Seek medical examination. Variable duration of this phase. The weight or mass of the striking and struck vehicle have significant influence on injury occurrence. At collision, larger car will slow to 10 mph and the smaller car will be instantly propelled backwards at 10 mph. Larger car has a 20 mph total velocity change while smaller car has 40 mph velocity change.
Smaller cars have more severe crash injuries.
This is of absolute importance at high speeds. At low speeds, the correlation between crash velocity and injury occurrence is less predictable aware if impending collision - Those occupants aware of the impending rear end crash had less severe injuries. Obviously this is helpful up to a certain speed. However, this is far from absolute. It may increase the risk of facet joint injury.