Within a few hundredths of a second after impact, the vehicle occupants in the
forward vehicle will press into the seat backs. The seat cushioning and structure will absorb
much of the energy related to the acceleration of the occupants. Consider a person sitting
in the front seat during a rear-end collision. Unless the head is well restrained by the head-
rest, it will move rearward over the top of the seat in a phenomenon called whiplash. There
are two components to the head movement: translation away from the body trunk and rotation about the neck. If the seat back remains rigid and quite vertical, the rotational component will likely be much greater than if the seat back structure yields. If the seat back yields, it absorbs some of the energy and reduces the rotational component. The resultant
force on the head from the two components can be quite sizeable even at fairly low-impact speeds. Depending on the speed at impact, the body may slide backward up the slope of the seat and impact on the rear structure of the passenger compartment.