Biomechanical response of human cervical spine to direct loading of the head

Currently, dummies are used to evaluate the potential for neck injuries under both inertial loading and direct contact loading, such as in the case of air bag loading. However, there is currently little known data on the biomechanical response of the human neck under direct head contact loading conditions. The objective of this preliminary research is to develop an experimental approach and analysis method that can characterize the response of human neck under low severity head impact loading conditions and compare the results with those for a dummy. The tests include three healthy adult male volunteers with average age of 22, as well as a Hybrid III (HY-III) test dummy. Low-level impact loads to the head and face of each subject were applied via a strap at one of three locations in various directions: an upward load applied to the chin, a rearward load applied to the chin, and a rearward load applied to the forehead. The resultant forces and moments at the occipital condyle were calculated using the measured applied loads, inertial properties of the head, and translational and angular accelerations of the head. For the volunteers, activation of the neck musculature was determined using EMG. Although two initial muscle activation conditions, relaxed and tensed, were investigated, only the relaxed results are presented in this study. For each of the test conditions, the three relaxed volunteers showed similar head rotation patterns, with the highest head rotational velocities observed for the chin-upwards loading condition. The head rotation for the relaxed volunteers was also substantial in the chin-upward and forehead rearward cases, up to 25 degrees. By contrast the dummy’s head rotation was small for all loading modes, less than 5 degrees. It is expected that as the intensity of the applied load increases and/or the volunteers tense the muscles in the cervical spine, these differences will decrease. The calculated forces at the occipital condyle were comparable for the volunteers and dummy, although the calculated moments in the dummy were somewhat higher than those for the relaxed volunteers. Although preliminary testing at low impact severity demonstrates that there may be differences in the moment-angle response between relaxed human volunteers and the HY-III dummy, it remains to be demonstrated if these differences will occur for higher-severity impacts associated with air bags or if these differences will occur for tensed human volunteers. However, once the overall moment-angle response of the head and neck to impact loading is characterized for both low and high levels, the next important step is to understand how the forces and moments are partitioned between the ligamentous cervical spine and the surrounding musculature. Thus, if differences exist, this data should contribute to the improvement of the physical dummy head/neck and its instrumentation to achieve better biofidelic interactions with the environment and to have a greater ability to accurately evaluate the potential for injury to the cervical spine and spinal cord.