Updated

Helen Zotikos had a strange reaction as she sweated away on an elliptical machine recently: hysterical laughter.

The 34-year-old mother of four was watching the goofy comedy “Superbad” in a darkened room with a big screen inside Retro Fitness in 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.”

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