The impulsive excitations during an underbody loading sequence in military scenarios exert a substantial axial load on the thoracolumbar spine causing severe injuries. There is a need for a biomechanical injury parameter to indicate the probability of injury of the spine in such high acceleration environments. The Dynamic Response Index (DRI), which is commonly used as the injury parameter for underbody loading scenarios, suffers from inherent disadvantages and has been reported to underpredict the chances of injury due to various reasons. A novel injury parameter based on the compression of lumbar spine capable of considering the posture of spine is obtained from a lumped mass MDOF model of lumbar spine is proposed. The model is capable of modeling axial, shear, and bending motions of individual vertebrae of lumbar spine. The effect of posture of the spine was studied for erect, normal, and slouched postures in seat ejection-type loading scenarios. © Springer Nature Switzerland AG 2020.