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Pedestrians are one of the least protected groups in urban traffic and frequently suffer fatal head injuries. An important boundary condition for the head is the cervical spine, and it has previously been demonstrated that neck muscle activation is important for head kinematics during inertial loading. It has also been shown in a recent numerical study that a tensed neck musculature also has some influence on head kinematics during a pedestrian impact situation. The aim of this study was to analyze the influence on head kinematics and injury metrics during the isolated time of head impact by comparing a pedestrian with relaxed neck and a pedestrian with increased tonus. The human body Finite Element model THUMS Version 1.4 was connected to head and neck models developed at KTH and used in pedestrian-to-vehicle impact simulations with a generalized hood, so that the head would impact a surface with an identical impact response in all simulations.

A Finite Element (FE) model of isolated head and neck complex was developed aiming to investigate the mechanisms of injury from axial impacts, in the sagittal plane, and the injury thresholds from experimental studies reported in the literature. The model was validated on a local and a global level, showing a significant correlation with experimental investigations and thereby having the potential to predict both reported injuries and dynamic buckling modes. The frequently reported Hangmans'' fracture was predicted to occur at an axial load of about 3.5 kN and at a local injury threshold of 191 MPa in the compact bone of C2. Also, when analyzing an experimentally designed inner roof of a vehicle, the FE model showed that an induced anterior translation of the head reduced both stress and forces of the cervical spine bone. Moreover, the recent FE model suggests that combined compression/flexion may result in less severe injuries compared to pure compression or compression extension.