PURPOSE: The aim of this study was to determine the kinematical characteristics of the pendular and the translational movements of each cervical vertebra at flexion and extension for understanding the mechanism of injury to the cervical spine. METHODS: Twenty volunteers, young men (24~37 years), with clinically and radiographically normal cervical spines were studied. We induced two directional passive movements and then took X-ray pictures. The range of pendular movement could be measured by measuring the variation of the distance between the center point of two contiguous cervical vertebrae, and the range of translational movement could be measured by measuring the variation of the shortest distance between the center point of a vertebra and an imaginary line connecting the center points of two lower contiguous cervical vertebrae. The measurements were done by using a picture archiving and communicating system (PACS). RESULTS: The total length of all cervical vertebrae in the neutral position was, on average, 133.66 mm, but in both flexion and extension, the lengths were widened to 134.83 mm and 134.79 mm, respectively. The directions of both the pendular and the translational movements changed at the 2nd cervical vertebra, and the ranges of both movements were significantly larger from the 5th cervical vertebra to the 7th cervical vertebra for flexion and combined flexion and extension motion (p<0.05). CONCLUSION: The kinematical characteristics for flexion and extension motions were variable at each level of cervical vertebrae. The 1st and the 2nd cervical vertebrae and from the 5th to the 7th cervical vertebrae were the main areas of cervical spinal injury. This shows, according to "Hook's law," that the tissues supporting this area could be weak, and that this area is sensitive to injury.