PHOENIX – During his 2,097-yard MVP blitzkrieg last season, Vikings star Adrian Peterson ran with the unbridled tenacity for which he's always been known, dishing out vicious shots as often as he absorbed them.

Peterson has excelled with that Pamplona bull style. But now, with the NFL in heated debate over a rule proposal that would penalize ball carriers for delivering contact with their heads, it's worth wondering whether some of Peterson's most impressive and physical runs could now occasionally result in penalties against the Vikings.

On Tuesday at the NFL's annual meetings at the Arizona Biltmore, debate continued over a proposed rule that would require a 15-yard penalty to be assessed if any ball carrier, while outside the tackle box and more than 3 yards down field, "initiates forcible contact by delivering a blow with the top/crown of his helmet against an opponent."

It's a pitch that has been greeted by many coaches as well-intentioned from a safety standpoint but perhaps misguided if enforced too aggressively.

The Vikings have made it clear they are wary of the rule. Owner Zygi Wilf spoke up twice during debate Tuesday morning. Coach Leslie Frazier, while noting the admirable safety objective, also raised an eyebrow at how to reasonably make such a rule work.

"I think it makes it tough for the running backs," Frazier said. "It's such an instinctive motion to duck that head to try to protect yourself.

"So I don't know what's going to happen, whether this will pass or not. But I think it puts those backs in a vulnerable state if they can't lower their pads. And the only way to lower your pads is to get your head down."

For the rule change to be adopted, it will require three-quarter approval in a final vote that has been tabled until Wednesday. At present, however, vocal opposition has been evident.

Frazier said he thinks the rule's intent to reduce head injuries could wind up having an adverse effect on player safety if ball carriers begin to run more upright without ability to protect themselves legally by ducking into collisions.

"When those defenders are coming at your legs, if you don't protect yourself and you don't get your pads down, now you run the risk of lower body injuries," Frazier said.

Yet Rams head coach Jeff Fisher, who doubles as an active member of the competition committee, has made it clear that if the rule were adopted, officials would be instructed to look for a very specific infraction.

"We want to bring the shoulder back to the game," Fisher said.

"We all know the helmet is a protective device. And it's not designed to be used like it's been used as of late."

Dean Blandino, the NFL's vice president of officiating, said part of the research that went into the rule proposal involved reviewing every play from Week 16 last season. In those 16 contests, Blandino said there were 34 instances of helmet-to-helmet contact between ball carriers and tacklers but only five that would have drawn penalties under the new rule.

"There is going to be helmet-to-helmet contact between a runner and a tackler," Blandino said. "That is not what this rule is designed to take out.

"It is where one player lowers the head and delivers a blow with the crown, the top of the helmet, lines up a player and hits him when we feel he has options to do something else."

Still, even with the competition committee and officials trying to zero in on "the obvious foul," coaches worry the speed of the game and the fluctuating discretion of each officiating crew could create too much confusion from week to week, series to series.

As of Tuesday morning, Pittsburgh coach Mike Tomlin didn't know whether he'd advise the Steelers to check the "For" box on their ballot or the one marked "Against."

"I'm interested in more discussion to be honest with you," Tomlin said. "I think that's the charge.

"I think it's easy to say we want to eliminate those type of ugly plays from the game for obvious reasons. But at the same time I think the discussion lies in how we do that."

Frazier wasn't sure if Peterson was aware of the proposal but was certain he knew which way the MVP would lean.

"He would not be in favor," Frazier said.

"Those guys have to be instinctive. They have to be allowed to do what they do."