A football helmet's health warning sticker is pictured between a U.S. flag and the number 55, in memory of former NFL player Junior Seau; new research suggests that the accumulation of subconcussive hits may have more significant long-term effects than concussions. (© Mike Blake / Reuters/REUTERS)

Connections between concussions and chronic traumatic encephalopathy have been the subject of debate and discussion from Capitol Hill to the NFL’s Park Avenue offices in New York, but a new study suggests that the mental, emotional and cognitive challenges faced by some former football players aren’t solely attributable to concussions they suffered during their playing days.

The study, published Thursday afternoon in the Journal of Neurotrauma, found a correlation between the cumulative number of hits experienced by youth, high school and college football players and their later-life clinical outcomes. Researchers developed a metric that found the total number of hits to be a better predictor than concussion history of later-life depression, cognition and behavioral regulation.

“There has been a tremendous amount of growth in the last several years in the prevention, detection, and management of symptomatic concussions across all levels of play and all sports. That’s fantastic,” said Robert Stern, a professor of neurology at Boston University, director of clinical research at the school’s CTE Center and one of the paper’s co-authors. “The problem is that the focus on concussion has taken away from an appropriate discussion about the more common subconcussive trauma.”

Researchers warned that the new study encompassed a limited sample size — 93 men who played football in high school or college — and that further research is needed. They utilized previous studies that used accelerometers to tabulate the number of hits players endure in a season, then extrapolated that data to estimate the total exposure to repetitive head impacts that their study subjects faced, taking into consideration the number of seasons played, positions played and the level of play. They called this figure the Cumulative Head Impact Exposure Index (CHII).

The mean number of total impacts for the men in the study was 7,742, and those with a higher CHII figure reported higher instances of cognitive, mood and behavioral impairments. Results suggested the CHII figure served as a better predictor of these later-life issues than other metrics, such as concussion history, starting age of football career or length of career.

“We need to take very seriously the notion that hitting your head over and over again may have long-term consequences,” Stern said.

Researchers were able to identify a threshold for players, and continued exposure to head trauma after that threshold resulted in a dramatic increase in risk. Going from 6,500 cumulative hits, for example, to 12,000 “increased the risk for objective cognitive impairment by a considerable twenty-five fold,” according to the study.

“The number of hits that are found to serve as a threshold are not meant to be interpreted as, ‘Wow, I’ve reached that magic number, so I should stop,’ ” Stern said. “There is no magic single number for any given individual. There might be some people who have many, many times the number of hits and do just fine later in life. And there may be many people who have far fewer hits and experience significant problems later in life.”

While researchers warn against projecting the study’s thresholds on a larger population or any individual, cumulative head impact should be monitored more closely, Stern suggests, not unlike the way a baseball coach scrutinizes a young pitching arm.

“When you think about it, we count the number of pitches and we do all kinds of things as parents and as a society to protect the health and well-being of our children . . . but then we drop our kids off at a football field and basically say, go ahead and hit your head as many times as you want,” he said.

While the new study is not focused on concussions or CTE, it is a further exploration of the cause-effect relationship between head trauma suffered on the football field and troubles experienced later in life, and that those issues aren’t restricted to the few who reach the NFL’s elite ranks.

“What we’ve been finding, it’s not just the concussions that ultimately put someone at risk for CTE,” Stern said. “It seems to be the overall exposure to brain trauma.”

A recent study by the Mayo Clinic published in the journal Acta Neuropathologica suggested that youth and young adults who participate in contact sports might be at an increased risk of developing CTE. Researchers examined more than 1,700 cases in the Mayo Clinic Brain Bank, and 1 in 3 of the 66 males who participated in contact sports when they were younger was found to have damaged brain tissue consistent with CTE.

“We don’t really know what percentage of any group has CTE,” Stern said. “What we do know is that of all the cases of neuropathologically confirmed CTE, there’s been one thing in common and that’s a history of repetitive head impacts.”

“I’m hoping this new study will spawn other research in this area. There really needs to be much more research focusing on overall head impact exposure and not just symptomatic concussions when it comes to improving our understanding of later-life brain health and disease.”