Adam Thielen can't quite wrap his head around a football player in today's climate not wanting his head wrapped by a helmet whose degree of safety has been tested and certified jointly by NFL- and NFLPA-appointed biomechanical engineers.

"I guess if a guy wants to wear a helmet that's not certified, then that's his deal," said Thielen, Vikings receiver and union representative. "Maybe he signs a waiver or something. But I don't know why he would want to. I think it's a new policy that's only for our benefit, our own health."

Raiders enigmatic receiver Antonio Brown is the notable vehement objector to the rule that players must wear only certified helmets in 2019 after being given a two-year grace period to comply.

Last week, Brown threatened to retire if he wasn't allowed to wear his Schutt Air Advantage. Monday, he challenged the policy in front of an independent arbitrator and lost because his helmet is more than 10 years old, too old to be certified. Tuesday, he took to Twitter looking for a newer version of a helmet that was discontinued in 2011, hoping he'll be able to get it tested and certified.

"That's his choice," Thielen said of Brown. "I can understand that you like a certain thing and you wore it your whole career. I can understand that you want to be grandfathered in. At the same time, it's all about player safety and you really can't argue about that."

Leaguewide, 32 players wore uncertified helmets last year and must switch this year. That includes six-time Super Bowl champion Tom Brady, whose Riddell VSR-4 is among 11 models not certified.

Raiders wide receiver Antonio Brown wanted to wear his old helmet but lost his appeal.

"I don't really love the one that I'm in now," Brady said Monday on WEEI's "Greg Hill Show." "But I don't really have much of a choice."

The Vikings have no players who need to change helmets this year. Six of them wore an uncertified helmet in 2017 — a year after NFL owners invested $60 million to improve helmet technology — before switching to certified helmets last year.

One of the six who switched last year was cornerback Xavier Rhodes, who sounded as if he did so willingly.

"Everything they do is to protect us," Rhodes said. "So, yeah, I like it."

There are 34 different helmet models that have been certified. They're ranked one through 34 in terms of degree of safety. Twenty-seven fall in the "green" category, which is the preferred level, while seven fall in the "yellow" category, which is considered acceptable.

Last year, all but seven Vikings wore helmets that fell in the "green" category.

Vikings equipment manager Dennis Ryan "is really good at telling you which ones have the best safety rating and getting you any helmet you want," safety Harrison Smith said. "Mine isn't at the highest level of safety, but it's within the limits."

And why not go with one of the top models for safety?

"I've tried on a lot of them and I honestly don't like how a lot of them feel," Smith said. "They just feel weird. And for me, there is definitely an aesthetic aspect. I have to feel good or I just don't like it. That's the case for a lot of guys."

Coming out of Notre Dame, Smith was wearing the same model as Brady.

"All I know is I had to change," Smith said. "I don't think it was an NFL [mandate] as much as it was Dennis being like, 'Hey, this helmet isn't safe enough.' "

Smith understands Brown not wanting to change helmets. He just wouldn't push the envelope the way Brown has.

"If I had my choice, I'd probably be wearing an old helmet that's not certified," Smith said. "But there have been a number of rule changes made for the good of the game long-term because of safety. I don't love them, but I understand them and think they're a good idea."

Mark Craig is an NFL and Vikings Insider. Twitter: @markcraigNFL. E-mail: mcraig@startribune.com