A concussion occurs when a force applied to your head, neck, or body causes your brain to move rapidly inside your skull. This movement stretches and strains brain tissue, triggering a cascade of chemical and metabolic disruptions that temporarily impair normal brain function. Importantly, you don’t need to be hit directly on the head. Any impact that transmits enough force to shift the brain can cause one.
What Happens Inside the Skull
Your brain floats in cerebrospinal fluid inside the rigid shell of your skull. When a sudden force hits your head or body, the skull accelerates or decelerates faster than the brain inside it. The brain slides, rotates, or compresses against the inner walls of the skull. This can cause injury at the point of impact (called a coup injury) and, as the brain rebounds, on the opposite side as well (a contrecoup injury). Simulations of head impacts have found that the rebound injury on the opposite side is often more severe than the injury at the initial impact site, and the rebound doesn’t always land directly opposite the blow.
Think of it like gelatin in a container. If you jerk the container hard enough, the gelatin shifts, deforms, and slams against the walls. Your brain responds the same way to sudden acceleration.
Rotational Force Is the Primary Culprit
Not all forces affect the brain equally. Researchers distinguish between two types of acceleration: linear (a straight push, like running into a wall) and rotational (a twisting or angular motion). Both contribute to concussions, but rotational acceleration is consistently the dominant factor in producing the tissue deformation that leads to injury. When researchers built models incorporating both types of motion, only rotational acceleration and rotational velocity reliably predicted the amount of strain inside brain tissue. This finding holds across studies of football, hockey, and other contact sports.
Duration matters too. The longer the acceleration event lasts, the less force is needed to reach dangerous strain levels. A sharp, brief rotation might not cause injury at low magnitude, but the same magnitude sustained over a longer window can produce enough tissue deformation to cause a concussion.
Estimated human tolerance thresholds for rotational forces sit around 20 to 30 radians per second for angular velocity and roughly 1,800 radians per second squared for angular acceleration, though these numbers vary between individuals and aren’t hard cutoffs. For linear acceleration, impacts above 40g have been flagged in head impact monitoring studies, but linear force alone is a poor predictor of concussion. The combination and direction of forces matter far more than any single number.
How Brain Tissue Gets Damaged
The rapid movement of the brain generates shear, tensile, and compressive forces within the tissue itself. White matter, the bundles of nerve fibers that connect different brain regions, is especially vulnerable. These fibers are long, tightly packed, and organized in parallel. When rotational forces twist the brain, individual fibers stretch and deform. This stretching disrupts the transport system inside nerve cells, causing transported materials to pile up and form swellings within hours of injury. Even in severe cases, only a small percentage of fibers in any given tract actually suffer this transport interruption, which is part of why concussions vary so much in severity.
The physical impact also damages tiny blood vessels throughout the brain, disrupting the blood-brain barrier. This barrier normally acts as a selective filter, keeping harmful substances in the bloodstream from reaching brain tissue. After trauma, the cells lining these vessels break down and the tight seals between them loosen. Plasma proteins leak into surrounding tissue, triggering inflammation. Immune cells activate, releasing molecules that cause further swelling and damage. This process, called cerebral edema, results from the combination of vessel damage, seal disruption, and the accumulation of fluid that follows.
The Energy Crisis Inside Your Brain
Beyond the physical stretching and tearing, a concussion sets off a chemical chain reaction that disrupts how your brain produces and uses energy. The initial strain on nerve fibers causes a massive release of glutamate, the brain’s primary excitatory chemical messenger. Glutamate floods the gaps between cells and binds to receptors on neighboring neurons, causing a wave of electrical activity. This triggers an imbalance of charged particles: potassium rushes out of cells while sodium and calcium rush in.
Your brain immediately tries to restore balance, and that restoration demands enormous amounts of energy in the form of ATP, the cell’s fuel molecule. To produce enough ATP, the brain enters a state of hyperactive glucose consumption. But here’s the problem: blood flow to the brain drops by as much as 50% after a concussion. The brain is burning through fuel at an accelerated rate while its supply lines are partially cut off. This mismatch creates a true energy crisis. The initial period of high glucose demand transitions into a period of depressed metabolism, during which the brain is running on empty and is especially vulnerable to further injury.
Elevated glutamate levels persist for 24 to 48 hours after the initial trauma, sustained partly by the damaged blood-brain barrier. Meanwhile, the influx of calcium into cells triggers the production of reactive oxygen species, unstable molecules that damage cell membranes, proteins, and DNA. This oxidative stress further weakens the blood-brain barrier, creating a feedback loop of leakage, inflammation, and cellular damage.
You Don’t Need a Direct Hit to the Head
One of the most misunderstood aspects of concussions is that a direct blow to the head isn’t required. A hard tackle to the chest, a rear-end car collision, or any sudden jolt that whips the head can transmit enough impulsive force to shift the brain inside the skull. During a whiplash event, for example, the violent back-and-forth snap of the head can cause the brain to accelerate and collide with the inner skull walls, producing a concussion with no external head contact at all.
The 2023 Amsterdam Consensus Statement on Concussion in Sport defines sport-related concussion as a traumatic brain injury caused by a direct blow to the head, neck, or body that results in an impulsive force being transmitted to the brain. That “or body” is key. A football lineman absorbing a hit to the torso, a cyclist thrown from a bike, or a skier who lands hard on their back can all sustain concussions from the force traveling upward through the spine and into the skull.
Why Symptoms Can Be Delayed
Concussion symptoms don’t always appear instantly. While some people feel dizzy, confused, or nauseous within seconds, others may not notice anything wrong for minutes or even hours. This happens because the chemical and metabolic cascade described above unfolds over time. The initial glutamate surge, the developing energy crisis, the progressive breakdown of the blood-brain barrier, and the inflammatory response all build gradually. Symptoms can evolve as each phase of the cascade takes hold.
In most cases, the acute symptoms of a concussion resolve within days to a few weeks. However, in a subset of people, symptoms like headaches, difficulty concentrating, and dizziness persist. Most persistent post-concussive symptoms emerge within the first 7 to 10 days and can last longer than three months. The severity of the initial metabolic disruption, the degree of blood-brain barrier damage, and whether the brain suffers a second injury before fully recovering all influence how long symptoms last.