Twelve volunteers participated in the experiment under the supervision of Tsukuba University Ethics Committee. The subjects sat on a seat mounted on a newly developed sled that simulated actual car impact acceleration. We selected impact speeds (4, 6 and 8 km/h), seat stiffness, neck muscle tension, and alignment of the cervical spine for the parameter study of the head-neck-torso kinematics and cervical spine responses. The effects of those parameters were studies without headrest. The muscle activity was measured with surface electromyography. The cervical vertebrae motion was recorded by cineradiography (90 frames/s X-ray) and analyzed to quantify the rotation and translation of cervical vertebrae at impact. Furthermore, the motion patterns of cervical vertebrae in the crash motion and in the normal motion were compared. Subject's muscles in the relaxed state did not affect the head-neck-torso kinematics upon rear-end impact. The ramping-up motion of the subject's torso was observed due to the inclination of seatback. An axial compression force occurred when this motion was applied to the cervical spine, which in turn developed the initial flexion, with the lower cervical vertebral segments extended and rotated prior to the motions of the upper segments. Those motions were beyond the normal physiological cervical motion, which should be attributed to the facet joint injury mechanism. Furthermore, the more rigid the seat cushion, the greater was the axial compression force applied to the cervical spine. On the other hand, the torso rebounding caused by the softer seat cushion tended to intensify the shearing force applied to the upper vertebrae. It was also deduced that the difference in alignment of the cervical spine affected the impact responses of head and neck markedly. Also the kyphosis of the cervical spine caused the upper travel of rotation center of the lower cervical vertebral segments and its rotational motion resulting in a higher neck injury incidence. Based on the differences in the alignment of the cervical spine between male and female occupants, it is also pointed out that the female neck injury incidence tends to become higher than that of male, as the female cervical spines lake the kyphosis position more often than the male cervical spines.
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