For anyone who has ever used a StairMaster at the gym, the time has come to really sweat—on a stepmill.

Stepmills look like moving staircases, and are so challenging that people brag about their workouts on social media with the tag #stairmonster. Gyms are adding them—and removing the classic stair climbers that have been a staple since the 1980s—as more people seek shorter, tougher workouts.

A traditional step climber’s pedals sink when you step on them and rise when you lift your feet, which can make workouts easier by shortening steps. Stepmill steps are a fixed height and move at a constant rate, pushing the user to keep up.

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Stepmills quietly have become the most-used cardio machine after treadmills at gyms across North America. That’s according to data from 967 clubs collected by Ecofit, a company in Victoria, British Columbia, that lets gym owners track equipment usage through a wireless platform. Over the past year, stepmills were used for about 18 times as many hours as two-pedal step machines, Ecofit says.

“It’s a really good workout in a fast amount of time,” said Samantha VanderPutten, a graduate student in physical therapy at New York University, as she climbed onto a stepmill at an NYU fitness center. “I get bored on the treadmill.”

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