More than 61 per cent of former NFL players had concussions during their playing days, and most of them said they were not sidelined after their injuries, according to a study issued Wednesday.
The report was based on a survey of 1,094 former players, ages 27 to 86, conducted in 1995-96 by the NFL Players Association.
Responses were analysed by Dr. Kevin Guskiewicz, research director of the University of North Carolina's Center for the Study of Retired Athletes.
Concussions have become an important issue in the NFL, as some of its marquee players, otherwise still fit to play, have been forced to retire because of head injuries.
Dallas Cowboys quarterback Troy Aikman retired in April after suffering 10 concussions in his 12-year career.
San Francisco quarterback Steve Young quit in 1999 after his fourth concussion in three years.
"We are not familiar with the report and therefore not in position to comment," said NFL spokesman Greg Aiello.
Thirty per cent of the players had three or more concussions and 15 per cent had five or more.
Overall, 51 per cent had been knocked unconscious at least once.
Seventy-three per cent of those injured said they were not required to sit on the sidelines after their head trauma.
However, they were not asked if they actually resumed playing after the concussion.
The study also found that 49 per cent of the former players had numbness or tingling; 28 per cent had neck or cervical spine arthritis; 31 per cent had difficulty with memory; 16 per cent were unable to dress themselves; and 11 per cent were unable to feed themselves.
Eight were diagnosed with Alzheimer's disease.
Alzheimer's has yet to be linked directly to head injuries, although evidence suggests the two are connected, Guskiewicz said.
"I think concussions are one of the biggest concerns of the league and the players association," said Frank Woschitz, director of the NFLPA's Retired Players Association.
For the last several years, the NFL has had a medical committee chaired by Dr. Elliott Pellman, the New York Jets team physician, studying the issue of concussions.
Based on recommendations of the committee, the league has funded a major research project to better understand and share information on the subject.
By John Marshall