The doctors who patrol the sidelines of NFL games to assess whether injured players can safely return to the field should report to the league and players' union rather than individual teams, said a Harvard University study released on Thursday.

That was one of the recommendations in a 500-page report by Harvard researchers who have worked with the National Football League Players Association union over the past two years to study the heavy physical toll that the sport takes on athletes.

The study comes amid heavy scrutiny for football, at all levels of play, over the high rates of concussion suffered on the field.

Repeated concussions are linked to the brain disease known as chronic traumatic encephalopathy, which can cause dementia. The condition has been diagnosed in former players including Hall of Fame linebacker Junior Seau and Pro Bowl safety Dave Duerson, who both committed suicide.

While emphasizing that the current structure is not deliberately unethical, it said team doctors, trainers and other medical staff may feel pressured to return injured players to the game more quickly than medically warranted to ensure the team remains competitive.

"NFL football has a storied history, and holds an important place in this country. The men who play it deserve to have their health safeguarded," said Glenn Cohen, a law professor who co-authored the study.

The recommendation would call for two groups of doctors to monitor players, one reporting directly to the teams and one to the league and players' union. The league currently employs about 175 doctors.

The NFL called the recommendations "untenable and impractical."

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"There is no higher priority in the NFL than the health and safety of our players, both during and after their playing careers," said league spokesman Brian McCarthy, in an e-mail.

He noted that the NFL has taken a number of steps in recent years to reduce the physical risk to players, which include changing rules to ban the most dangerous helmet-to-helmet hits and changing rules on practice and training.

Indeed, the report cited the decision to place independent doctors on the sidelines charged with evaluating players for signs of concussion, as an example of the value of changing the reporting structure for medical personnel.

The Harvard report recommended further strengthening the rules regarding concussions, including shifting diagnosed players onto short-term injured reserve lists, which would allow teams to fill their spots on their 53-man rosters.