The properties of seat with head restraint are important parameters for the optimum design of the seat system performance during rear-end impacts. The aim of the current study is to verify the influence of different seat properties on human cervical vertebral motion using X-ray cineradiography and head-neck-torso kinematics under low speed rear-end impacts.

Six volunteers participated in the experiment under the supervision of an ethics committee. The subject sat on a seat mounted on a sled that simulated actual car impact acceleration. Impact speeds (4, 6, and 8 km/h), and seat stiffness (hard or soft) without headrest were selected as the influence parameters for the cervical vertebrae at impact. The cervical vertebrae motion was recorded by 90 f/s X-ray cineradiography. it is also analyzed to quantify the cervical motion and the head-neck-torso motion under different impact conditions with the combination of influence parameters.

From the current study, it is said that the difference in seat characteristics affects the timing of the straightening of the spine, which in turn markedly affects the load to be applied to each cervical vertebral segment. In case of higher stiffness of seat, the motion ot opper torso in the initial phase of impact becomes sharp, and the axial compression force on the cervical spine tends to become greater. Even if the stiffness is low, however, the rebound of the upper torso is greater in the latter half of impact.

Therefore, it is-necessary to verify more clearly the straightening of the spine and cervical vertebral motion with respect to the difference in seat characteristics in order to design a seat system that can reduce minor cervical injury.