How can a car accident cause so much pain?  While this part of the equation is just beginning to be understood, there are some things we know.  In general, different things happen depending on where and how hard your car got hit, your position in the car, the position of your head, type of seat, position of the headrest, your pre-accident medical condition, and other things.

Start this lession by looking at an animated gif (complements of Wayne State) of a neck while being rearended...

Notice how far the head travels backwards.  Also, look at the neck.  Notice how much stretch there is on the ligaments in the front and how the facet joints in the back get crunched.