The Dynamic Response Index (DRI) is a measure of the likelihood of arising from a such as might be encountered in a military environment (i.e., during a mine blast, or in an ejection seat). The DRI is a dimensionless number which is proportional to the maximum spinal compression suffered during the event. In this equation, X denotes the spinal compression, and d2z/dt2 denotes the time-dependent shock acceleration (the input). The coefficients ω and ζ are the lumped spinal frequency and damping, ω = 52.90 radians/second, ζ = 0.224.