There are currently two accepted criteria for assessment act exposures. Our studies have shown an interaction between the deformation velocity and level of compression during impact, resulting in a greater compression tolerance for low-speed impact than for high-speed loadings. High-speed thoracic impact can cause critical or fatal injury in physiologic experiments before exceeding the acceleration or compression tolerance. The velocity-sensitive tolerance is represented by the maximum product of velocity of deformation and compression, which is derivable from the chest compression response. As the magnitude of this “viscous” response increases, the risk of serious or fatal injury increases. This paper discusses the analysis of available literature and results from our laboratory and demonstrates the need for a viscous tolerance criterion to assess chest impact protection in high-velocity impact.