This article describes the design and testing process of a new pair of legs for the iCub. The iCub is an opensource humanoid robot that is being used by more than 20 institutes and research centers worldwide. Although the legs of the original iCub allow the robot to perform basic balancing and crawling locomotion the extension of its locomotion skills to bipedal walking could give to the iCub community additional possibilities for a wider range of experimentation. To extend the iCub locomotion capabilities to bipedal walking we thus undertook a design revision of the legs. A major novelty of the new design is the introduction of series elastic actuators (SEA) at the knee and ankle joints. Additional modifications were applied at the ankle joint to improve both the torque capability and range of motion. The present article will describe the conceptual design of the new system, and will present some preliminary data regarding the testing of the elastic module.