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In this paper, we extend the running controller of a passive one-legged hopper to a planar biped robot with torso, and evaluate the controller on simulations. The controller is derived based on an energy-preserving strategy and it actually preserves mechanical energy at touchdown. Interestingly, zero dynamics of decoupling controller (dynamics about a pair of controlled legs) is found to be stable. Combining the simple attitude controller at stance phase generates stable periodic running gaits of arbitral period. The control performance is shown to be better than a simple PD-feedback control of leg placement.