A note on the description of articulating joint motion

An expression for the relative rotation matrix for motion between two body segments forming an articulating joint is derived. Local coordinates on each segment of arbitrary orientation is assumed, as well as an initial state of the standard anatomical position and an arbitrary final state. This rotation matrix thus eliminates influence of initial axes orientation relative to the body segment and relative to global coordinates, eliminates whole limb rigid body motion, and includes only the motion component of rotation of the distal segment about the proximal segment. Orthopedic angles are derived from the rotation matrix.