Sports concussion study challenges assumptions about dangerous head impacts

A study of athletes across seven sports has challenged an 85-year-old theory about concussion, finding that straight-line head acceleration predicted the injury better than rotation.

The findings contradict the theory that head rotation is the main cause of concussion and suggest that helmet safety standards may underestimate the risks associated with straight-line impacts.

Researchers analysed data from motion-sensing mouthguards worn by 203 male and female athletes, identifying 47 impacts associated with concussion and 3,804 that did not result in the injury.

The study, conducted by Stanford Medicine scientists and collaborators, found that peak linear acceleration was the strongest single predictor of concussion.

Linear acceleration measures changes in the head’s straight-line motion, while rotational acceleration measures changes in how quickly the head turns.

Adding rotational acceleration to the researchers’ mathematical model did not improve its predictive accuracy.

Including peak rotational velocity, which measures how quickly the head rotates, produced a slight improvement.

However, only 47 of the recorded impacts were associated with concussion, limiting the evidence available to estimate injury risk.

The researchers also acknowledged that the impacts may have involved greater linear movement than rotational movement, potentially influencing which measure performed better.

Jessica Towns, a graduate student in bioengineering and lead author of the study, said: “Not only is the linear acceleration predictive of concussion, the threshold is lower than people had previously thought in terms of the levels that are unsafe.”

The results challenge a long-established theory of head injury biomechanics dating back to Scottish physicist A.H.S. Holbourn during the Second World War.

Holbourn proposed that rotational impacts were particularly damaging because they caused shearing, where different parts of the brain stretch and strain as they move at different speeds.

Straight-line movement was considered less dangerous because it was not thought to produce the same forces.

Subsequent animal research appeared to support the theory, and many helmet safety rating systems prioritise reducing rotational acceleration.

David Camarillo, associate professor of bioengineering at Stanford and senior author of the study, said earlier understanding had relied heavily on mechanical theory rather than direct measurements of head movement.

“Holbourn’s reasoning was based just on mechanics; it wasn’t biology,” he said.

Until recently, reliable measurements of head impacts in athletes were difficult to obtain.

Earlier research relied on helmet-mounted sensors, which can move independently of the head and affect measurement accuracy.

For more than a decade, Camarillo’s laboratory has helped develop sensors incorporated into mouthguards to measure head motion more directly.

The latest analysis included athletes from seven sports, including lacrosse, mixed martial arts and American football.

Impacts were recorded on video, with concussions identified either by medical staff at the scene or through subsequent video analysis of visible signs such as disorientation and balance disturbances.

The researchers had expected the data to support the established theory about rotational movement.

“But we found the opposite,” Towns said.

“We found that linear head motion was more predictive of concussion than rotational head motion.”

The analysis also suggested that the level of straight-line acceleration associated with concussion may be lower than previously estimated.

Further studies are needed to confirm the findings, particularly given the relatively small number of concussion events and the possible differences in the types of head movement recorded.

The researchers also want to establish whether changes to protective equipment and safety standards could reduce concussion risk.

Camarillo said: “I would say this is the beginning, not the end.

“This is an 85-year-old theory, and this is the first paper that has really challenged it.”