This thesis explores the use of physics based human motion models in the context of video-based human motion estimation and scene analysis. Two abstract models of human locomotion are described and used as the basis for video-based estimation. These models demonstrate the power of physics based models to provide meaningful cues for estimation without the use of motion capture data. However promising, the abstract nature of these models limit the range of motion they can faithfully capture. A more detailed model of human motion and ground interaction is also described. This model is used to estimate the ground surface which a subject interacts with, the forces driving the motion and, finally, to smooth corrupted motions from existing trackers in a physically realistic fashion. This thesis suggests that one of the key difficulties in using physical models is the discontinuous nature of contact and collisions. Two different approaches to handling ground contacts are demonstrated,one using explicit detection and collision resolution and the other using a continuous approximation. This difficulty also distinguishes the models used here from others used in other areas which often sidestep the issue of collisions.