Usually head injuries are associated with contact sports like football, boxing or hockey. But there's growing evidence that there are risks in soccer, too.
Take the example of former Brazilian star Hilderaldo Luiz Bellini, known simply as Bellini to soccer fans. He played on the nation's first World Cup champion team in 1958 and was known for his acrobatic leaps and fearless headers. When he died earlier this year, an obituary from the Guardian called Bellini "a powerful, resolute, and sometimes abrasive central marker."
His death came after more than a decade of showing signs of what doctors and his family initially thought was Alzheimer's.
But new evidence suggests Bellini was instead suffering from a degenerative brain disease known as Chronic Traumatic Encephalopathy, or CTE.
"When I looked at his brain I found strong evidence of these changes," said Lea Grinberg, who led the team which studied Bellini's brain. She said she was asked by his family to do the study and make the findings public.
One issue in years past was the heavier ball, which was traditionally made of leather.
"When it was wet it would absorb the water and it was like a brick coming on the head," she said. Today, the ball is much lighter, but the game is more physical and athletes are stronger and faster, posing risks, especially to young athletes whose neck muscles are not yet fully developed, said Grinberg.
Her study, she said, showed association between CTE and Bellini's injuries, but stopped short of providing clear causation. Grinberg said more research will be needed in the field to determine clearly what kinds of effect collisions and headers have on soccer players.