Several of the speakers talked about what is likely and not likely to cause head injuries in soccer. Sports medicine expert and former soccer player Dr. Donald Kirkendall delved into whether repetitive heading might cause brain injury. He said that if the heading was being done properly, the ball's impact with the head is not usually forceful enough to cause a concussion. Proper heading involves contracting the neck muscles so the head is more rigidly fixed to the trunk of the body and hitting the ball squarely with the forehead near the hairline.

Although soccer balls can be kicked to speeds as high as 70 miles per hour, even most professional players cannot kick a ball that fast and most soccer players would not attempt to head a ball moving that fast, Dr. Kirkendall said. He also added that youths rarely have enough force to kick a ball to speeds higher than 40 miles per hour. He calculated the impact of a soccer ball on the head of youths of various sizes, based on the likely speed of the ball, and concluded that the force of impact is well below the force that is thought to be necessary to cause a concussion in heading a soccer ball.

But he added that concussions do occur in soccer when the ball hits an unprepared player in the head. He also gave examples of concussions occurring when players accidentally knock their heads into other players while attempting to head the ball, particularly if they are attempting to flick the ball backwards. He noted that soccer professionals do not usually attempt such backwards headings unless they are running to the ball such that there is enough space between themselves and the players behind them. But youths often don't have enough space when they attempt to flick the ball backwards with their heads and they run the risk of knocking their head into the player directly behind them.

As neuropsychologist Dr. Jeffery Barth pointed out, “We sometimes speak in very broad strokes about the problems of soccer heading and that it may be causing significant neurologic problems, but it may not be the heading that is doing this. It may be the fact that two players go up for the same ball and hit their heads together, which clearly would cause significantly more impairment.”

Several speakers noted that players who frequently head the ball tend to be aggressive players and their aggressiveness may make them more susceptible to head collisions with other players. Other frequent causes of concussions in soccer players are head collisions with other players or goalposts or falls where their heads hit the ground, according to Dr. Kirkendall.

Compared to other contact sports, head injuries are common in soccer. In neuropsychologist Dr. Jill Brooks' study of high school soccer players, she found that more than one quarter of them had experienced one or more concussions. Neuropsychologist Dr. Ruben Echemendia reported that in his study of college athletes, over 40 percent of the soccer players had at least one concussion prior to attending college. By comparison, only 30 percent of the incoming football players in the same study reported having had a concussion.

Dr. Kirkendall . . . calculated the impact of a soccer ball on the head of youths of various sizes, based on the likely speed of the ball, and concluded that the force of impact is well below the force that is thought to be necessary to cause a concussion in heading a soccer ball.

Dr. Brooks found that many high school soccer players neglected to report experiencing a concussion, because they didn't think it was serious or wanted to continue playing in a game. “Most concussions go unreported,” she said.