The associated press has reported that nearly 40 percent of NFL players last season wore a helmet model that got the second-lowest rating for reducing the risk of concussions in a study by Virginia Tech researchers.

Riddell’s VSR-4 helmet received just one star in a study of football helmets led by Virginia Tech professor of biomedical engineering Stefan Duma and released Tuesday. Another Riddell model — the Revolution Speed — was the only helmet that earned five stars, the top rating.

Five models — two made by Riddell, two by Schutt and one by Xenith — received four stars.

According to Riddell, 38 percent of NFL players wore the low-rated VSR-4 in 2010. The company also said 39 percent wore one of the various models that fall under the Riddell Revolution name or the new Riddell 360.

“It is our hope that based upon this and other independent research, that players and teams at all levels will continue to migrate to the Revolution family of helmets,” Riddell Sports president Dan Arment said in a statement emailed to The Associated Press by an outside PR firm that represents the manufacturer.

Each NFL player is allowed to choose which brand and model of helmet he wears.

In November, the NFL told the AP it estimated that 75 percent of helmets used in 2010 were made by Riddell, which has had a licensing/sponsorship agreement with the league since 1990. The league said 23 percent were made by Schutt, 1 to 2 percent by Xenith, and a handful by Adams USA.

A helmet made by Adams was the only one that received a rating of “not recommended” by Duma’s study.

Duma said in a telephone interview that this is the first time comparative test results on football helmets have been made public.

He said the star ratings provide “a prediction of concussion risk.”

“All of these helmets protect you from skull fracture, so what we’re doing is going to the next level and looking at how they protect you from brain injury,” Duma said.

“We’re basing this analysis off a million impacts we’ve collected,” added Duma, who called the study the culmination of about eight years of research. “We know how players are hit. … It’s much more elaborate than anyone’s ever looked at, in terms of evaluating the performance of helmets.”

Concussions occur when the brain moves inside the skull from an impact or a whiplash effect, but it’s still an injury that doctors — not to mention players and executives in various sports – are learning about. There is general consensus that concussions cannot be eliminated from football. But as more links have emerged between head injuries and disease later in life, the NFL has stepped up its efforts to educate players about concussions and protect them from the injury.

NFL spokesman Greg Aiello said the league’s head neck and spine medical committee would review the Virginia Tech research. He also noted that the league and NFL Players Association let players, trainers and equipment managers know last year that three helmet models — two made by Riddell and one by Schutt — met all three criteria for qualifying as a top-performing helmet when 16 were tested.

The league and NFLPA also told players and teams then: “In general, some of the modern helmets performed better in these tests than did the older helmets.”

Riddell stopped selling the VSR-4 in 2010. The Revolution models include significant changes from the VSR-4, such as covering the jaw area and having padding there, as well as adding padding to increase the space between the head and the shell.

The Virginia Tech ratings “show that there is a difference between helmet brands and models and that strong research and development can make a significant impact on the protective performance of football helmets,” Arment said in an emailed statement.

Duma wants to run the same sort of testing on baseball and cycling helmets.