A new finite element model (FEM) of the lower extremities has been developed to study the effects of muscle tension on knee-thigh-hip (KTH) injuries. This model was created by remeshing the MADYMO human lower-extremity FEM to include representative femur cross-sectional geometry, trabecular bone, cortical bone with directionally dependent mechanical properties and Tsai-Wu failure criteria, articular cartilage, and improved lower-extremity muscle geometry. The dynamic response of the model was validated using results biomechanical tests documented in the literature. Simulations of knee-to-knee-bolster impacts conducted with and without previously reported levels of muscle activation for braking/bracing suggest that muscle tension decreases the force applied to the flexed knee that is required to cause KTH fracture. Muscle tension also increases the likelihood of femoral shaft fracture by increasing bending moments in the femur. More accurate data on activation levels of muscles in the lower extremities during vehicle braking and bracing are needed before the effects of muscle tension on KTH injury can be fully understood and quantified.