The results of an independent study commissioned by the NFL and the players' union show modern helmets meet all national safety standards, though it stressed that no helmet can prevent concussions and more studies are necessary.
The results were forwarded to teams on Friday, along with a memo from commissioner Roger Goodell outlining the study and a question-and-answer brief designed for players.
The results come roughly three months after a congressional committee criticized the NFL's research into equipment, particularly helmets, and questioned whether player safety is indeed being given top priority in an "infected system that needs to be cleaned up."
"The study should be regarded as an initial step in learning more about the effectiveness of safety equipment," Goodell said in his memo, which was obtained by The Associated Press. "It is not a definitive statement on helmet performance."
The study, which began last year, involved two independent labs that tested each of the 16 commercially available helmets for impacts similar to what would be felt by an NFL player in the open field. Eight locations on each helmet were tested using four speeds of impact, based on an analysis of game film and computer models in an attempt to replicate actual hits.
The raw data was examined by two independent biomechanical engineers and a statistician, and reviewed by the NFL's new Head, Neck and Spine Committee and doctors from the players' union.
The results showed that all 16 helmets met or exceeded national standards to protect players against traumatic head injuries, and none performed worse than a reference group of helmets from the 1990s. Three of the modern helmets - the Riddell Revolution, the Revolution Speed and the Schutt DNA Pro - were singled out as the top-performing helmets.
NFL players can choose to use any helmet that meets national safety standards.
"This was an opportunity to get the information to the players," NFL spokesman Brian McCarthy told The Associated Press. "The helmets that were tested all performed well. The three that were called out did qualify as top performers, but all the helmets met safety standards."
The information sent to each NFL team and its training staff included a line in bold-faced print that said "no helmet, including these top-performing helmets, can prevent concussions or reduce the risk of concussion to any specified degree."
Concussions in the NFL have recently become a major topic of concern.
Last year, Goodell and others from the NFL testified before Congress about concussions, and the league has already implemented new return-to-play guidelines for players who sustain head injuries in practice or a game. Teams must now consult with an independent neurologist whenever a player sustains a head injury.
The results of the helmet study are based solely on NFL conditions and are not applicable to collegiate, high school or youth football. The conclusions also cannot be drawn about helmet performance in specific types of collisions.
"Potential contributing factors for concussion, such as playing position, concussion theory, type of collision and helmet fit were not studied as part of this initial laboratory-based study," the results said. "Thus, this research represents an initial step in measuring helmet performance, and is not a definitive statement on performance."
NFLPA assistant executive director George Atallah did not return a message seeking comment.
McCarthy said the testing is ongoing. The next step could involve additional laboratory work involving more speeds and impact points, or perhaps on-field analysis of performance. The NFL and the union are also considering expanded research to include other kinds of equipment.
"This has been a joint project throughout, and the intent was to take a look at existing helmets," McCarthy said. "It's an initial step in learning the effectiveness of the equipment, and we hope the manufacturers themselves can use the information to continue to improve."