Biomechanical Response of the Lower Leg under High Rate Loading

Severe lower extremity injuries have been observed in military vehicle occupants in an underbody blast (UBB) event. Characterization of how these loading conditions, different from those seen in automobile collisions, affect the response of the lower limb provides valuable understanding of the relevant injury mechanisms from a UBB. This study investigates the effect of a high-rate axial load to post-mortem human surrogates' (PM HS) lower extremities. Eighteen PMHS were tested on a drop-tower test fixture designed to simulate the occupant loading environment and an unconstrained knee boundary condition. Strain-time histories found on the lateral and medial sides of the tibia showed inhomogen eous deformation patterns along the length of bone. The response of the lower leg to high rate loading showed significant local stiffness at the distal tibia-fibula region k1 = 2028.38 ± 525.02kN/m and k2 = 9058.35 ± 2043kN/m. The 50% survivability for lower extremities subject to these loads and load rates was calculated at 7.7kN for the distal tibia and 6.8kN for the proximal tibia. Additionally, an analytical three-mass model of the lower leg was developed for the tested condition and the predicted model fit compared to the measu red response.