A method for creating a frame of reference of a leg of a subject for subsequent 3-D kinematic analysis of the leg with non-invasive trackable references secured to the femur and the tibia of the leg. The method comprises the steps of: i) obtaining tibial and femoral axes with respect to the trackable references by tracking points and motions of the leg; ii) registering a known posture of the subject with respect to the trackable references; iii) obtaining a desired posture of the subject with respect to the trackable references as a function of the known posture by tracking a predetermined motion of the leg; and iv) setting a frame of reference to said tibial and femoral axes of the leg in the desired posture of the subject with respect to the trackable references.