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 configured to determine an edge of a target in the depth images; 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 at least by: dividing the mathematically representable contour into a plurality of segments, each contour segment comprising an inner side corresponding to lower depth values and an outer side corresponding to higher depth values; and matching an orientation of each contour segment against a corresponding edge segment of the determined edge in the depth image by comparing the depth values on each side of the determined edge segment and determining that the depth values on a side of the determined depth segment that overlaps with the inner side of the matched contour segment are lower than the depth values of a side of the determined edge segment overlapping with the outer side of the matched contour segment; 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.