This thesis work deals with the problem of increasing performances in biped locomotion. We analysed a recent study in biped robot walking and running, inlcuding other values of stiffness in knee joint and comparing these results with the literature review. Power and torque reduction, during walking and running tasks, are our final objectives. Giving the same input to the motors, we modified the stiffness of the knee joint in order to increase biped robot performances in locomotion.