Kinetics of a Whiplash Injury

"Every object continues in a state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces acting upon it." -Sir Isaac Newton 1643-1727

In other words, if a body is at rest it will tend to stay at rest; if a body is in motion it tends to stay in motion. It is this principle that underlies the whiplash injury.

In a rear-end collision, the car accelerates forward causing the front seat to be pushed into the occupant. This force causes the trunk to be thrust forward. The unrestrained head stays at rest causing extension at the neck as the trunk moves forward. This backward bending of the neck continues until the head strikes the head-rest. This can strain soft tissues of the front of the neck.

Many patients will describe the impact as being so great that their car seat was broken. The head rebounds forward after the car stops, straining soft tissues of the back of the neck and into the shoulder girdles. The extent of damage seen in head-on collisions is similar and depends on the position of the head at the time of impact.

On impact there is tearing of muscles, ligaments, and other soft tissues. Bruising, bleeding, and inflammation results as would occur during any soft tissue injury.

A motor vehicle accident takes approximately 600 msec to occur.

		Prior to impact - The head and body travel at the same speed as the vehicle
		0-100 msec - Back and torso load the seat - Torso acceleration begins - High shear forces develop in the neck
		100-250 msec - Head snaps into extension, extends over the head restraint, and may collapse it.
		250-400 msec - The head begins its motion forward - The seat back bounce increases occupant velocity 30-70% more than that of the car. - Slack is taken out of the shoulder harness.
		400-600 msec - Full deceleration of head, neck and torso. - High tension and shear forces in spine. - High brain stem, spinal cord, and nerve root tension. - Posterior ligamentous tension in the spine.