Quadrupedal running is an efficient form of locomotion found in nature, which serves as an inspiration for robotics. We believe that a resonance-based approach is the path towards energy-efficient legged locomotion and running robots. The first step in working towards this goal is creating an energy-efficient one-legged hopper. Such a one-legged hopper was designed and constructed. The impact efficiency of the mechanism is calculated analytically, determined in simulation and measured with the prototype. The impact efficiency as calculated from the experiments is found to be in agreement with the analytical expectation and simulation results. Finally, using an electric motor to inject energy by creating a virtual spring with reverse hysteresis, hopping is achieved.