Correlation of Strain and Loads Measured in the Long Bones With Observed Kinematics of the Lower Limb During Vehicle-Pedestrian Impacts

The purpose of this study is to determine the loads in the long bones of the lower extremities during vehicle pedestrian impact tests, and to correlate load data with observed kinematics in an effort to understand how stature and vehicle shape influence pedestrian response. In tests with a large sedan and a small multi-purpose vehicle (MPV), four postmortem human surrogates (PMHS) in mid-stance gait were struck laterally at 40 km/h. Prior to the tests, each PMHS was instrumented with four uniaxial strain gages around the mid-shaft cross section of the struck-side (right) tibia and the femora bilaterally. After the tests, the non-fractured bones were harvested and subjected to three-point bending experiments. The effective elastic moduli were determined by relating the applied bending loads with the measured strains using strain gage locations, detailed bone geometry, and elastic beam theory. Using the strains measured in the vehicle-pedestrian tests and the calculated effective elastic moduli, the axial load and bending moments in the instrumented bone cross-sections were calculated. Peak longitudinal strains in the mid-shaft cross-sections approached 1% in the right tibiae and exceeded 0.5% in the right femora with peak strain rates of 200%s^-1-750%s^-1 in the right tibiae and 100%s^-1-170%s^-1 in the femora. While peak axial forces were consistent for both vehicles and ranged from 1 kN to 3 kN, bending moments in the right lower extremity exceeded 300 Nm in the sedan impacts but were substantially lower in impacts with the MPV. The right tibia bent predominantly in the medial direction during the impact whereas bi-modal patterns were observed in the sagittal bending moment time histories of the femora. Stature differences caused variations in hip and knee impact locations relative to the hood edge and bumper of each vehicle that may have been a contributing factor resulting in more severe struck-side lower extremity injuries in the tall subject tested with the MPV, and more severe struck-side lower extremity injuries in the shorter subject tested with the sedan.