Accident data shows that driver's kinematics response in real accidents can be significantly different from that in dummy or cadaver tests because of driver's muscle contraction. In this study, a finite element (FE) human body model consisting of an upper body of a dummy model and a lower limb-pelvis biomechanical model with 3D active muscles was developed to in-depth investigate lower limb injuries. Driver's emergency reaction during frontal impact was simulated by modelling muscle active contraction based on a series of volunteer experimental tests. Besides, a realistic impact environment with the response of the restraint system and the invasion of the driver's compartment was established in this study. The results show that muscle contraction can cause extra loads on lower limbs during the impact, which can increase the injury risk of lower limbs. As for the femur injury, muscle contraction caused an additional 1kN axial load on the femur and the femur resultant bending moment of active models was also higher by about 10-40Nm. Besides, the tibial index of the model with muscle activation was about 0.1 higher. Additionally, the results indicate that the femur injury is strongly related to the combined action of both axial force and bending moment. And the variation of the injury tolerance along the tibia shaft should be considered when evaluating the tibia injury. Overall, the current lower limb injury criteria can be still lack of robustness.