The human body is not designed to absorb certain types of impact, including that caused by automobile collisions. The reason for this is physiological. The human head weighs roughly 10 - 15 pounds, and rests upon the most flexible and least reinforced part of the spine: the neck. Your neck contains 32 joints (among other muscle and ligaments) and is easily stretched and injured if too much force is applied in any direction. When the joints of the neck are stretched or strained beyond their normal limitations, they can lose their ability to return to their original length.
Automobiles, on the other hand, are heavy, solid objects weighing roughly two tons. These structures are designed to absorb impact and minimize energy transfer, however the amount of force exerted when one automobile is struck by another is equal to thousands of pounds - much of which is exerted upon the neck and spine of the passengers. The neck cannot support or control the weight of the head under such circumstances, and the head is forcefully thrust in the direction of the vehicle causing the impact. The sudden movement of the head, backwards, forwards or sideways, is referred to as whiplash. It is more accurately called cervical acceleration/deceleration (CAD) trauma or syndrome, which describes the rapid movements that can injure the vertebrae of the neck and the muscles and ligaments that support them. As the muscles and other tissues within the neck then stretch toward their physical limits, extreme tension builds and the muscles "snap" (or "whip") with a violent spasm in the opposite direction. During this violent process, the soft discs between the spinal bones can stretch or tear, and vertebrae can be forced out of their normal position, reducing range of motion. The spinal cord and nerve roots in the neck can get stretched and become irritated. The result is injury to the muscles, ligaments, nerves (soft tissues) and the joints within the neck, which will generally cause headaches, dizziness, pain in the shoulders, arms and hands, reduced ability to turn and bend, and low back problems. Inasmuch as bruising of the brain can also sometimes occur in auto accidents and similarly severe causes of whiplash, some victims have experienced blurred vision, ringing in the ears, nausea and numbness.