Concussions in football happen when a hit causes the brain to move rapidly inside the skull, stretching and disrupting nerve cells. The sport’s combination of speed, mass, and frequent contact creates the perfect conditions for this type of injury. But the specific forces involved, the plays that cause the most damage, and which players face the greatest risk are more nuanced than most people realize.
What Happens Inside the Skull on Impact
Your brain floats in cerebrospinal fluid inside a rigid skull. When a football player takes a hit, the skull accelerates or decelerates suddenly, but the brain lags behind. This mismatch creates two types of force that injure the brain in different ways.
Linear acceleration, the straight-line force of a head-on collision, creates sudden pressure changes inside the skull. These pressure gradients tend to cause more localized injuries like bruising on the brain’s surface. Rotational acceleration, the twisting or angular force from an off-center hit, causes the brain to shear against the inside of the skull. This relative motion between brain and bone stretches nerve fibers across wide areas, producing the diffuse damage characteristic of concussion.
Most football hits involve both types of force simultaneously, but it’s the rotational component that does the most concussion-specific harm. When nerve cell membranes get physically stretched by shearing forces, tiny defects open in their walls. Charged particles that are normally kept on one side of the membrane flood through: potassium rushes out of cells while sodium and calcium rush in. This triggers a cascade of abnormal electrical activity across the brain, something researchers describe as a “spreading depression-like” state. That wave of disrupted signaling is what produces the immediate symptoms of concussion: confusion, slowed thinking, dizziness, and sometimes loss of consciousness.
The Plays and Collisions That Cause Concussions
Not all hits are created equal. Biomechanical and video analysis of diagnosed concussions at the high school and youth levels found that helmet-to-helmet contact was the most common cause, followed by helmet-to-ground impacts and then helmet-to-body contact. In one case series published in Neurology, five of nine analyzed concussions came from helmet-to-helmet collisions, three from the head hitting the ground, and one from a hit to the body.
That breakdown reflects what the sport looks like in practice. Tacklers leading with their heads, ball carriers lowering into contact, and defenders colliding at full speed all create the high-acceleration, rotational-force scenarios that injure the brain. But the ground is a major factor too: a player driven backward whose helmet strikes the turf can experience a sharp rotational snap that’s just as dangerous as a direct hit from another player.
Kickoffs have historically been among the most dangerous plays because of the long running distances and high closing speeds between coverage and return teams. The NFL redesigned kickoff rules in 2024, moving to a “dynamic kickoff” format that reduces the distance players can build up speed before contact. Data from the 2024 season showed just four concussions on kickoffs league-wide, though the full picture of whether those numbers represent sustained improvement is still developing.
Which Positions Face the Most Risk
The answer depends on how you define risk. Research consistently shows that different positions face different types of head impact exposure, and the picture changes depending on whether you’re counting diagnosed concussions, unreported concussions, or total head impacts.
In the NFL, defensive backs have historically had the highest overall number of diagnosed concussions, largely because they’re involved in open-field tackling at high speeds. Quarterbacks, meanwhile, have the greatest risk per play: they take fewer total hits, but each one is more likely to result in a concussion because they’re often struck while vulnerable and unprepared for contact. Studies of professional football have also flagged running backs as a high-risk group for the same reason, since they absorb hits on nearly every carry.
Offensive and defensive linemen present a different, arguably more concerning pattern. A study of NCAA Division I players found that while linemen didn’t have significantly more diagnosed concussions than other positions, they reported far more undiagnosed concussions and “dings,” those brief moments of seeing stars or feeling foggy that players often shake off. Offensive linemen, in particular, reported significantly higher numbers than nearly every other position group. These players engage in head contact on virtually every snap, absorbing dozens of lower-intensity but repetitive impacts per game.
Why Sub-Concussive Hits Matter
The concussions you hear about on injury reports may not be the whole story. Researchers have grown increasingly concerned about sub-concussive impacts: hits that don’t produce obvious symptoms but still transfer force to the brain. A football lineman might absorb hundreds of these impacts in a single season without ever being pulled from a game.
Emerging evidence suggests these accumulated hits may lead to changes in brain structure and function over time. Cross-sectional studies in football and other contact sports have found associations between repetitive head trauma and both tissue-level changes in the brain and later-life cognitive problems. Some researchers believe that concussions, or a combination of concussions and sub-concussive impacts, may contribute to chronic traumatic encephalopathy (CTE), mild cognitive impairment, and depression, though the exact mechanisms connecting these impacts to long-term disease are still being worked out.
This is part of why the position-specific data on linemen is so notable. Players at positions that experience more frequent but lower-intensity hits report more post-impact symptoms and more often continue playing despite those symptoms. The culture of the position, where contact on every play is expected, makes it easy for accumulating damage to go unaddressed.
How Protective Equipment Reduces Force
Helmets are designed primarily to prevent catastrophic injuries like skull fractures, not to eliminate concussions. No helmet can stop the brain from moving inside the skull. However, newer equipment is making a measurable difference in reducing the forces that reach the head.
The most notable recent development is the Guardian Cap, a soft-shell cover that fits over a standard helmet. The NFL required certain position groups to wear Guardian Caps during preseason practices starting in recent seasons. An analysis of NFL preseason practice data from 2018 to 2023 found that after the Guardian Cap requirement was implemented, the concussion rate among affected positions dropped by 54% to 62%. That’s a substantial reduction, driven by the cap’s ability to absorb and distribute impact energy before it reaches the helmet shell. The NFL has since expanded Guardian Cap use to regular-season games for select positions.
Recognizing a Concussion After Impact
A concussion doesn’t require a loss of consciousness. In fact, most concussions don’t involve blacking out at all. The symptoms can be subtle: headache, feeling “in a fog,” difficulty concentrating, balance problems, sensitivity to light or noise, or just feeling “off.” Some symptoms appear immediately, while others develop over minutes or hours.
Sideline assessment typically involves a physical examination, questions about symptoms, and cognitive tests that evaluate memory, attention, concentration, and processing speed. These tests compare a player’s post-impact performance to their baseline. Most athletes don’t need a brain scan after a concussion, since CT and MRI imaging usually appears normal. Concussion is fundamentally a functional injury, a disruption in how nerve cells communicate, rather than a structural one visible on standard imaging.
The metabolic disruption inside the brain doesn’t resolve the moment symptoms fade. After the initial ionic flooding, the brain enters an energy crisis as it works to restore normal cell chemistry. During this recovery window, the brain is especially vulnerable to additional injury. A second impact before recovery is complete can produce dramatically worse outcomes, which is why return-to-play protocols require a gradual, symptom-free progression through increasing levels of activity before a player is cleared for contact.