This paper presents manageable models of an occupant and restraint system. Low-order design models are essential in the development of active restraint systems, which aim at lowering certain injury criteria by real-time control of the occupant motion with a restraint actuator. The design models are constructed from first principles, using knowledge obtained from a sensitivity analysis of a reference occupant model. The models can easily be employed for controller and observer design, and can be used in real-time injury prediction. The design model is validated in a variety of frontal impact simulations, and the most important biomechanical responses are compared to the responses of the reference model.