1. A human-computer interaction system for visually tracking human movement comprising: an active depth sensor for capturing depth images of human body parts; a processing unit coupled to the active depth sensor for receiving the depth images, the processing unit comprising: an edge detection module; a tracking module coupled to the edge detection module to determine a best fit of a mathematically representable contour with respect to one or more body parts to determine a position of the one or more body parts within the captured depth images; and a body-lean determination module coupled to the tracking module to receive the determined position of the one or more body parts and configured to calculate a distance of each of the one or more body parts with respect to the active sensor and to relate the distance of each body part with respect to other body parts to determine lean data including the position of the body parts with respect to each other.