Don’t be fooled by the advertising. Just because “Leatherheads” looks, feels and smells like a wacky romantic comedy, that doesn’t mean it isn’t also a football flick. Sure the film, opening Friday, features 1920s gridiron stars, played by George Clooney and “The Office”s” John Krasinski, fighting for the affections of tough-as-nails newspaper reporter Renee Zellweger. But it’s also filled with the genre touches that have made the football film a favorite with audiences: downtrodden, or underachieving, guys who have to prove their worth by opening up a big can of whup-tail on the field of glory.

And it doesn’t matter if those guys are convicts (”The Longest Yard”), part-time bartenders (”Invincible”) or barely literate Cajun sideline workers (”The Waterboy”); ultimately, manhood is won on the gridiron. And so, for the most part, is the girl.

“Football is perhaps the most macho of all the sports, so men are put on the line to prove their masculinity,” says Irv Slifkin of moviesunlimited .com. “In most cases, the principal characters are not only proving they can play with the big boys, but also must win over the woman on the sideline and have the men on their team — usually former adversaries — on their side, rooting for them. So there are really three conflicts going on, which adds a complexity to what could be a simple genre film.”

Not all football films have this complex trifecta, but there’s little doubt that since the dawn of the forward-pass era, gridiron classics have mostly been about the ways in which men bond, achieve greatness and find their inner testosterone. Sure, there have been flicks like “Friday Night Lights” (2004), which examine the rotten core of competitive athletics. But mostly, filmgoers have flocked to, and cheered, rah-rah films like “Knute Rockne All American” (1940), in which the Notre Dame team — and, by transference, every red-blooded American — wins one for the Gipper.

Not that the football film is all that different from any other classic sports flick, which is “about the underdog getting his shot,” says Glen Macnow, co-author of the forthcoming “The Ultimate Book of Sports Movies.”

“‘Rocky’ is about the underdog getting his shot,” Macnow says. “‘Hoosiers,’ ‘The Longest Yard.’ Half the sports movies are about this. It’s because it works and audiences like it. Our heart is always with the underdog; that’s just human nature. We all identify.”

But “Leatherheads” is also something a bit different. Call it, if you will, the sports film as metrosexual genre exercise. In other words, it’s been cannily crafted to appeal to those who couldn’t care less about head-knocking male humanoids. So it’s not just about a football hero played by Clooney who’s trying to establish his team as a major league enterprise. It also tells how Clooney, as Jimmy “Dodge” Connelly, recruits a war hero and star athlete (Krasinski), then finds out that Golden Boy is after his gal. Football and foolin’ around. A winning combination.

“There is a new form of sports movie, which is the sports movie/date flick,” Macnow says. “I think this started with ‘Bull Durham’ and maybe peaked with ‘Jerry Maguire’ — ‘You had me at hello’ — and has developed a market that could exploit. In this movie, what you have is the old-time beginnings of the NFL, fat guys in undersized uniforms, leather helmets. And to make it safe for women, you get the romance thing.”

So what else is new? Fact is, except for a film like 1932’s “Horse Feathers,” in which the Four Marx Brothers wreak their particular brand of havoc on the gridiron, football flicks are, in many ways, like every other film out there.

“Nearly every movie is about the guy trying to get the girl, or vice versa,” says “Leatherheads” co-screenwriter Rick Reilly. “Every movie is about the protagonist trying to prove himself. Football just happens to be a perfect place to watch him. It’s ‘Saving Private Ryan’ with better helmets.”