The current state of neck injuries sustained in car-to-car rear-end collisions were investigated according to recent automobile accident statistical data in Japan. To clarify the neck injury mechanisms for low impact speed car collisions, the newly developed impact sled experiment which simulates actual car impact acceleration (and which does not involve any risks) was performed using human subjects *). In order to measure and analyze the physical parameters such as human head rotational acceleration, neck bending moment, shearing and axial forces, the component measurement method with six degrees of freedom (using head accelerometers mounted on the subject’s head) was applied and demonstrated. Furthermore, relationships among the physical parameters, impact speeds, sitting positions, headrest heights, and neck muscle tones applied on the subject’s head and neck system were analyzed. These analyses would enable us to comprehend the conditions of the neck muscle tone (occupant’s tension) and the effects of the sitting postures including headrest height, factors which are of vital importance to the understanding of neck injury mechanisms.