There are many factors involved in how or why a person may be injured, even when the car shows little or no damage. Rear impact collisions setup the neck for a whiplash injury. The car moves forward, the seat pushes your body forward and your neck lags behind, then the tissues stretch out until it’s pulled forward very quickly (much quicker than the speed at which the car was hit). The body is stopped by the seat belt, then your head continues forward until again pulled back by your stretching tissues.
The body tissues in the neck like muscles and ligaments injured in whiplash are generally durable, but it’s a matter a timing. They can handle heavier loads if the load is applied in a safe and slow fashion. The quicker the load is applied, the less of a load the tissues will be able to handle before failure. Imagine a stunt person jumping off a building. They can handle the load of the impact by slowing their body from falling speed to stopped either a) over a long period of time by landing on a fancy air bag or b) over a short period of time by hitting the pavement. Obviously, loads applied over shorter periods of time are more damaging.
Think about this crash test discussed by Dr. Arthur C. Croft in his Whiplash and Mild Traumatic Brain Injury: ”The average head weighs about 8-10 lb [under normal gravity]. This is the force her neck muscle must support in the horizontal position. If we increased this force to 12.8 times that of gravity [what happened when her head was accelerated and pulled forward over a very short period of time], her head would effectively weight about 102 to 128 lb…So, the volunteer in this crash test had approximately that load placed on her neck over a span of just two tenths of a second.” That’s enough weight over a short enough period to surpass the tissue’s ability to resist damage, especially if the person is unaware of the impending collision and cannot brace for impact.
Don’t let an undamaged bumper fool you. If you get rear-ended, get their insurance information, because you may be injured even if you don’t feel it immediately. If you are injured, please get treatment to improve your chances of better recovery and reduce the likelihood of permanent injury.