League officials said they considered the testing of 16 helmet models to be a first step toward improving helmet technology in an attempt to prevent players from suffering concussions. The study, which was conducted by two independent laboratories, was supported by the NFL Players Association, which prepared a written summary of the testing results jointly with the league.

"We thought there was a substantial reason to believe this was valuable data, and the players' association felt this was important and should be shared with the players," said Jeff Pash, the NFL's executive vice president of labor. But "this is not the final word or anything close to the final word on helmets."

NFL Commissioner Roger Goodell wrote in a two-page memo to teams that the league and union "believe that the information contained in the summary should be shared with your players."

Goodell also wrote that the study "should be regarded as an initial step in learning more about the effectiveness of safety equipment. It is not a definitive statement on helmet performance. A number of follow-up studies are planned. Our hope is that manufacturers will use the information generated from these studies to continue to improve the quality of equipment used by NFL players."

The NFL sponsored the study and testing was conducted at labs in Tennessee and Canada, Goodell wrote. He added that the study design, testing procedures and data were reviewed by two independent engineers and an independent statistician analyzed the data.

Helmet models were evaluated for their performance in various impact tests. The performances of the helmets in the tests were compared to the testing performances of helmets worn by the players in the 1990s, and the league and union wrote in their summary that "no contemporary helmet performed worse than the helmets" from the '90s.

The league and union wrote that three current helmet models -- the Riddell Revolution, the Revolution Speed and the Schutt DNA Pro -- qualified as "top performing" helmets.

NFL players are permitted to wear any helmet certified by the National Operating Committee on Standards for Athletic Equipment.

"We're not telling players to wear any particular helmet," Pash said. "Based on this study, you can't do that. This was a first step. There's more work to do."