As New York City heats up for Fashion Week, fresh faces are flooding casting calls. Modeling agency Wilhelmina says the male model trend is shifting away from an ultra-thin rocker look and towards a healthier, classic male physique. (Zoeann Murphy/The Washington Post)

The men who walk the runways of fashion’s capital cities, who pose in photo spreads for magazines such as GQ and who hawk everything from underwear to cologne are also the men who define and sell a modern definition of masculinity.

When New York Fashion Week begins Thursday, two masculine archetypes will be engaged in a lively debate on the subject of manliness: what is beloved and what is rebuked; what is romanticized and what is demonized; what is hot and what is not.

One of these guys is beanpole skinny. He isn’t classically handsome. He might simply be an odd duck — someone with a perfectly imperfect face that is impossible to ignore. His longtime dominance of the fashion conversation is being challenged by the return of a man with muscles and swagger who exists in a cloud of intoxicating testosterone.

If these two extremes have anything in common, it is this: Both types are white. After so many years, that remains the default choice.

While beefcake endured at labels such as Versace, where a certain oiled hunkiness has always defined the brands, by the turn of the 21st century, muscles had fallen out of favor with many of the most influential design houses, photographers and stylists.

Instead, for more than a decade — an eternity in fashion years — the masculine ideal has been as young as 17 — tall, pale and lean. Practically gaunt. Even when these gamine boys were old enough to vote, their appearance still telegraphed such extreme youthfulness that it suggested jail bait.

If their jaws were chiseled, such jutting splendor had less to do with exceptional bone structure than body fat: They had next to none. In fashion parlance, they were interesting looking. A current favorite is Roberto Sipos — 6-2 with a 29-inch waist — who has walked in some 60 shows since February, which is akin to being ubiquitous. He has short, fine blond hair, blue eyes and a flat, angular face filled with freckles.

At influential labels such as Prada, this lanky ideal typically looked like Peter Pan in a slim-cut business suit. At New York’s Patrik Ervell, he resembled a teenage tech titan in a windbreaker. And at Saint Laurent this summer, the models looked like indie rockers who subsist on tobacco and kale juice.

But fashion’s long swoon for the lightweight may be ending.

“A year ago, the guys who were more athletic were on the sidelines,” says Jim Moore, longtime creative director of GQ magazine. “What’s interesting now is the variety of the models chosen.”

Packing some 25 pounds more muscle and as old as 28 — which isn’t very old at all — the big boys are returning to favor. To the numbers: 6 feet tall, 40-inch chest, 32-inch waist.

With a more defined and athletic physique, in the right lighting these guys look a bit like Thor pumped up to hurl his giant hammer across the sky. They are reminiscent of the quintessential male Adonises, the ones who dominated during the 1990s when Marcus Schenkenberg posed in Calvin Klein skivvies and Tyson Beckford made history as the first African American model to serve as frontman for Ralph Lauren.

The big boys — or at least the more muscular ones — have returned to the spotlight to challenge the waifs.

The role of models is only becoming more pronounced as men pay more attention to fashion.

“I walk out on the street and see guys better dressed than they were five years ago. They’re paying attention to themselves and what they wear,” Moore says. “They’re paying attention to the runway and how that will influence how they’ll dress.”

Since the late 1990s, menswear sales in the United States have marched progressively upward. In 2013, they increased 5 percent over the previous year to reach $60.8 billion, according to a report by the NPD Group. The menswear market is only about half the size of the women’s market, but it had stronger sales last year. Men were buying pants and outerwear, and even stocking up on socks — $2.8 billion worth of socks, to be precise.

The men’s side of the fashion business is offering guys $5,000 skinny suits and $365 slim-fit dress shirts; $1,000 sweatshirts and $500 sweatpants. The physique of the models reflects the divergent aesthetics of the clothes, and the two conspire to shape cultural perceptions, desires and, perhaps, how time is spent in the gym. The shift is delivering to men a modicum of the pressure to mimic the fashion aesthetic of the moment that women have felt for generations.

“A guy who knows his body type might say, ‘I can’t be that thin, but I can do more cardio or I can do less heavy weights,’ ” says Brent Zachery, who has worked as a model for 17 years. “I think [fashion] affects how he works out or if he wants to beef up.”

The downsizing of male models began at a time when Prada was ascendant; Helmut Lang’s street style had fashion insiders rapturous. Hedi Slimane was casting fat-free men in his Dior Homme shows. There was a backlash to pumped-up magnificence and artifice. Cool was epitomized by slack insouciance. A cultish devotion to CrossFit hadn’t broken into the mainstream.

“We were inundated with the same look in models. Even the hair was parted on the same side,” Moore says. “Everything was stripped down, and [designers] wanted these perfect coat hangers.”

Why has the aesthetic begun to shift away from that puny sameness? It is being moved by influential designers such as Givenchy’s Riccardo Tisci, who gravitates to a guy with a hint of street credibility who measures in at 6-2 or more and has an athletic body.

When Filip Hrivnak, a former hockey player, began modeling a few years ago, his significant athletic bulk was a problem. “At the beginning it was a bit of concern, so . . . I had to lose some weight,” Hrivnak says. “But then [a] few months ago, I received a request from [photographers] Mert & Marcus to build up more muscle.” Hrivnak, by the way, credits Tisci with helping to launch his career.

Like many fashion shifts, this one mostly grew out of a quest for something new, by a general impatience with the status quo. “I don’t know if there’s a macro reason,” says New York-based menswear designer Michael Bastian. “It’s just a cycle.”

And models, who want to be relevant, have accommodated the change. “For me personally, I’ve been evolving from ‘the skinny guy’ in high school and university to the more muscular guy,” says model Braeden Wright.

“I want to be someone who can look like a Greek warrior one minute, a gritty rocker the next,” Wright explains. In other words, he wants to have it both ways and keep working regardless of fashion’s whiplashing preferences. “I’ve been told things varying from that I have a very classic look and that I look very Old Hollywood to that I have a ‘beautiful caveman face’ — as my [primary] agent decided to eloquently put it when I first walked into his office.”

“I take that as a compliment,” Wright adds with a laugh.

For Bastian, fashion is simply coming back around to an aesthetic to which he has been steadfast. Bigger guys “look better in the clothes,” he declares. “They’re the kind of guys that most guys want to look like.”

The glory days of the lean, rocker look were not kind to models of color.

While a few black models, such as the lanky Armando Cabral — who is a regular in J. Crew advertising — thrived, many were stymied by an unspoken tendency to connect black men with urban style or athleticism. “When a designer has 30 outfits and one pair of shorts and the model needs to be shirtless, that’s when they’ll cast a black model,” says Zachery, who is black.

“Being on the inside and seeing how hard models of color have it, you feel like you’re representing the race on every job,” he says. “I can count on one hand the number of times when, if it hasn’t been a BET fashion show or a ‘black’ fashion show, that there’s been more than one black male model on the shoot.”

So do they fit in now?

“The body standard has changed, but the beauty standard is the same,” notes Jason Kanner, founder of New York’s Soul Artist Management, a modeling agency. The male side of the modeling industry, he says, remains stubbornly enamored with young men who, right now, happen to be Scandinavian or Dutch.

“They’re really sticking for a moment,” Kanner says of their popularity. “It’s the skin coloring, the flatness of the faces, the sandy blond or brown hair, the blue eyes.”

Pure economics — the emergence of China as a market for luxury goods — has leavened that dominance, a bit. The handful of Asian models represented by Kanner’s agency are generally booked solid, he says. But the two dozen black models might work two to four times a week.

The recent vigorous conversations, both inside and out of the fashion industry, about diversity on the runway have focused on women. But the men’s side has been awash in homogeneity as well. In a recent Details cover story on the rise of male models, no people of color were featured. And in an echo of the thinking behind some womenswear brands, menswear designers have referred to “aesthetics” to explain their choice of models. Kanner considers the word akin to racism by a more palatable name.

“If you’re making an assumption about race [and body type],” says Bastian, “you’re not looking hard enough.”

Kanner says the onus is on agents to “push the agenda forward. Even when you’re asked for white models, you need to suggest [black ones].”

As the spring 2015 fashion season opens in New York, masculinity is represented in a multitude of ways, from the intellectual aesthete to the swashbuckling hero. The door is open for eccentrics and geeks and has reopened for muscleheads. But black models, whether scrawny or muscular, are still only barely in the room — or more precisely, on the runway.

“If people were to book me less in order to display some of the amazing beauty that diversity brings,” says model James Gatenby, who is white, “I wouldn’t mind.”