West Orange, N.J. Only in this theater, plush velvety seats were replaced with treadmills, ellipticals and stationary bikes.

Ms. Zotikos says she is “not a workout person,” but that the cardio theater has become her go-to spot for exercise four days a week. “I don’t think about working out. I think about the movie,” she says. “And I also like how it’s dark and I don’t feel like everyone’s looking at me while I’m working out.”

Cardio theaters are a quietly persistent niche in the $22 billion health-club industry. Fans say they are a welcome distraction from exercise. Club owners say they provide shy exercisers an escape from the harsh lights of gym floors.

Retro Fitness, a rapidly expanding chain centered in the Northeast, has a cardio theater in all 125 locations. Gold’s Gym trademarked “Cardio Cinema” a few years ago and now has the theaters in more than a quarter of its 421 U.S. locations, Gold’s President Aaron Watkins says. Smaller gym chains from Rocky River, Ohio, to Amityville, N.Y., also have drawn crowds with cardio theaters.

“We’re in the business of distraction,” says Eric Casaburi, founder and CEO of Retro Fitness. “Fitness comes from that. If you don’t get distracted, you’re going to hate it. Nobody likes to sweat and feel pain.”

It might seem retro to open a movie theater in an era when movie-ticket sales have stagnated and audiences feast on Netflix and TV-on-demand.
But health clubs are finding that cardio theaters appeal to movie buffs, as well as new exercisers who would rather be part of an audience than feel as if people are watching them.

In the U.S., most fit people already belong to a gym. Industry growth depends on persuading novice exercisers to sign up—and use the facilities enough to make their membership worth keeping. So far, it’s working: The share of Americans age 6 and older who belong to a health club increased to 18% in 2013 from 16% in 2009, according to the International Health Racquet & Sportsclub Association.

Gyms as inexpensive as $10 a month have proliferated in recent years, lowering the barrier to entry for the reluctant. Cardio theaters thrive particularly in $20-to-$50-per-month clubs that want to offer as many basic fitness options as possible.

At most cardio theaters, a movie plays on repeat each day and through the evening. Some gym managers take movie requests from members or follow themes like showing an older movie on “Throwback Thursday.” To show the movies legally, each club must pay an $820 annual fee per location to the Motion Picture Licensing Corporation.
Several club owners say their members favor action movies. “If I play ‘The Notebook,’ there probably isn’t going to be a ton of people,” says Mark Fine, general manager of Gold’s Gym in Milwaukee, of the 2004 tear-jerker. “I don’t know a lot of people that want to cry and run at the same time.”

Bryan Bullock, general manager of Onelife Fitness in Kansas City, Mo., says members look forward to the release of the next month’s movie schedule. Although many gyms show only movies rated PG-13 and tamer, Onelife plays R-rated films, because the gym caters to adults. “We try to push it a little bit, but keep it in the realm of what’s socially acceptable,” Mr. Bullock says.

The gym showed horror movies before Halloween. As he did yoga, Mr. Bullock glanced at a TV screen outside the theater that shows the movie playing. “Someone was getting sawed in half with a chain saw,” he recalls. “I thought, ‘Maybe I took this a little too far.’ ”

Mike Arteaga’s Health & Fitness Centers in Poughkeepsie and Highland, N.Y., have cardio theaters. Mr. Arteaga says the Poughkeepsie location attracts everyone from older people to students at nearby Marist and Vassar colleges. Members sometimes occupy all 20 of the room’s cardio units at once.

At several of these theaters, sound is broadcast to the room just like in a regular movie theater. Ben Bonaventura, general manager at a Retro Fitness soon to open in lower Manhattan, says he sells the theater to future members as a place to “take the earplugs out for at least 45 minutes.”

Even some cardio-theater fans complain that it’s difficult to keep moving through a full 90-minute or 120-minute movie. But others say that films inspire them to spend more time than they had planned to work out.

LeeAnne Taylor, a 28-year-old hairstylist in San Francisco, works out at a Crunch Gym housed in a 1926 theater where dozens of cardio machines now fill the balcony.

Ms. Taylor recalls watching “Cinderella Man,” a boxing biopic, as her usual elliptical workout stretched from 30 minutes to 90. “I was almost late for work because I was so into it,” she says.

Bruce Carter says that about 10% of the 480 clubs his firm has designed in the past 35 years have had cardio movie theaters. Mr. Carter, owner of health club design firm Optimal Design Systems International, says the theaters can set apart clubs that don’t have basketball gyms or swimming pools.
Cardio theaters are part of a larger evolution of health clubs away from macho shrines bristling with weight machines to more inclusive places.

Many clubs are pushing weights away from the entrance and shrinking or removing the mirrored walls of the bodybuilding era.

In some cases, clubs are reinstalling changing cubicles in women’s locker rooms to make new members feel less self-conscious, Mr. Carter says. Clubs also are reducing the lighting throughout the facility, not just in cardio theaters. “If you’re not happy with how you look, the last thing you want to do is go into an area with a lot of mirrors and a lot of bright lights,” Mr. Carter says.

Mr. Casaburi of Retro Fitness says the chain’s core market is people new to exercise. He says the company tries to position cardio theaters as close as possible to locker rooms so new members don’t have to parade through the club.

Cardio theaters aren’t thriving everywhere. Work Out World, a 12-club chain in New Jersey, plans to repurpose its six cardio theaters into smaller studios or areas for small-group training. Owner Steve Roma says the theaters aren’t as well-used as they were five or six years ago.

Mr. Carter says that although some clubs swear by cardio theaters, everyone must adapt to a shifting market. “If we’re doing our job of making the entire club less intimidating, then we don’t need as much to have spaces where people can try to hide away,” he says.