Pedestrian Shoulder and Spine Kinematics and their Influence on Head Kinematics

Licentiate thesis, 2013

Pedestrian to vehicle crashes still represent a major health issue worldwide. In order to prevent fatalities and injuries during the in-crash phase of a pedestrian accident, a sound understanding of pedestrian in-crash kinematics is required and Human Body Models (HBMs) are valuable tools when studying such kinematics. Since injuries to the head feature frequently in severe and fatal injuries in these types of crashes, it is crucial to study the head boundary conditions that govern the kinematics during the crash. This thesis is focussed on the influence of pedestrian shoulder and spine kinematics and the effect elbow and shoulder impacts have on head linear and angular kinematics. A new full-scale experiment was carried out to establish 3D linear and 3D angular displacements of the head, spine, and both scapulae. Three past full-scale experiments were re-analysed to establish spine curvature. A comparison between the responses of a finite element pedestrian HBM, the Total Human Model for Safety (THUMS) Version 4, and previous shoulder impact experiments comprising volunteers and PMHSs were made. The results were used to study shoulder impacts displaying boundary conditions similar to shoulder impacts in full-scale pedestrian experiments. The recent full-scale pedestrian experiment provides novel and valuable 6 degrees of freedom (DOF) kinematics suitable for use in HBM evaluations. Head kinematics are governed by neck flexion which is controlled by torso flexion and head inertia. When the pelvis impacts the vehicle, torso flexion away from the vehicle is introduced, influencing the head kinematics. Elbow impacts have considerable influence on head kinematics and elevate the position of the shoulder before shoulder impact. The head impacts the vehicle shortly after the shoulder. Since the load transfer from shoulder impact to the head takes longer than the time between shoulder and head impact, shoulder impacts does not influence head kinematics before the first contact of the head with the vehicle. The load transfer from shoulder impacts into the torso is governed by the scapular motion over the thorax. Limited scapular motion increases the overall shoulder stiffness and leads to higher head linear and angular displacements compared to lower shoulder stiffness. Elevated shoulder positions limit scapular motion over the thorax while shoulder impacts from supero-lateral directions, which were observed in the pedestrian full-scale experiments, slightly increase scapular motion over the thorax. THUMS Version 4 is suitable for studying head linear displacements following shoulder impacts although THUMS compares better to tense volunteers than relaxed volunteers. In THUMS, head rotation around the anteroposterior axis is slightly lower and head twist is considerably higher than seen in the volunteers.