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In the present study, we investigate a control strategy for hopping motions of an articulated leg that is driven by series elastic actuation. A highly compliant spring in the knee joint allows the exploitation of periodic energy storage but creates a major control challenge by severely limiting the bandwidth of closed-loop position or force control. This handicap is intensified by slow actuators, substantial delays, and the kinematic coupling of the articulated design. With classic closed-loop control strategies failing, an adaptive open-loop control algorithm is presented, that, over a series of jumps, estimates the compression of the actuator springs, and gradually modifies the motor inputs in order to minimize slipping and create a purely vertical motion.