During a rear-end impact, the force of the crash causes several things to happen very quickly. First, the car seat is pushed toward the person, and the upper body hits the seat. This pushes the body forward. The head snaps backward against the head restraint. The vehicle impact is over, but the upper body continues to move forward. Finally, the head bounces off the head restraint and comes forward again. As the head moves forward, the upper body returns to rest against the seat back.

Recent studies have shown that during this type of accident, the lower part of the neck is thrust forward while the head remains level. This action causes a shearing motion of one neck bone moving forward under another. The result is abnormal movement of the neck joints. This stretches the tissue around the joint and puts pressure on the nerves.

Neck pain, stiffness, and muscle soreness a day or two later are referred to as whiplash. Doctors and therapists may also refer to this type of injury as a flexion-extension injury.