Forward kinematics use Denavit–Hartenberg (D–H) notation to propagate link transformations from the base to the end effector. The reference coordinate is defined with origin at the intersection point of joints. Inverse kinematics are applied to simplify the calculations. The ankle joint is positioned so that the bottom of the foot rests on the floor surface. The leg's orientation is specified by the angle of rotation of the reference coordinate in the X-Y plane, so that foot's position can be described by a single variable.