Articulated tracking of humans is a well-studied field, but most work has
treated the humans as being independent of the environment. Recently,
Kjellstrom et al. showed how knowledge of interaction with
a known rigid object provides constraints that lower the degrees of freedom in
the model. While the phrased problem is interesting, the resulting algorithm
is computationally too demanding to be of practical use. We present a simple
and elegant model for describing this problem. The resulting algorithm is
computationally much more efficient, while it at the same time produces
superior results.