In this paper, we present the design and the results of a control scheme aimed at moving an underactuated double pendulum simulating a leg. The system is actuated at the hip and includes a spring at the knee. It interacts with the grou nd, which is in fact a treadmill, through a telescopic foot made of a linear spring and a damper. The control scheme is different in the 2 phases, swing and stance, and a repetitive step is achieved by switching between these two controls. The paper describes the used models, the control algorithms and their implementation. It presents also experimental results of the approach.