Concussions in football happen when a hit delivers enough force to make your brain shift and twist inside your skull. This can occur during tackles, blocks, collisions between players, or even from hitting the ground. The force needed is surprisingly high, around 96 to 100 g’s of linear acceleration, but football regularly produces impacts in that range, especially at full speed.
What Happens Inside Your Skull
Your brain floats in cerebrospinal fluid inside the skull. When your head takes a hit or suddenly changes direction, the brain doesn’t move in sync with the skull. It shifts, compresses against the inner wall at the point of impact (called a coup injury), and then rebounds to strike the opposite side (a contrecoup injury). The contrecoup side often sustains more damage because the brain, which is slightly less dense than the fluid around it, gets displaced in the opposite direction of the initial hit while the denser fluid moves toward the impact site.
Rotational forces add another layer of damage. When your head twists during a collision, the gray matter and white matter inside the brain move at different speeds because they have different densities. This shearing effect tears the tiny connections between nerve cells, particularly at the junctions where gray and white matter meet. That widespread tearing is what disrupts normal brain function and produces the symptoms you feel after a concussion.
The Forces That Cause Injury
Researchers have used accelerometers embedded in helmets to measure exactly how much force it takes. In high school football, linear acceleration above 96 g’s and rotational acceleration above 5,582 radians per second squared represent the bottom thresholds where concussion risk starts climbing. The average concussion-causing hit in college athletes registers around 103 g’s, and in professional players about 98 g’s. Those numbers are remarkably consistent across levels of play.
At 106 g’s combined with high rotational acceleration, a player faces roughly an 80% chance of concussion. But force alone doesn’t tell the whole story. Where the hit lands matters too. Impacts to the front, side, or top of the helmet carry higher risk than hits to the back. When researchers combined rotational force, linear force, and impact location into a single prediction model, the probability of concussion rose from about 2% (for rotational force alone) to nearly 7% (adding linear force) to over 13% when the hit landed on the front, side, or top of the head.
How Concussions Happen on the Field
The most common scenarios break down into a few categories, each with different force profiles.
Open-field tackles are where some of the highest-magnitude impacts occur. A defensive back closing on a running back at full speed, or a linebacker meeting a receiver over the middle, produces exactly the kind of sudden deceleration and rotational whipping that the brain handles worst. These collisions involve two players moving in different directions, often with heads lowered, creating combined closing speeds that generate extreme forces.
Line-of-scrimmage collisions work differently. Offensive and defensive linemen collide on nearly every play, but these are typically shorter-distance, lower-magnitude impacts. The tradeoff is volume. Linemen absorb more total hits per game and per practice than any other position group. Offensive linemen report the highest frequency of full-contact practice reps, significantly more than most other positions. While each individual hit may fall below concussion thresholds, the cumulative load is a concern in its own right.
Ground impacts are often overlooked. A player doesn’t need to collide with another player to get a concussion. Being driven to the turf, especially when the back of the head strikes the ground, can produce the rapid deceleration that sends the brain into the skull wall. Quarterbacks are particularly vulnerable to this when sacked from behind.
Kickoff returns remain one of the higher-risk plays despite rule changes. The NFL saw concussions on kickoffs rise from 8 to 35 after introducing its Dynamic Kickoff format, though the rate per play actually stayed below the old format because return volume more than doubled with over 1,100 additional returned kicks. The play’s danger comes from players sprinting at full speed from opposite directions, creating the closing-speed problem that maximizes impact forces.
Which Positions Face the Most Risk
In college football, concussions are spread across positions more evenly than most people expect. Offensive linemen account for about 17.5% of head-impact-related outcomes, followed by defensive backs at 16.7%, linebackers at 15.1%, and defensive linemen at 14%. Running backs come in at 9.7%, wide receivers at 11.6%, and quarterbacks at 5.5%.
The surprise is offensive linemen topping the list. While skill-position players tend to experience fewer but harder hits from full-speed, open-field collisions, linemen absorb a relentless stream of moderate impacts. Offensive linemen also reported significantly higher numbers of undiagnosed concussions and minor head-impact events (“dings”) than most other position groups. This suggests that the repetitive, play-after-play nature of line play creates a different kind of risk, one that’s easier to dismiss because no single hit looks dramatic.
Why Symptoms Don’t Always Show Up Immediately
One reason football concussions go unrecognized is that symptoms often develop on a delay. While some players feel dazed or confused right away, concussion symptoms typically appear within one to two days after the hit. They can emerge gradually over hours, which means a player might finish a game feeling fine and wake up the next morning with a headache, mental fog, or sensitivity to light.
This delay is partly why self-reporting is unreliable, especially among linemen who experience high-frequency impacts and may normalize the feeling of being hit in the head dozens of times per practice. The brain’s inflammatory response to injury builds over time, so the full picture of a concussion often isn’t clear until well after the game ends.
How Equipment and Technique Reduce Risk
Modern helmets have made measurable progress. More than 98% of NFL players now wear top-performing helmets that have been independently rated for impact absorption. The league is also prioritizing facemask redesigns to catch up with improvements already made in helmet shell technology. At the youth and college levels, padded helmet covers (sometimes called Guardian Caps) add a soft outer layer that reduces the peak force transferred during helmet-to-helmet contact.
Tackling technique is arguably more important than equipment. “Heads-up” tackling, where the player keeps their head up and makes initial contact with the shoulder rather than the crown of the helmet, reduces the chance that rotational and linear forces channel directly through the skull. Programs that train players to tackle with their heads to the side of the ball carrier, keeping the head out of the direct collision path, have gained traction at every level of football. The physics are straightforward: if your head isn’t the point of impact, the forces acting on your brain drop dramatically.
Rule changes reinforce these techniques. Targeting penalties, limits on full-contact practice time, and restrictions on certain blocking techniques all aim to reduce either the number or severity of head impacts. High school players face a concussion rate of about 0.66 per 1,000 game exposures, compared to just 0.08 per 1,000 practice exposures, a gap that reflects both the higher intensity of games and the protective effect of limiting contact in practice.