This work deals with the modelling of the planar motion of a walking four link robot as described in Figure 1. The system is composed of four links, a mass m O attached to the joint at r 2 , and a kind of a ''shoe'' plate that is attached at the joints of each of the points r 0 and r 4 . These plates are used to maintain the lateral balance and to help to induce torques at these points. The role of this articulated system is to imitate a walking biped robot. In this work, we confine the discussion to the modelling and control of one step.