A hybrid controller that induces stable running gaits on a monopedal robot is developed. The robot features a rigid leg with a revolute knee and a heavy torso with center of mass located far from the hip. The torso houses a novel powertrain that provides series compliance in the compression direction of the leg. The proposed control law is developed within the hybrid zero dynamics framework and it acts on two levels. On the first level, continuous within-stride control asymptotically imposes

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... rtual) holonomic constraints reducing the dynamics of the robot to a lower-dimensional hybrid subsystem. On the second level, event-based control stabilizes the resulting hybrid subsystem. The controller achieves the dual objectives of working harmoniously with the system's natural dynamics and inducing provably exponentially stable running motions, while all relevant physical constraints are respected.