An Oslo University medical student says he's determined that soccer players who use their heads to strike at balls are risking brain damage.

Thirty percent of retired Norwegian soccer players were found to have signs of minor brain damage from "heading" balls, according to the survey medical student Alf Thorvald Tysvaer conducted for his doctoral thesis.Tysvaer said in an interview with the news agency NPK that 3 percent of the active Norwegian soccer players he studied also showed symptoms of brain damage. Soccer balls can travel at more than 60 mph.

But Willy Simonssen, assistant head of Norway's Soccer Federation, said such widespread damage appeared unlikely.

"I have not noticed it in my meetings with players," he said.

In the study, former players complained of headache, dizziness and irritability and some had problems with pain in the neck, Tysvaer was quoted as saying. He said those were symptoms of chronic, or permanent, brain damage.

But the brain changes he recorded were not as severe as those suffered by boxers, Tysvaer said.