Stanford study’s new data about sports concussions

By Stephen Beech

A major study of concussion injuries in sports could lead to better protective equipment and treatments, say scientists.

American researchers analyzed data from motion-sensing mouthguards worn by athletes in seven different sports.

Concussions, mild traumatic brain injuries, are among the most common sporting injuries with an estimated 3.8 million sports-related concussions every year in the United States alone.

The Stanford University research team discovered that straight-line motion of the head, measured by linear acceleration, is more predictive of concussion than turning motion of the head, measured by rotational acceleration.

The findings, published in the journal Annals of Biomedical Engineering, suggest that the prevailing understanding of the forces that lead to concussion may be missing the mark.

Linear acceleration tends to dominate when an impact force is directed to the head’s center of mass. For example, a collision to the top of the head when the head is lowered, or a fall backward and the head hitting the ground.

The new findings turn nearly 85 years of thinking in the field — which held that rotational motion was the primary driver of concussions — on its head.

The researchers say, if confirmed by further studies, it could have implications for helmet design and safety guidelines in sports.

Study lead author Jessica Towns 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.”

Head injury biomechanics can be traced to work by the Scottish physicist A.H.S. Holbourn.

He proposed that impacts that rotate the head, such as an uppercut to the chin in boxing, are particularly injurious to the brain as a result of “shearing” - the stretching and straining of the brain as different parts of the heavy but gelatinous organ rotate at different speeds.

Linear motion doesn’t cause shearing and was considered less dangerous.

Today, the most commonly used rating system for helmets in several sports prioritize reducing rotational acceleration.

Study senior author David Camarillo said: “Holbourn’s reasoning was based just on mechanics; it wasn’t biology.”

He said that until recently the only available data was from helmet-mounted sensing systems, but as the helmet moves around a lot it doesn’t give a direct measurement of head movements.

For more than a decade, Camarillo’s lab has helped develop motion-sensing technology that can be incorporated into mouthguards, where they can more accurately capture head motion.

Variations of the mouthguards are now used in several sports to help monitor for concussions.

Camarillo said: “The question we started out with many years ago was, ‘What are the forces required to cause concussion?’

“If we understand the nature of those forces and the limits of the human brain, then it might be possible to prevent concussions.”

The new study analyzed the largest and most diverse set of mouthguard data to date, including 203 male and female athletes spanning seven sports, including lacrosse, mixed martial arts (MMA) and football.

It included 47 impacts that led to concussions and 3,804 impacts that did not.

The impacts were videotaped and concussion was determined either by on-site medical staff or by later review of video for visual signs of injury, such as disorientation or balance disturbance.

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When the team set out to determine the forces that contribute to concussion using the new data, they expected to confirm the Holbourn hypothesis.

But Towns, a doctoral candidate at Stanford University, said: “We found the opposite.

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

Specifically, in their mathematical model, the researchers found that peak linear acceleration was the single most predictive factor in whether an impact would lead to concussion.

Camarillo said that, in real life, almost every collision results in both linear and rotational head motion.

He noted that the new findings do not mean that rotational motion isn’t harmful, only that it was less predictive of concussion than linear motion.

But he said the findings suggest that safety guidelines for helmets, from football to cycling, could be underestimating the risk of linear acceleration.

The researchers found that head impacts with a linear acceleration of 100 g — 100 times the standard acceleration because of gravity — carry a 50% risk of concussion.

The average head impact in football is about 25 g.

Towns said: “If 100 people were each hit at 100 g, half of them would get a concussion.

“That’s important because the thresholds that have been used for helmets to be sold on the market are usually much higher than 100 g.”

An often-cited threshold for 50% concussion risk, derived from helmet-mounted sensors, is 192 g.

In contrast, the new mouthguard data showed that a collision of 150 g nearly guarantees a concussion, with a risk of more than 99%.

Camarillo said: “There are still too many concussions happening.

“Why are there so many concussions even when people are wearing helmets?

“Part of the answer could be in this paper, which is that the linear accelerations that we previously thought were safe, we now see are much more dangerous.”

The researchers also found the threshold for concussion is likely much lower for children.

Camarillo said: “There’s an urgent need to get more of this data on kids because that might end up being the biggest problem.

“If linear acceleration is its own distinct mechanism of concussion and it’s very common, then we’ve been missing a lot.

“We haven’t tried as hard to optimize for this variable.

“It gives me some hope that there’s a new opportunity for preventing concussions.”

He added: “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.”