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Players did not show elevated levels of tau, a protein typically found after severe concussion.

Research suggests that players in contact sports such as hockey suffer detectable changes in brain function at the end of a playing season, even when they don’t suffer a diagnosable concussion, said van Donkelaar, who specializes in sport-related concussion.

Photo by Nathan Denette / Canadian Press files

“Football and hockey players that don’t get diagnosed with a concussion nevertheless suffer a number of head or body impacts that are going to potentially disrupt brain function,” he said.

In game conditions, a soccer ball may travel at speeds of up to 80 km/h.

“In soccer, the player may purposely make a head impact to direct the ball … yet players don’t wear protective headgear,” he said. ‘Although there are a growing number of studies evaluating the wisdom of this, ours is the first to measure blood biomarkers of cell injury.”

A study at the Albert Einstein College of Medicine found that players who headed the ball 45 times over two weeks showed poorer performance on tests of psychomotor speed, attention tasks and working memory, at least temporarily.

A second study recently found that female players are far more susceptible to brain damage from repeated heading than men.

Canada Soccer instructs players not to head the ball while recovering from a concussion and instructs coaches not to train players younger than 12 to head a soccer ball.

“They are not to be trained to head using a soccer ball,” said Canada Soccer spokesman Brad Fougere. “There are sponge balls or balloons that can be used to teach a child where on the head is the safest place to head a ball.”

The U.S Soccer Federation has banned heading for players under 10 years of age.

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