Concussions happen when a force to the head, neck, or body causes the brain to move rapidly inside the skull. This movement stretches and strains brain cells, triggering a cascade of chemical disruptions that temporarily impair normal brain function. You don’t have to be hit directly on the head to get one. A hard fall, a car crash, or a collision on a playing field can all transmit enough force to shake the brain and cause injury.
What Happens Inside the Skull
Your brain floats in cerebrospinal fluid inside your skull, which provides some cushioning. But when your head accelerates or decelerates suddenly, the brain can slam against the inside of the skull at the point of impact (called a coup injury) and then bounce against the opposite side (a contrecoup injury). This back-and-forth motion is common in car accidents, falls, and direct blows to the head.
The real damage, though, happens at the cellular level. When brain tissue is stretched or sheared by rapid movement, the nerve fibers (axons) connecting brain cells get strained. Research on nerve cell tissue suggests that stretching axons by roughly 10% of their length can cause reversible injury, the kind associated with a typical concussion. Stretching them by about 20% or more starts causing cell dysfunction and death. This type of widespread nerve fiber damage is called diffuse axonal injury, and it’s particularly linked to rotational forces, where the head twists rather than just moving straight forward and back.
Once those cells are strained, a chemical crisis unfolds. The impact causes brain cells to release a flood of signaling chemicals, which triggers a massive shift of charged particles across cell membranes. To restore balance, cells burn through their energy stores at an accelerated rate, demanding far more glucose than usual. At the same time, blood flow to the brain drops by as much as 50%. The brain is essentially running a sprint while being starved of fuel. This energy mismatch is why concussion symptoms can linger for days or weeks, even though imaging scans often look normal.
The Most Common Causes
Falls are the single biggest cause, accounting for nearly half of all TBI-related hospitalizations. This includes everything from a toddler tumbling off a piece of furniture to an older adult slipping on ice. Adults 75 and older are especially vulnerable: they account for about 32% of TBI-related hospitalizations and 28% of TBI-related deaths, and falls are a primary reason.
Motor vehicle crashes are the next major cause. The forces involved in even a moderate-speed collision are well above the threshold for concussion. One study reconstructing NFL helmet impacts found that concussed players experienced peak head accelerations around 94 g (where 1 g equals the force of gravity). A car crash can easily produce forces in that range or higher, especially without a seatbelt or in a rollover.
Assaults, including punches and blows to the head, round out the leading causes. The rotational component of a punch to the jaw, for instance, is particularly effective at generating the kind of brain-twisting motion that injures axons.
Sports With the Highest Concussion Rates
Among high school athletes, boys’ tackle football tops the list, followed by girls’ soccer, boys’ lacrosse, boys’ ice hockey, and boys’ wrestling. Girls’ lacrosse, field hockey, basketball, and softball also rank in the top ten. Across all high school sports, more than two out of three concussions result from athlete-to-athlete collisions.
The specific mechanism varies by sport. In football, tackling causes nearly two out of three concussions. In ice hockey, body checking accounts for about one in three. Soccer concussions most commonly occur during heading, but the concussion itself usually results from colliding with another player while going for the ball, not from the ball itself. In wrestling, takedowns are the leading cause. Cheerleading concussions are almost exclusively linked to stunts involving tosses or lifts.
Some mechanisms are less obvious. In baseball, about one in four concussions come from being hit by a pitch. In field hockey, six out of ten concussions involve contact with equipment like a stick or ball rather than another player. In volleyball, digging (diving low to return a hard-hit ball) is the activity most commonly linked to concussions.
You Don’t Always Need a Direct Hit
One of the most misunderstood aspects of concussions is that you don’t need to be struck on the head. A hard tackle to the chest, a whiplash-type motion in a car accident, or even a blast wave from an explosion can transmit enough force through the neck and spine to jolt the brain inside the skull. Any sudden acceleration or deceleration of the body can do it, as long as the force reaches the head.
There’s also no single force threshold that guarantees a concussion. Individual tolerance varies based on factors like age, neck strength, the direction of impact, and whether the person braced for the hit. A blow that causes a concussion in one person may not in another.
Who Is Most at Risk
Children and older adults face the greatest risk, though for different reasons. Children’s brains are still developing, their necks are weaker relative to their head size, and they’re more likely to encounter falls during play and sports. Children in rural areas are more likely to sustain a TBI and to die from one compared to children in urban areas, likely due to differences in healthcare access and the types of activities common in rural settings.
Older adults face elevated risk because of balance problems, medication side effects that increase fall risk, thinner blood vessels, and often the use of blood thinners that can make even mild brain bleeding more dangerous. People 75 and older have the highest rates of TBI-related hospitalization and death of any age group.
Anyone with a previous concussion is also at increased risk for prolonged symptoms after a subsequent injury. A prior brain injury is one of the strongest predictors of persistent post-concussive symptoms, which can include headaches, difficulty concentrating, and mood changes lasting weeks or months.
Why Symptoms Can Be Delayed
Concussion symptoms sometimes appear immediately, but they can also take hours or even days to show up. This delay catches many people off guard. You might walk away from a collision feeling fine, only to develop a headache, fogginess, or nausea later that evening or the next morning. The metabolic energy crisis in the brain builds gradually as cells struggle to restore their chemical balance, which helps explain why symptoms don’t always track with the moment of impact.
This is part of why concussion diagnosis remains a clinical judgment call rather than something a scan or test can definitively confirm. Standardized assessment tools like the SCAT6 help clinicians evaluate symptoms, balance, memory, and concentration, but a normal score on these tools does not rule out a concussion. No single test can. The diagnosis depends on the full picture: the mechanism of injury, observable signs, and how the person feels in the hours and days that follow.
If you take a significant hit during a sport or activity, the safest approach is to stop playing immediately. Returning to activity before the brain has recovered exposes it to further injury during a period when it’s already in an energy deficit and uniquely vulnerable.