Almost three decades ago, S. Futamura  devised the deformation index concept for determining the control parameter for the viscoelastic response of deformed elastomeric materials. We have extended this to impact mitigation, in which typically material hardness and energy dissipation both affect the behavior. Laboratory tests were carried out on a series of elastomers with independently varied storage and loss moduli to deduce the deformation index. We then extended the experiments to more complex conditions, including material failure. The utility and limitations of this approach are discussed.