The neck does not simply snap back. For a fraction of a second it bends into a shape it was never built to make, and that shape is where the injury lives.
Say a stopped car is struck from behind. The seat drives the body forward before the head can follow. Because the head has weight and wants to stay where it was, it lags for an instant while the shoulders are already moving. In that instant the spine in the neck is doing two opposite things at once. The lower joints are forced backward into extension while the upper joints are still bent forward, and the neck momentarily takes the shape of a letter S. It is not the graceful arc the neck makes when a person looks up at the sky. It is a sharp, uneven bend that the anatomy was never designed to hold.
The whole thing happens faster than a person can brace. The lag, the S, and the snap that follows unfold in well under a fifth of a second, long before the muscles that guard the neck can tighten to protect it. The occupant does not get to flinch. By the time the brain registers the hit, the loading is already over.
The harm concentrates at the small paired joints at the back of each level of the spine, the facet joints, and at the tough sleeve of tissue that wraps each one, the facet capsule. During that S-shaped moment those capsules are pinched and stretched in a way normal motion never produces, and the tissue can be strained past the point where it recovers cleanly. Researchers have loaded these joints in the laboratory to the same pattern a rear impact produces and shown the capsule reaching injurious strain.
Here is why a trial lawyer has to understand this. A strained facet capsule is soft tissue and a joint sleeve. It does not break a bone and it does not usually show on an X-ray, and often not on a standard scan either. The defense reads a clean film and calls the case nothing. But a clean film is not the same as an uninjured neck. The film shows bone. The injury is in tissue the film was never going to see. The evidence that the injury is real is the mechanics of what the joint went through, not a picture that was only ever going to show the parts that did not break.
So when the story is a low-speed rear-end and the other side says the neck is fine because the pictures look fine, go back to the S. Ask what the joints did in that fraction of a second. The bones held. The sleeve around the joint is where the pain came from, and the pain is not imaginary just because the camera could not find it.
On the cervical facet joints and their capsules as the site of whiplash injury, see Facet Joint Kinematics and Injury Mechanisms During Simulated Whiplash (Spine, 2004) and Mechanical Evidence of Cervical Facet Capsule Injury During Whiplash (Spine, 2001). This is general information, not medical or legal advice.
