depth pixels of the one or more frames to identify the articulated body portions of the object; accessing a model, the model comprising articulated body portions corresponding to the articulated body portions of the object; matching the articulated body portions of the model to the identified articulated body portions of the object to provide an initial match; evaluating an extent to which distance constraints are violated in the initial match by the articulated body portions of the model, the distance constraints comprise constraints on distances between the articulated body portions of the model; and adjusting the model in an attempt to satisfy the distance constraints, including adjusting a length of at least one of the articulated body portions of the model based on the extent to which the distance constraints are violated.