A concussion happens when your head experiences a sudden acceleration or deceleration that causes the brain to move, stretch, or twist inside the skull. This can result from a direct blow to the head, but it can also happen without any head contact at all. A hard hit to the chest or a whiplash motion from a car crash can transmit enough force through the neck to shake the brain and cause injury. Falls are the single most common cause, followed by being struck by an object (often in sports), motor vehicle crashes, and assaults.
What Actually Happens Inside Your Skull
Your brain floats in cerebrospinal fluid inside a rigid skull. When your head suddenly accelerates or stops, the brain doesn’t move in perfect sync with the bone surrounding it. For short, sharp impacts, the brain lags behind the skull, stretching the blood vessels that bridge the gap and bumping against the inner surface of the skull. For longer-duration forces, the brain moves with the skull but twists internally, creating shearing stress deep in the tissue, particularly in the dense bundle of nerve fibers connecting the two hemispheres.
Rotational acceleration, the kind of spinning or twisting motion your head undergoes when hit from the side or at an angle, is the dominant force behind most concussions. It deforms brain tissue far more than a straight-line impact of the same magnitude. As the duration of that rotational force increases, the threshold needed to cause injury actually drops. A longer, slower whip of the head can be just as dangerous as a faster, shorter one.
Once the tissue stretches beyond a safe limit, a chemical cascade begins almost immediately. Ions flood across damaged cell membranes in the wrong direction, and the brain scrambles to restore balance. That restoration demands a surge of energy at the worst possible time: blood flow to the brain can drop by as much as 50% after a concussion, starving cells of the fuel they need to repair themselves. This mismatch between energy demand and energy supply is the core of the “metabolic crisis” that follows a concussion, and it’s why symptoms can linger for days or weeks even though imaging scans often look normal.
There Is No Safe Threshold
One of the most persistent myths about concussions is that they require a specific amount of force. The NFL once commissioned research suggesting that hits above 85g (85 times the force of gravity) would likely cause concussions, while anything below probably wouldn’t. Real-world data quickly challenged that. In a five-season study at UNC-Chapel Hill, researchers recorded concussions from hits as low as 60g and 63g. One running back concussed at 60g had some of the worst symptoms recorded during the entire study. Meanwhile, other players absorbed hits over 100g without obvious injury.
The takeaway is that individual vulnerability matters enormously. Factors like the angle of impact, the direction your head was turned, your history of prior concussions, and even your neck strength all influence whether a given force crosses the line into injury. Two people can experience the same collision and walk away with very different outcomes.
Falls: The Leading Cause Across All Ages
Falls account for more traumatic brain injuries than any other single cause, and the pattern varies sharply by age. Infants under one year are most often injured when they’re dropped or roll off a surface. Toddlers and preschoolers fall while climbing, walking, and exploring, usually from relatively low heights. School-aged children fall from playground equipment, bikes, and skateboards. In older adults, a simple trip on a curb or a slip in the bathroom can generate enough force to cause a concussion, particularly because aging reduces the brain’s cushioning fluid and makes blood vessels more fragile.
How Concussions Happen in Sports
Among youth athletes, boys’ tackle football has the highest concussion rate per athletic exposure, followed by girls’ soccer and boys’ lacrosse. The specific mechanism varies sport by sport, and the details are worth knowing if you or your child plays one of these.
- Football: Tackling causes nearly two out of three concussions at the high school level.
- Soccer: Heading the ball is the most common activity linked to concussions, but the injury usually comes from colliding with another player during the header, not from the ball itself. About one in three concussions in girls’ soccer and one in four in boys’ soccer happen during heading.
- Ice hockey: Almost two out of three concussions result from player-to-player collisions, with body checking responsible for about a third.
- Lacrosse: About three out of four concussions in boys’ lacrosse come from collisions between athletes.
- Basketball: Nearly two out of three concussions among boys and half among girls result from player collisions.
- Cheerleading: Almost all concussions are linked to stunts involving tosses or lifts.
- Baseball: About one in four concussions result from being hit by a pitch.
- Wrestling: Takedowns are the most common cause.
- Field hockey: About six in ten concussions involve contact with a stick or ball rather than another player.
Girls’ soccer ranking second overall is notable. Girls tend to report higher concussion rates than boys in sports where both sexes compete under similar rules (soccer, basketball, baseball/softball). The reasons likely involve a combination of neck strength differences, reporting patterns, and hormonal factors, though no single explanation fully accounts for the gap.
Car Crashes and Whiplash
You don’t need to hit your head on the steering wheel or window to get a concussion in a car accident. In a rear-end crash, your torso accelerates forward with the seat while your head snaps backward into the headrest and then whips forward. That rapid back-and-forth generates enough rotational force to strain brain tissue. Crash simulations have shown that a rear-end collision at a speed change as low as 15 km/h (roughly 9 mph) can produce brain tissue strain comparable to impacts that caused concussions in NFL players, particularly when the headrest sits low enough for the head to wrap over it.
This is why concussion and whiplash injuries frequently overlap. The neck motion that strains your cervical spine is the same motion that shakes your brain.
Assaults and Blast Exposure
Violence is a significant cause of concussions, including intimate partner abuse, physical assaults, and shaken baby syndrome in infants. These injuries are often underreported and undertreated because the circumstances surrounding them can prevent people from seeking care. Blast waves from explosions are another mechanism, particularly relevant for military personnel. The pressure wave passes through the skull and creates rapid compression and expansion of brain tissue, producing a concussion even without any physical impact to the head.
What Makes Some People More Vulnerable
Prior concussions are the single strongest predictor of future concussions. Each one lowers the threshold for the next, meaning less force is needed to cause injury. The brain’s metabolic recovery after a concussion takes longer than symptoms suggest. Someone who feels fine after a week may still have cellular repair underway for weeks beyond that, and a second impact during this window can be far more damaging than the first.
Children and adolescents are more susceptible than adults for several reasons. Their heads are proportionally larger relative to their bodies, their neck muscles are weaker, and their brains are still developing. Recovery also tends to take longer in younger patients, with children under 12 often needing more time before safely returning to activity than teenagers or adults.
Neck strength plays a protective role regardless of age. A stronger neck reduces the degree of head acceleration after an impact by absorbing and distributing force before it reaches the skull. This is one reason why athletes who brace for a hit tend to fare better than those caught off guard, and why strengthening the neck is increasingly emphasized in concussion prevention programs.