"[A]lthough the magnitude of the acute changes observed was small,” the researchers note, “it is the presence of the effect that is of interest. This measure was previously shown to be altered in confirmed concussion, but the acute changes . . . following the sub-concussive impact of football heading raise concerns that this practice, routine in soccer, may affect brain health.”

Changes in motor response and memory were observed in the five women and 14 men participating in the study. Each was asked to perform a rotational header — redirecting the soccer ball — 20 consecutive times during 10-minute sessions. The researchers found that immediately following these sessions, subjects' error scores on both short- and long-term memory tests were significantly higher than subjects' baseline performances.

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Even after just a single session of heading, memory-test performance was reduced by as much as 67 percent, though the alterations appeared to clear within 24 hours. The researchers caution against taking this temporary disruption as a sign of no long-term damage.

Other studies over the past few years have also found:

* Soccer players are prone to traumatic brain injury, with 22 percent of all injuries being concussions.

* Changes in brain anatomy of soccer players — particularly, a thinning of the cortex — is associated with slower cognition.

* Molecular markers of brain damage are significantly elevated in response to heading by male professional soccer players.

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The latest research, which used transcranial magnetic stimulation to measure brain function, builds on similar work that has found biochemical markers of brain injury in soccer players suffering the accumulated effects of sub-concussive head impacts. In these players, an initial injury triggered a pathological process, a cascade of cellular events, that led to brain degeneration.

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Soccer is the most popular sport in the world, with more than 265 million amateur and professional players. Competitive players head the ball an average of six to 12 times per game, according to experts, and at ball speeds far greater than those in practice drills.

“For the first time, sporting bodies and members of the public can see clear evidence of the risks associated with repetitive impact caused by heading a soccer ball,” Angus Hunter of the University of Stirling in Scotland said in a statement accompanying the new research results. “We hope these new findings will open up new approaches for detecting, monitoring and preventing cumulative brain injuries in sport. We need to safeguard the long-term health of soccer players at all levels, as well as individuals involved in other contact sports.”