As part of the development of a simulation model for side impact accidents, it has been necessary to develop a model of the human thorax. This is required to provide both the correct dynamic interface with the simulation of the car structure and a reasonable criterion of injury probability. In terms of designing structures (such as additional padding) that will maximise occupant protection, it is important to have a good understanding of the dependence of injury on the time history of the impact forces.It is only possible to develop a complete model of a system as complex as the thorax in terms of a step-by-step simulation. However, an analytical model has also been developed to describe the third order mass/spring/damper system that forms a major subsystem in the step-by-step simulation. This differs from the ordinary second order mass/spring/damper system by having an additional spring in series with the damper, which gives a softer response to a sudden impact. Detailed characteristics of the system have been derived, including an analysis of the dependence of peak deflection (probably a good measure of injury probability) on the duration of an applied impulse force.