The objective of this work is to purpose a more efficient joint for the knee of a bipedal robot. Indeed, the biomechanical studies proved the human knee joint is a complex structure which allows a rolling and a sliding movement of the femur on the tibia in the sagittal plain . These movements are guided by the cruciate ligaments and the articular contacts. These motions cannot be reproduced by one or two revolute joints. Differents authors introduced polycentric knee joint with a four-bar structure to approach the human knee movements. A cross four-bar knee with respect to a revolute knee joint can reduce the energy consumption of a biped. Moreover, several papers paid interest to the effect of spring equipping the bipedal robot joints. We compare the performance of a bipedal robot which uses cross four-bar linkages on the knee joints and we compare the energy consumption of this biped during a walking gait with and without springs on the knee joints. We develop an optimization problem to design optimal walking trajectories with and without springs on the knees. These sets of optimal trajectories show the benefit of springs on the energy consumption for the bipedal robot with cross four-bar knees.