Lower extremities are the most frequently injured body regions in vehicle-to-pedestrian collisions and such injuries usually lead to long-term loss of health or permanent disability. However, influence of pre-impact posture on the resultant impact response has not been understood well. This study aims to investigate the effects of preimpact pedestrian posture on the loading and the kinematics of the lower extremity when struck laterally by vehicle. THUMS pedestrian model was modified to consider both standing and mid-stance walking postures. Impact simulations were conducted under three severities, including 25, 33 and 40 kph impact for both postures. Global kinematics of pedestrian was studied. Rotation of the knee joint about the three axes was calculated and pelvic translational and rotational motions were analyzed. Pedestrian in walking posture exhibited larger knee bending angle (40% for ipsilateral knee joint) and pelvic rotation angle (27.5% for Z-direction pelvis rotation angle) with less constraint due to isolated single-leg interaction with vehicle and nonplanar characteristic from the leg swing. The walking posture increased the injury risk of soft connection tissue about 20-30% and reduced the internal force in bone structure about 25% regardless of impact severity. Two-leg interaction, inertial effect, anatomical features of the knee and pelvis exhibited a coupled influence on lower extremity kinematics. Injury predictors such as tibia stress and collateral ligament stretching ratio was found to be associated with kinematics. The trade-off of injury risks induced by kinematics with different pre-impact postures is a challenge for vehicle front-end structure design. Further research efforts are necessary to include more loading scenarios and to quantify the lower extremity injury risk in detail.