This paper presents the design and preliminary evaluation of a quasi-passive lower limb exoskeleton for walking efficiency improvements. The exoskeleton recycles the negative work performed by the knee joint in late swing phase and the ankle joint in mid-stance phase, to assist ankle push-off in late-stance phase when a burst of positive power is needed. The exoskeleton consists of a torsion spring as an energy storage element, and two clutches attached to both ends of the spring to control the timing of recycling and releasing energy in a gait cycle. The two clutches are actively controlled by two small servo motors with very low power consumption based on the plantar pressure. The novelty of this exoskeleton is it makes the extra kinetic energy dissipated at the knee joint reusable, by transferring it to the ankle joint to assist positive power generation during push-off, for the first time. Eight male subjects walked with the exoskeleton engaged (EXO_ON), disengaged (EXO_OFF), and without the exoskeleton (NO_EXO). Inverse dynamics analysis demonstrated reduced negative biological work at the knee joint during late swing and at the ankle joint during mid stance, as well as reduced positive biological work at the ankle joint during late stance comparing the EXO_ON to EXO_OFF conditions. These results prove the effectiveness of the exoskeleton at joint level.