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A feedback model of biped dynamics is proposed where the internal and external forces which act on the skeleton are unified as forces of constraint, some intermittent and some permanent. It is argued that these forces are, in general, functions of the state and inputs of the system. The inputs constitute gravity and muscular forces. This model is particularly suited for understanding the control problems in all locomotion. It encompasses constraints that may be violated as well as those that cannot be violated. Applications to motion in space, locking of a joint, landing on the ground, and Initiation of walk are discussed via a simple example. A general projection method for reduction to lower dimensional systems is provided where, by defining an appropriate coordinate transformation, a prescribed number of forces of constraint are eliminated. Finally an application of the model in estimating inputs (joint torques) is briefly discussed.