Abstract—Reducing metabolic cost of walking has long been a challenge for exoskeleton researchers. A recent break-through reported a passive exoskeleton that yielded signifi-cant energy saving with a clutch-spring mechanism working in parallel with calf muscles. In this study we asked whether the same bio-mechanism exists in the hip and knee joints. We employed OpenSim, an open source platform, and the MATLAB Optimization Toolbox to optimize the engaging and disengaging timings and the stiffness of the springs for walking energetic efficiency. When applied to the ankle joint, the proposed approach yielded results that agreed with the reported one. We then extended this method to the hip and knee joints, respectively. The simulations showed the springs could save up to 6.38%, 4.85% and 7.63% for the ankle, knee and hip joints at the optimal stiffness of 8.20kN/m, 7.35kN/m and 4.15kN/m, respectively.