Underbody blast (UBB) results in lumbar spine injuries in 35% of military-vehicle casualties, resulting in disability and reduced quality of life. A multibody model of a lab-simulated UBB on a full-body cadaver was developed using geometric and inertial properties acquired from a CT scan of the same cadaver. The model comprises a skull, individual vertebral bodies, and a sacrum. Vertebral levels were connected by spring-dampers. Stiffness and damping values were taken from literature of the intervertebral disc and optimized to calibrate the model. The sacrum acceleration recorded in the experiment was input to the model sacrum, and the optimization algorithm worked to maximize the CORA (ISO18571) score of the head and T1 vertebra axial acceleration. The peak accelerations at T1 in the experiment and optimized model were 128 g and 111 g and the times-to-peak were 13.8 ms and 13.9 ms, respectively. The CORA score of both the head and T1 was 0.645 (fair). Stiffness in flexion increased by two orders of magnitude, while other degrees of freedom were scaled by values <100. This study developed a simple, fast-running, subject-specific model to predict injury across the spine. The vision is to assess the probability of injury of any seat configuration, in any vehicle.