Collective power of head hits, not just number, increases the likelihood of CTE

When Jeffrey Vlk played running back in high school in the 1990s and then safety in college, he took and delivered numerous tackles during full-contact football practices. Spanking was a mainstay, as were injuries, including concussions.

When he became a coach at Buffalo Grove High School outside of Chicago in 2005, Vlk did as he was taught: he had his players hit and tackle during practices to “toughen them up.”

However, by the time he became head coach in 2016, he saw that many of his players were so worked up from a week of hitting in practice that they missed games or were more prone to injury in those games.

So as of 2019 Vlk has eliminated full contact practices. Players wore shoulder pads once a week, on Wednesday, what he called contact day. That’s when they hit tackle bags and crash pads and bagged teammates but didn’t throw them to the ground. Vlk said no starting player had been injured in his training in four years.

“Injuries like that can stay with you for a long time,” he said, “and knowing that I’m keeping the kids safe not only in our program but outside of the program is reason enough to go this route.”

Vlk’s approach to limiting players’ hits is slowly spreading in the football world, where much of the effort is focused on preventing and treating concussions, which often have observable symptoms and are followed by sports competitions.

But researchers have posited for years that the more blows to the head a player receives — even subconcussive ones, which are usually not tracked — the more likely he is to develop cognitive and neurological problems later in life.

A new study published Tuesday in the scientific journal Nature Communications added a critical wrinkle: A football player’s chances of developing chronic traumatic encephalopathy, or CTE, are related to the number of head bumps absorbed, but also to the cumulative impact of all those hits.

The study, the largest to look at the causes of CTE to date, used data published in 34 studies that tracked the number and magnitude of head hits measured by football helmet sensors. Using the data, which goes back 20 years, the scientists estimated the number and force of head thrusts absorbed by 631 former football players who donated their brains to studies overseen by Boston University researchers.

The paper sought to address one of the most intractable challenges facing brain trauma researchers: identifying which aspects of head bumps contribute most to CTE. They looked at the number of blows to the head, the number of years playing football, the strength of those blows and other factors.

The best predictor of brain disease later in life, the study found, was the cumulative force of the head punches absorbed by the players over the course of their careers, not the number of concussions diagnosed.

“We’re now getting a better understanding of what causes CTE pathology, but we’re also getting a better understanding of what doesn’t cause CTE pathology,” said Dr. Daniel Daneshvar, an assistant professor at Harvard Medical School and the study’s lead author. “And in this case, it’s the largest study of CTE pathology ever, and concussions were essentially noise.”

Of the 631 brains examined, 451 players, or 71 percent, were found to have CTE, while 180 did not. The players estimated to have absorbed the greatest cumulative power had the worst forms of CTE, which have been associated with symptoms such as memory loss, impulsive behavior, depression, and suicidal ideation.

Eric Nauman, a professor of biomedical engineering at the University of Cincinnati who was not involved in the study, said the results reinforced the idea that an accumulation of subconcussive blows, rather than concussions, was the driving force behind long-term cognitive decline .

The most recent data “seems to support the idea that, yes, all these hits matter, they all add up,” said Dr. Nauman. “If you build up damage faster than the body can repair it, you have a problem.”

He said the analysis points the way to obvious changes that could make football safer, such as eliminating hitting during training and developing helmets that absorb more impact, especially on the back of the head.

Dr. Nauman noted that the new study included the brains of players with and without the disease, sparing the common concern that the researchers were only looking at the most damaged brains.

It also found links between the estimated number and type of hits players took during their career and their health many years later, a factor Dr. Nauman would make it more difficult for opponents to argue that players may have suffered unknown injuries in the decades since. they stopped playing football, which may have caused their later cognitive problems.

Dr. Nauman said the new research was still subject to limitations. The study counted all head impacts detected by helmet sensors, except those caused by crowding or incidental movement. But previous research suggested that key hits appeared to be above a certain threshold, a distinction the study was unable to make.

Because the NFL has not published its helmet sensor data, the study used college sensor data as a proxy for professional players.

Helmets have improved in recent years and it is likely that players whose careers predate the improvements have taken more of the impact of a given hit. But football players in decades past were, on average, smaller and slower than today’s football players, making each hit less powerful, said Dr. Nauman.

“That’s certainly a caveat, but it’s not something that would make me think the basic conclusions are wrong,” he said.

Joseph J. Crisco, a professor at Brown University who helped develop a sensor used in Riddell helmets, said the study sought to overcome a fundamental challenge — that researchers had not tracked how many hits the brain donors had during their careers. had collected.

Instead, the study used helmet sensor data from a more recent group of players to estimate the number and strength of headbutts for the older players, based on what positions they played, at what level of the sport and for how long.

While research was needed on the actual impact of players’ heads over their lifetimes, the findings suggest that “the players who are hit the hardest and most often have a greater chance of developing CTE in the future.”

Steve Rowson, who studies the impact of helmets and concussion risk at Virginia Tech, said the study’s emphasis on the strength and hit rate players take is consistent with how scientists understand brain injuries.

“Every time you hit your head, your brain is going through some load, and there’s going to be a pressure response and a brain tension response — a stretching of the brain tissue,” he said. “And if you cross a certain threshold, you would expect an injury response, and the severity of that injury response will depend on the acceleration values.”

Researchers have been able to pinpoint some factors that explain different players’ exposure to headbutts, he said. For example, he said linemen are most likely to get hit on the front of their helmets, while quarterbacks are more likely to get serious hits on the back of their helmets.

But, Dr Rowson said, it would be a mistake for people to think they can now use the findings to predict a person’s chances of long-term cognitive problems.

“What I think we can’t do right now is look at one person and get a really good sense of their exposure to a head impact versus another,” he said, “because there’s a huge difference from person to person that we can’t pretty well before.”

The study notes that future research should explore different thresholds for hit counting, an advancement that Dr. Rowson was important. Some head thrusts, he said, are mild enough that the brain can probably tolerate them. But at what point the effects become harmful is not clear, he said.

“Not all effects are created equal,” he said. “Trying to figure out which effects are most important, I think, can really help with this kind of analysis.”