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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.