Strain rate dependent bone fracture thresholds are investigated to improve the understanding of dynamic lower extremity injury criteria for threats at different rates of compressive impact. Cortical bone specimens were extracted from eleven bovine femur bones and compressed at constant strain rates (10-4 s-1, 10-3 s-1, 10-2 s-1 and 3×102 s-1). The experimental bone response is fitted with a viscoelastic constitutive model with non-linear strain rate dependence. Probability of failure is estimated (from the ultimate compressive specimen stress) with a Weibull cumulative distribution function and discussed with respect to strain rate.