This study hypothesized that a lack of lower extremity muscle activation explains why midshaft femur fracture cannot be reproduced in frontal impact experimental tests by impacting the knees of unembalmed cadavers. To test this hypothesis, a new lower-extremity finite element model (LX FE Model) of a midsize U.S. male was developed that included regional variability in cortical bone thickness, directionally dependent cortical-bone material properties, and a complete set of lower-extremity muscles with associated muscle mass. This model was validated by simulating biomechanical tests in which whole seated cadavers and cadaver segments were impacted using loading conditions similar to those produced in frontal motor-vehicle crash testing. Muscle forces for use in simulations with the LX FE Model were estimated using a commercial musculoskeletal model that was validated using EMG and reaction force data from a series of maximum and 50% maximum one-foot braking tests performed by subjects in a laboratory seating buck.

Simulations of knee-to-knee-bolster loading during frontal crashes using the validated LX FE Model with and without the muscle forces predicted by the musculoskeletal model for maximum braking indicate that muscle tension increases the effective mass of the occupant in response to knee-impact loading by increasing the coupling of muscle mass to the skeleton, and thereby increases the forces applied to the knees. Muscle tension also preferentially couples soft-tissue mass to the KTH distal to the hip, which increases the percentage decrease in force between the knee and the hip. The magnitude of this increase is that force transmitted to the hip without muscle tension is similar to force transmitted to the hip with muscle tension despite the higher knee-impact forces with muscle tension. Most importantly, relative to the study hypothesis, simulation results indicate that muscle tension has a meaningful potential to shift fracture location from the hip to the shaft of the femur by increasing bending moments in the femoral shaft.