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A three-link planar model of a biped is studied in the frontal plane. When this model is subjected to certain on-off constraints, the forces of constraint are derived as functions of the system state and the input, and can be shown in a simple block diagram. Linear state-variable feedback is designed to stabilize and decouple the system for small motions about an equilibrium state. The motion of this biped is implemented as a sequence of state transitions between stable equilibrium states. Ramp state and input references are used to guide the biped system from one equilibrium state to the next. Single-support postural stabilization and side step and return motions were investigated by computer simulation in order to evaluate the effectiveness of this control strategy.