It takes far more force to injure your brain than to injure the ligaments at the top of your neck. Reconstructed football concussions cluster near 100 Gs of head acceleration, while cervical soft tissue is injured at a small fraction of that. The gap is why an impact can leave your brain scan clean and your upper neck genuinely damaged.
What Do the Actual Force Numbers Look Like?
Two large studies put real numbers on what it takes to concuss someone.
Guskiewicz and colleagues instrumented collegiate football helmets with accelerometers and captured impacts as they happened on the field. Clinically diagnosed concussions occurred at a **mean of 102.8 Gs**, with a range of 60.5 to 168.7 Gs (Guskiewicz et al., 2007).
Pellman, Viano and colleagues took a different approach, reconstructing NFL concussions in the laboratory using crash-test dummies matched to game film. They found concussion required **98 ± 28 Gs** delivered over roughly 15 milliseconds (Pellman et al., 2003).
Two independent methods, two different populations, and both land near 100 Gs.
It's worth noting the threshold varies by how you measure it. Work comparing video reconstruction, instrumented helmets, and computational modeling has placed the 50%-risk level anywhere from roughly 65 Gs to 192 Gs (Rowson & Duma, 2012). The exact number is debated. What isn't debated is the order of magnitude — injuring brain tissue takes a substantial hit.
My working figure for the neck side is roughly **4.5 Gs** to begin tearing the ligaments of the craniocervical junction, which puts the upper neck on the order of thirteen times more vulnerable than the brain to the same force.
What Do Those Numbers Feel Like in the Real World?
Numbers in Gs are hard to feel, so here are two reference points.
Two full-sized football players colliding helmet-to-helmet generate roughly 90 Gs. That's in the neighborhood of the concussion threshold, which is why serious impacts in contact sports genuinely can damage brain tissue.
Now the other end. **4.5 Gs is roughly a 7 to 10 mile-per-hour fender bender.** A low-speed parking lot collision — the kind where both drivers get out, look at the bumpers, agree it's nothing, and drive away.
That is the level at which the ligaments holding your skull onto your spine begin to be damaged.
Why Is the Brain So Well Protected?
The brain's high threshold isn't an accident. It is genuinely well defended.
It sits inside a rigid bony vault. It is surrounded by cerebrospinal fluid, which distributes force rather than concentrating it. And it has no job requiring it to move independently, so nothing about its design trades protection for mobility.
If you were engineering a container for something fragile and irreplaceable, you would build roughly what a skull is.
Why Isn't the Upper Neck Protected the Same Way?
The craniocervical junction was designed under completely different requirements, and those requirements explain why it fails at such low force.
Nearly all of your orientation sensors are mounted on your head — your eyes, your ears, your inner-ear balance apparatus. Every one of them is directional, so their usefulness depends on aiming them precisely and quickly. When you hear something behind you, you need to turn your head toward it accurately, without conscious effort.
That requires an extraordinarily mobile joint at the top of the spine. And mobility and stability trade against each other like a seesaw — the more freely a joint moves, the less inherently stable it is.
Your upper neck sits at the far mobile end of that trade. There are no intervertebral discs between the skull and the first vertebra, or between the first and second. The joint surfaces are curved and angled rather than interlocking. The second vertebra has a bony peg — the dens, from a root word meaning *axle* — that the first vertebra and your entire head rotate around. The C1–C2 segment alone accounts for roughly half of all rotation in your cervical spine.
All of that produces beautifully precise aiming, and it means the structure holding your head onto your body is held there almost entirely by ligaments, with very little inherent bony stability.
It was built for precision. It was not built to absorb impact. And it sits at the exact spot where any force delivered to your head has to pass through — which is why it belongs in any evaluation after a [concussion or mild traumatic brain injury](condition-concussion-and-mtbi.html).
Does Speed Matter as Much as Force?
Yes, and this is the factor most often left out.
Ligaments respond very differently depending on how quickly they're loaded. Apply force slowly and a ligament has substantial tolerance — it stretches, accommodates, and returns. Apply the same force rapidly and it tears far more easily. Think of a rope: pull steadily and it takes the load, but snap it hard and suddenly and it's much more likely to fail.
Acceleration and deceleration injuries happen within a fraction of a second. It's the **rate of change**, not simply the magnitude of force, that overwhelms the ligament's ability to adapt. Which is why a low-speed collision — where total force is genuinely modest but your head's direction reverses almost instantaneously — can do more ligament damage than the numbers alone suggest.
Direction matters too. The alar ligaments, running from the second vertebra up to the skull, are under maximum strain when the head is rotated. Biomechanical testing of head-turned rear impacts confirms that alar, transverse, and apical ligament strain increases substantially when the head is turned at the moment of collision (Panjabi et al., 2006). So a [whiplash](condition-whiplash.html) sustained while you were checking a blind spot or looking at a passenger loads those ligaments in their most vulnerable position.
Is Everyone Equally Vulnerable?
No, and this rarely gets mentioned to the people it affects most.
Muscle mass around the neck absorbs some strain before it reaches the ligaments. A larger, more muscular person has more of that buffer, so the same collision transfers less load to tissue that has no give.
As a result, women tend to sustain more ligamentous damage than men in comparable car accidents. Not because of anything about how they were driving or positioned, but because there is less muscle available to intercept the force before it reaches the ligaments.
If you're a woman whose symptoms started after a collision everyone called minor, that pattern is worth knowing about.
What Does This Mean for Being "Cleared"?
Put the pieces together, because the conclusion follows directly from the arithmetic.
Any impact severe enough to make anyone worried about your brain has, by simple comparison of thresholds, almost certainly exceeded the threshold for injuring your upper neck — and exceeded it long before approaching the brain's tolerance. The neck doesn't need a large hit. It needs a fraction of one.
Yet the standard workup after a head injury images the brain, confirms there's no bleed, and sends the person home cleared.
We check the fortress and skip the gate.
That's not carelessness. It follows logically from the assumption that a mild traumatic brain injury is an injury to the brain — [an assumption worth examining on its own](post-is-a-concussion-actually-an-injury-to-the-brain.html). If that's what you believe is damaged, that's what you image. But it means the most vulnerable structure in the collision is routinely the one nobody looks at.
A normal brain MRI can be completely accurate and still miss the actual injury, because the brain genuinely wasn't damaged and the imaging was never pointed at what was.
Why Does an Upper-Neck Injury Cause So Many Symptoms?
An unstable or misaligned upper neck is not merely a mechanical inconvenience.
That junction carries the largest share of the position information your brain uses to know where your head is in space. It routes blood to the back of your brain through the vertebral arteries. It sits directly against the drainage pathway fluid uses to leave your head.
So a structure damaged at low force, in an accident everyone agreed was minor, is positioned to produce [headaches](condition-headaches.html), [dizziness](condition-dizziness.html), [brain fog](condition-brain-fog.html), pressure in the head, and a nervous system that will not settle — symptoms that then get attributed to a brain injury nobody can find.
The force numbers say where to look first. They have been saying it for a long time.
Did Your Symptoms Start After an Impact Everyone Called Minor?
If headaches, dizziness, or brain fog began after a collision or fall that seemed too small to matter, the force thresholds suggest a specific place to investigate — and it isn't the one that usually gets imaged.
Examining the craniocervical junction directly, with imaging built to show it, is the work we do at Cerebral. If you'd like a real evaluation, we're here.
References
- Guskiewicz KM, Mihalik JP, Shankar V, et al. Measurement of head impacts in collegiate football players: relationship between head impact biomechanics and acute clinical outcome after concussion. *Neurosurgery*. 2007;61(6):1244–1253. https://pubmed.ncbi.nlm.nih.gov/18162904/
- Pellman EJ, Viano DC, Tucker AM, Casson IR, Waeckerle JF. Concussion in professional football: reconstruction of game impacts and injuries. *Neurosurgery*. 2003;53(4):799–814. https://journals.lww.com/neurosurgery/abstract/2003/10000/concussion_in_professional_football_.4.aspx
- Panjabi MM, Ivancic PC, Maak TG, Tominaga Y, Rubin W. Alar, transverse, and apical ligament strain due to head-turned rear impact. *Spine*. 2006;31(6):631–638. https://pubmed.ncbi.nlm.nih.gov/16540865/
- Panjabi MM, Ito S, Pearson AM, Ivancic PC. Injury mechanisms of the cervical intervertebral disc during simulated whiplash. Soft tissue injury threshold during simulated whiplash: a biomechanical investigation. *Spine*. 2004;29(11):1217–1225. https://pubmed.ncbi.nlm.nih.gov/15105668/
- Rowson S, Duma SM. Brain injury prediction: assessing the combined probability of concussion using linear and rotational head acceleration. *Annals of Biomedical Engineering*. 2012;41(5):873–882. https://pubmed.ncbi.nlm.nih.gov/23299827/