© Scott A. Miller/Associated PressA team at a Pop Warner game in 2009. More than 285,000 children, ages 5 to 15, compete in Pop Warner leagues.
In response to growing concerns over head injuries in football, Pop Warner, the nation's largest youth football organization, announced rule changes on Wednesday that will limit the amount of full-speed collisions and other contact allowed in practice.

The issue of brain injuries sustained on the football field has forced a reckoning at all levels of the sport in recent years. Pop Warner's new rules, which will affect hundreds of thousands of youth football players, some as young as 5 years old, were seen as the latest acknowledgment that the nation's most popular sport poses dangers to the long-term cognitive health of its athletes.

As scrutiny of the impact of football has escalated, studies have shown that younger players can face repetitive brain trauma similar to that sustained at the college level - and perhaps even more acutely, because their brains are not fully developed and require longer rest periods after injury. Pop Warner officials said they were persuaded to alter their rules by research earlier this year showing that players as young as 7 are exposed to collisions as severe as those at the college level.

Under its new rules, effective for the coming season, which starts in August, contact will not be allowed for two-thirds of each practice - a move prompted by research showing that most of the hardest hits in youth football occur not in games, but in practice. The organization is also forbidding all drills that involve full-speed, head-on blocking and tackling that begins with players lined up more than three yards apart, as well as head-to-head contact.

"The science shows that this should be done," said Dr. Julian Bailes, chairman of the Pop Warner medical advisory board and co-director of the NorthShore Neurological Institute in Illinois. "We think right off the bat that with this change we can eliminate 60-plus percent of the brain impacts or concussions."

More than 285,000 children ages 5 to 15 play in Pop Warner football leagues. The program says it has produced more than two-thirds of the players now in the National Football League.

A stream of studies has linked collisions on the field to long-term cognitive problems, and several former college and professional players have been posthumously found to have a brain disease similar to Alzheimer's that is caused by repeated head trauma. More than 2,000 former N.F.L. players have filed suit against the league and the helmet manufacturer Riddell, alleging that they deliberately hid critical information about the dangers of concussions and hits to the head. Their suits were recently consolidated into one master complaint.

The N.F.L. has changed some rules and stiffened penalties for hits to the head and neck area. The league's new contract with the players union also includes provisions that limit the number of full-contact practices and off-season workouts. At the college level, the Ivy League last year sharply reduced the number of allowable full-contact practices that teams can hold.

Research has shown that the damage from concussions can be cumulative, and that the brains of younger athletes may be particularly susceptible. Pop Warner and other football leagues came under increasing pressure, which pushed the organization to institute a rule two years ago that only medical experts, not coaches or parents, could make return-to-play decisions for concussed athletes.

The group was persuaded to take its more drastic step when a study of second-grade football players published in February - which used data from sensors installed in helmets - showed that the average player sustained more than 100 head impacts during the course of about 10 practices and 5 games. Though most of those hits were moderate, some exceeded a force equivalent to a big hit in college football.

"This is absolutely a step in the right direction," said Chris Nowinksi, the executive director of the Sports Legacy Institute, which seeks to advance the study, treatment and prevention of brain trauma in athletes. "Limiting contact during practice is the single easiest way to reduce the risk of concussions and sub-concussive injuries."

Jon Butler, the executive director of Pop Warner, said that while the organization's officials wanted to reduce head-to-head collisions, they stopped short of more drastic measures "because there has to be some full-speed contact in practice so players are prepared for it when they get into a game."

Dr. Matt Grady, a pediatric sports medicine specialist at the Children's Hospital of Philadelphia, said the new rules, while a good start, did not go far enough, and that the emphasis in football for players who have not yet reached high school should be on developing skill and technique, not learning how to tackle.

"Playing tackle football at 10 years old doesn't translate to being a pro athlete," he said. "I think the ability to catch and run and throw translates to being a pro athlete. Players should develop these skills, and then we can add in the collisions later."

Because multiple concussions are more likely to cause permanent declines in cognitive function than one or two, Dr. Grady added, leagues should do more to prevent them at the lowest levels.

"If a player is going to get a concussion," he said, "I prefer it not to be at 11."

Dr. Bailes, chairman of Pop Warner's medical advisory board, said the organization was open to additional safety measures, like banning linemen from getting set in a three-point stance, in which a player's hand touches the ground, because it results in head-to-head collisions. But first, Dr. Bailes said, the league will wait until the 2012 season is over, and then review how the new rules influenced players and coaches.

"I think when you make a change like this, you want to study it," he said. "And we'll ask: 'Will this be enough? Do we need to work further to take the head out of the game?' "

Mark Mueller, a Pop Warner coach in Hoffman Estates, a suburb of Chicago, said the rules would affect coaches more than players, forcing them to re-evaluate some of their time-honored drills. One that came to mind, he said, was a drill called "bull in the ring," in which teammates encircle a player and repeatedly rush him.

The drill is intended to teach young players how to recognize where a block is coming from.

"But most of the time the kids in these drills are just getting knocked around like a pinball," he said. "None of us are doctors, so when you're having the medical advisory board of Pop Warner saying these are some things you have to think about in the long term, most coaches are going to think about whether these things are good and safe."