Human body motion is represented by a skeletal model derived from image data of a user. The model represents joints and bones and has a rigid body portion. The sets of body data are scaled to a predetermined number of sets for a number of periodic units. A body-based coordinate 3-D reference system having a frame of reference defined with respect to a position within the rigid body portion of the skeletal model is generated. The body-based coordinate 3-D reference system is independent of the camera's field of view. The scaled data and representation of relative motion within an orthogonal body-based 3-D reference system decreases the data and simplifies the calculations for determining motion thus enhancing real-time performance for multimedia applications controlled by a user's natural movements.