Fig. 3. Coordinate axes for the hip and knee joints. The hip was modeled as a ball-and-socket joint with three orthogonal axes of rotation. The center of rotation was fixed and located within the femoral head. Rotation about the z-axis was termed hip flexion (counterclockwise) and hip extension (clockwise). Rotation about the y-axis was termed hip adduction (counterclockwise) and hip abduction (clockwise). Rotation about the x-axis was termed hip internal rotation (clockwise) and external rotation (counterclockwise). The kinematics about the z-axis of the knee joint was modeled by a planar, rolling joint. Clockwise rotation about the z-axis of the knee joint was termed flexion, and counterclockwise rotation was termed knee extension.