For years, researchers studying chronic traumatic encephalopathy, or CTE, believed that the primary cause of the condition was repetitive blows to the head. They figured that the more frequently a person sustained hits to the head, whether or not these resulted in concussions, the more likely these individuals were going to develop neurological and cognitive conditions.

But a collaborative effort by researchers found that it wasn’t the number of hits to the head that triggered brain disease later in life, but rather the cumulative force of hits to the head that caused damage.

According to Neuroscience News, scientists from Boston University and Harvard Medical School analyzed brains donated to BU’s UNITE Brain Bank for their study that was published in Nature Communications. The study is the largest to date to investigate the causes of CTE, characterized by memory loss, impulsive behavior, depression, and suicidal thoughts.

The researchers used data from 34 published studies that tracked blows to the head of 631 former football players. The impact was measured by sensors inside of football helmets of living players and projected onto brain donors based on their duration, level, and position of play, noted Dr. Jesse Mez, senior author of the study and an associate professor of neurology at BU’s Chobanian & Avedisian School of Medicine.

 The study found that 71% of the brains examined — 451 of the 631 ─ had some level of CTE, while 180 showed no signs of disease. The researchers used the “cumulative head impact index-g-force” or CHII-G, a measure that accounts for both the number of hits and how hard those hits were over the course of a player’s career, to establish cumulative force, said Mez.

“Anything that reduces the number of hits and force could be beneficial,” noted the expert. “This would include starting to play at an older age, playing fewer games, not hitting in practice, or reducing drills that encourage hard hitting.”

Mez told Neuroscience News that while his research clearly shows the volume and force of hits contributes to CTE in football players, there is growing evidence of a similar relationship in other contact sports.

Last year, Mez released a preliminary study linking CTE with playing ice hockey. His research found that each additional year of playing ice hockey may increase a person’s change of developing CTE by about 23%.

“Sports like ice hockey, boxing, MMA, rugby and soccer need systemic investigations as well,” he said. “I’m hopeful that leagues and coaches take notice. For at least some of these sports, changes could substantially improve long-term neurodegenerative outcomes without dramatically changing the nature of the sport.”