High speed craft are used by civilian agencies and the military for rescue, interdiction and for rapid insertion and extraction of forces. ISO-2631 Part 5 has been shown to be as good as or better than any other injury criterion for assessing lumbar spinal injuries in high speed craft, although limitations still exist. This study develops a new low-order dynamics metamodel for predicting vertical impact to the human spine, based on a Madymo (TNO, Inc) simulation of a seated occupant subjected to predominantly vertical impacts. A data set of volunteer occupant and Hybrid III was collected and used for model validation. Craft vertical acceleration time histories were used as input to both the metamodel and to the current ISO-2631 Part 5 spine dynamics model. Though the ISO 2631 Part 5 neural net predictions generally agree with the experimental data up to 4g, they diverge for larger accelerations. In contrast, the output of the metamodel corresponds well with the experimental lumbar acceleration response over a large range of high speed planing craft impacts (0–14 g) and is recommended for assessment of lumbar injury from repeated impact in high speed craft.