A whiplash injury occurs when the head and neck are thrown very quickly in one direction and then rebounds in the opposite direction. Because the second, snapping motion happens faster then the spinal structure can recover, the muscles, ligaments, and bones in the neck are damaged. This type of injury can also occur in hard falls and diving accidents.
The greatest amount of injury in a typical rear-end collision is from the shifting about of the victim's spine. The more the vehicle is accelerated and propelled forward, the more "whipping" action occurs.
Studies indicate that injury can occur at impact speeds of 8 mph where there is little or even no damage to the car. Even in these types of accidents, the head can receive up to 2 1/2 times more force than the car itself.
When the head is rotated (as in talking to someone) or inclined, (as it may be looking up and out through the rear view mirror), the severity of the injury increases substantially.