A properly used seat belt fits snugly over the pelvis and across the chest. In the event of a frontal collision, the lap and shoulder belts restrict the occupant’s forward motion to prevent potentially injury-producing contacts with the dashboard, steering wheel, and windshield. The restraining forces applied to the occupant by the seat belt are spread across the strong, bony structures of the body.
The bones of the pelvis are exceptionally strong and can withstand considerable force without fracture. The rib cage is quite flexible but, at a certain point, the ribs will no longer continue to bend, and may break. Fractures to the ribs may result in serious injuries to underlying soft tissue such as the heart and lungs. This is a particular concern for the elderly who have more brittle bones and less tolerance to high collision forces.
A load limiter is designed to allow the seat belt force applied the the chest to rise only to a point where serious injury is unlikely. The seat belt is then allowed to extend in a controlled manner, maintaining a constant restraining force to absorb energy. The occupant’s forward motion is finally arrested by the front air bag that deploys in the crash.