In the National Football League, there are rules and restrictions on everything from how much white can show on a player’s socks to what sort of sweatbands can be worn on the wrists. Yet when it comes to the most critical piece of equipment — the helmet — the league provides little guidance and essentially leaves the decision up to each player.

Even as head injuries have become a major concern, the N.F.L. has neither mandated nor officially recommended the helmet models that have tested as the top performers in protecting against collisions believed to be linked to concussions. Some players choose a helmet based on how it looks on television, or they simply wear the brand they have been using their whole career, even if its technology is antiquated. As a consequence, despite lawsuits related to head injuries and the sport’s ever-increasing speed and violence, some players are using helmets that appear to place them at greater risk.

“Frankly, the league has been far more aggressive about thigh and hip pads than they have about ensuring that every player has access and information regarding helmets,” said DeMaurice Smith, executive director of the N.F.L. Players Association.

The rules governing helmets are not complex. The league stipulates only that any helmet certified by the National Operating Committee on Standards for Athletic Equipment, or Nocsae, may be worn. That broad standard allows players to use models that may be archaic or did not score well on specific impact tests.