After a few committed months of hot yoga at a studio in New York, Christina Rice had found her niche. So when the studio announced that it was offering teacher training, she signed right up.

It was only when she arrived with her mat that she noticed something striking.

There were 54 other women and men in the 10-week course, and not one of them looked like her. She was the lone African-American in the class.

"I did bond with some of the other students," says Rice. "But I did feel very isolated at times. There were no teachers of color. I didn't have another woman who looked like me, who understood my struggles, my insecurities."

Boutique workout studios - specialized, exercise-specific gyms - are exploding in gentrifying urban areas. They include not only hot yoga but also CrossFit, which is everywhere; Barry's Bootcamp (in Los Angeles, Nashville and Washington, along with other major cities); SoulCycle (nearly 20 markets); or Orangetheory (hundreds of studios nationwide).

They are the modern answer to the sprawling, soulless gym, which insists on financial commitment but doesn't really care whether you actually work out. In the boutique world, you make reservations. You're greeted with smiles. You're served an ice-cold glass of the "spin class is self-care" Kool-Aid.

According to the International Health, Racquet and Sportsclub Association, the industry trade group, more than 18 million people now claim membership in a boutique studio (though with the studios' class-by-class reservation model, "allegiance" might be a better term). In a relatively short period - CrossFit had been around for decades but truly caught fire around 2012; Barry's Bootcamp began expanding in 2011 - they have claimed half as many exercisers as traditional gyms.

But some people have begun to question the stark differences between the studios and the neighborhood YMCA. Like the prices: In Washington, a single 50-minute Barry's Bootcamp class is $34. Spinning studio Flywheel charges $30. Solidcore, a Pilates-like workout, can run as much as $37, or about half the cost of a monthly membership in most urban gyms.

And like Rice, other fitness junkies have begun to notice who isn't coming. Sweat through a class in one of these studios and it's very possible that you'll see it, too: many, many lithe young white bodies and very few people of color. Or older or heavier exercisers.

Leticia Long has regularly attended classes such as Orangetheory and SoulCycle because she owns Wired Cycling, a studio offering cycling and TRX in D.C. Her daughter, she says, loves Barry's.

But Long, who is African-American and Hispanic, is struck by what she sees there. "What is their messaging saying about aging? What is fit? What is beauty? I look at everything from the signage to the marketing materials," she says. All she sees is millennials. "How open and inclusive is that?"

"There hasn't been a time in our collective history where people have been as integrated as they are now," adds Jessamyn Stanley, a North Carolina-based yoga teacher and author. And yet, pick a class, any class, she says. "Is this reallllly everybody, or just everybody that can afford to go?"

Daniel T. Lichter, a Cornell University sociology professor and demographer, agrees that cities are more integrated, but he sees the rise of boutique businesses such as juice bars and studios - with their specific clientele - as a trend in keeping with larger demographic shifts. "We've seen this return of the white-middle class, minority professionals, and professional immigrants. There's more money in the city now," he says. "There's now a large enough clientele that they can cater to and specialize in."

Some have made efforts to foster diversity. In an email, SoulCycle chief executive Melanie Whelan described her company's effort to maintain a team of instructors that give "riders a range of genders, races, backgrounds and personalities to identify with." The company also maintains an inclusivity and diversity council and offers underserved youths in some markets 12-week scholarships to take classes and learn nutrition. Barry's Bootcamp declined a request for comment, while Flywheel did not respond to a request.

But Stanley, who has gained some fame with her criticisms of boutique workout and yoga culture's lack of diversity, describes the studios she has visited outside the big-city bubble as anything but diverse. She tackles the subject in her book "Every Body Yoga." The issue, she says, extends beyond race.

She's often the only fat woman in the room as well, she says. And if you're looking for a mature crowd, you'll have to keep looking, too: By the health club association's reckoning, the average age of studio exercisers is 30.

"The messaging," says Stanley, "is essentially: You're allowed in this space if you are white, slender, able-bodied and less than 45, cis-gender and heterosexual. And if you're not, then you're not welcome."

Christine Hewitt / Washington Post Jessamyn Stanley is a yoga teacher and author who is outspoken about the lack of diversity in studio culture. She says messaging is a large part of where studios fail prospective students. Jessamyn Stanley is a yoga teacher and author who is outspoken about the lack of diversity in studio culture. She says messaging is a large part of where studios fail prospective students. (Christine Hewitt / Washington Post) (Christine Hewitt / Washington Post)

Todd Miller is director of George Washington University's Weight Management and Human Performance Laboratory and has researched commercial gyms. He sees the CrossFit boom as the beginning of boutique workout culture, and its origin story (the founder is said to have concluded his first attempt at the achingly demanding workout by vomiting) as a marketing tool that separates these classes from hopping on an elliptical for a half-hour and calling it a day.

"They're trying to cater to a specific kind of person: someone who works out to the point that it feels like it's going to kill them," Miller says.

His research into workout habits reveals that "people want to be around others who are like them. That's almost universal."

It all speaks to the subtle selling point of the specialized studios. It's not the way their slim, exquisitely carved instructors can execute a side plank, a sumo squat or a warrior pose. It's their very cliquishness.

What makes such studios appealing, a spokeswoman for the health club trade group wrote in an email, is "the sense of belonging, where everyone is 'like them.' " What they foster is known in the business as "tribes," spandex-clad warriors who feel a special kinship after enduring a couple of dozen burpees together.

The tribe model has a distinct upside, says Miller. The sense of community encourages exercisers to keep exercising - something conventional gyms haven't been successful at.

But it has a downside as well.