Brain injury and the lasting effects of concussion in sport have become a major health issue in recent years, especially in such hard-hitting sports as American football. Although the thump of a soccer ball on a forehead seems fairly innocuous, compared with a crashing tackle on the three-yard line, a soccer player may "head" the ball hundreds or even thousands of times during the course of the season. The cumulative effect of many "sub-concussive" blows to the brain has been unknown and unstudied until now.

"We chose to study soccer because it is the world's most popular sport," says the report's lead author Michael Lipton, associate director of the Gruss Magnetic Resonance Research Center at the Albert Einstein College of