Characterization of the lower limb soft tissues in pedestrian finite element models

Current finite element (FE) models of the human lower extremity lack accurate material properties of the soft tissues (flesh, fat, and knee ligaments), which are needed for computational evaluation of pedestrian injuries. Medial collateral ligament (MCL) is the most frequently injured ligament in lateral impacts. Therefore, the accuracy of the viscoelastic mechanical properties of the MCL FE model is of crucial importance in modeling pedestrian impacts. During automotive impacts, the flesh and fat get compressed, absorb part of the impact energy, and transfer and distribute the rest of energy to the skeleton. Therefore, the compressive response of these soft tissues can affect the accuracy of bone fracture predictions and as a result the overall kinematics of the FE pedestrian model. Quasi-Linear Viscoelastic (QLV) constitutive material models were assumed for MCL, flesh, and fat. Their global properties in terms of material parameters were derived using uni-axial step and hold tests on cadaveric specimens. The material models coefficients were derived by optimization. The flesh/fat models were validated in lateral leg impact tests at 2.5 m/s. The force-deflection results of the impactor, compared to other models, showed more biofidelity with respect to the cadaveric and volunteer data.