Are you cheating on your gym membership?

As upscale fitness boutiques are exploding in popularity, some of the nation's largest big box gyms are opening small studios with high-end amenities, "it" trainers and specialized workouts that consumers can pay for on a class-by-class basis.

Big box gyms make their money off recurring monthly memberships with an average monthly fee of $54, but many say they saw an opening in the studio market where consumers are willing to pay between $25 and $35 for a single spinning, yoga or boot camp class.

Equinox, the leader in upscale gyms with a track record for starting trends through its innovative classes (Equinox acquired SoulCycle), says its new offering, called Project, in New York's trendy NoLita neighborhood, has classes in everything from high-intensity interval training to yoga for $35 each. The enterprise promotes rising fitness stars, encouraging them to create passion projects and work with other trainers to form unique mash-ups like a dance-pilates hybrid.

Equinox, which declined interview requests, is tapping popular trainers like Bec Donlan, who has more than 42,000 Instagram followers, to create their own workouts. Donlan is teaching a booty-band class at Project called Babes & Bands.

Membership in traditional fitness clubs grew by 5 percent, compared with over 70 percent in studios between 2012 and 2015. And the percentage of members who visited more than one facility in 2015 increased in all segments, ranging from 28 percent of traditional gyms to 86 percent of studio members, according to a study by the International Health, Racquet & Sportsclub Association.

"Today's consumers, driven primarily by millennials, do not demonstrate a strong allegiance to one type of fitness program or provider. Therefore multiple memberships is becoming (or has become) the norm," the report noted.

Experts call it the tribe mentality, led by millennials who aren't afraid to spend more money to feel like they're part of a shared experience and community. ClassPass, which contracts with fitness boutiques across the country and allows consumers to try different studios for a monthly fee, has capitalized on this phenomenon.

"Some people go to the gym and they know what they're doing ... but there's a segment that wants to have an experience and feel something going to those classes," said Michelle Ryan, chief marketing officer of New York Sports Club. "They're motivated by people feeling like, 'Hey, why weren't you here yesterday?'"

The brand, owned by Town Sports International Holdings Inc. (TSI), recently launched NYSC Lab in two locations in New York and one in Boston. The Lab features trendy open-space studios with upscale finishes and a rotating roster of six classes that includes strength, cardio and high-intensity interval training. Celeb trainer Kira Stokes is at the helm of the Chelsea studio.

"We were just listening to what the consumers wanted," said Ryan, who noted the company is also exploring online streaming options, which are also popular at many fitness boutiques.

In 2015, approximately 31 percent of members said they participated in at least one small group class, from a low of 4 percent in sports-specific studios like yoga to a high of 36 percent in personal and small group training studios, paying an average of $35 per class, according to the study by International Health, Racquet & Sportsclub Association.

Still, experts say most gym-goers still want to put on headphones and work out on their own at a low-fee monthly membership gym.

"I wouldn't expect disrupted wholesale business models over the next five years. We're going to try to see the big players compete in these areas incrementally over time," said Emile Courtney, an analyst for S&P Global Ratings.

Discretionary spending on fitness has remained steady with increased spending among higher-income consumers willing to pay for $25 to $40 per class offerings because "they have the income to do it, the time to do it and they are adventurous enough to do it," he said.

That's what Crunch gym is capitalizing on with its new SweatShed boutique offering, where the motto is fitness meets entertainment.

SweatShed's app allows consumers to pick the type of class, instructor and even where they want to stand.

Crunch, known for unique classes like gospel house aerobics and urban attitude dance, ignored the boutique bubble at first but by mid-2015, company executives noticed the market was saturated.

"That's when we started to say, 'Hey, let's look at some of the things we're doing and stay current,'" said Mike Spiegel, Crunch's East Coast regional director of fitness. "The best brands are the ones that adapt."

Shannon Geraghty has been working out at one of Crunch's New York gyms for two years, but also hits up two other boutique classes three times a week — SoulCycle and a high-intensity interval training class.

The 25-year-old merchandise planner for Ralph Lauren says she's definitely increased how much she spends on fitness in the last few years.

"I try to tell myself that spending money on myself and my health and well-being is better than spending $50 on a pair of shoes," said Geraghty.