Student Loretta Douglas, left, performs a surfing-like move with instructor Jen Frankel in a SurfSet class in Tysons Corner. (Michael S. Williamson/The Washington Post)

The most important thing to bring to the gym? An active imagination.

It’s what turns an indoor cycling class into a stage of the Tour de France, or a treadmill session into a marathon triumph. Add some California dreaming to a new piece of exercise equipment — the RipSurfer X, a 70-inch-long surfboard perched on a short, wobbly base — and you’ll be on the beach, ready to ride the ocean.

That’s the premise of SurfSet, a new group fitness concept that transforms squats, push-ups and planks into new moves by adding a touch of instability and fantasy.

“We’re keeping the fun of surfing but taking away the water, the sharks and the stinging coral,” says Jen Frankel, group fitness director for Crunch’s two Northern Virginia gyms. When the Tysons Corner and Reston locations opened in December, Frankel — a master trainer for SurfSet — made sure both were ready to help members catch a wave.

Crunch’s shtick is offering unusual classes, Frankel explains. Although SurfSet has had national (and even some international) success since it was featured as a winning business idea on the TV show “Shark Tank” in the fall of 2012, it hasn’t made much of a splash in our area yet.

Only a few other outfits nearby have gotten on board. Baltimore’s Canton Club has featured SurfSet for a year, which is how Mantra Fit owner Carleen Birnes fell in love with it. Birnes, who teaches stand-up paddleboarding fitness in Severna Park, realized the indoor classes could fill the off-season when it’s too chilly to be out on the river. In December, she opened a small studio that’s devoted to SurfSet classes.

“It’s exactly what we’d do on the paddleboards, but we’re not worried about falling in when we do one-legged moves,” says Birnes, who has crafted a schedule with several different formats: cardio, strength, balance and yoga. A SurfSet junior class for ages 8 to 15 starts in March.

Although aquatic experience can come in handy for SurfSet, it’s not necessary, Frankel says. She has never surfed a day in her life. She has, however, spent decades dancing as a Rockette and in Broadway shows. And she relies on her acting chops to make every class feel like a day at the beach. Just not a particularly relaxing one.

Before launching into the warm-up, Frankel introduces her students to the RipSurfer X units. There are bands on either side on the board that control how much it teeters, and for beginners, she recommends hooking both in to provide stability. (Advanced exercisers can lose one or both bands to boost the balance challenge.) Most of the top of the board is covered with black matting, which is more comfortable on the knees and wrists. It also indicates where you can step safely; no mat and you’re likely to go splat.

When the moves are done correctly, wipeouts are rare. But to avoid turning a misstep into an injury, Frankel recommends keeping the area to the sides of each board clear. She also advises against wearing socks, which can lead to slips.

The first moves on the board are simple: Stand sideways, squat and let your feet rock from toes to heels. Pull up your surfer arms — the front one extending straight out and the other bent into your chest — as you step side to side, getting a feel for how to balance on the board. Then turn so you’re facing front and squat again. If it feels shaky, that’s normal, Frankel says.

It’s why she has each of her students grab a weighted bar. They can plant one end on the ground for extra stability or pick it up for more of a strength and balance challenge. Her favorite forward-facing move: water walkers. Stand on one leg while you dip the other one into the air, as if you were sweeping your toes through the sea foam.

Many SurfSet exercises play off similar visualizations. In “hot sand,” you quickly hop on and off the board, pretending that the beach is burning your soles. For “Jaws shark kicks,” you sit on the board facing forward, lean back and grip the edges. Then you imagine that a shark has appeared at your feet and you need to fend it off by smacking it with your heels. That core strengthener is also practical advice — if you ever happen to find yourself face-to-face with a great white. “If you strike a shark on the nose, it’s supposed to retreat,” Frankel asserts.

Almost any move you can do on land is also possible on a RipSurfer X — only it’s more difficult because of the wobble. So a lot of standard exercises may not feel quite so familiar: push-ups, overhead presses, downward dog. “Wave runners” are the SurfSet version of mountain climbers. And board burpees are downright cruel — although pretty similar to the pop-ups real surfers perform. (Too tough? Try it with your feet on the floor and your hands on the board.)

“Because you’re on the board, you’re doing things differently and using every muscle in your body,” says Loretta Douglas, 39, who has become a SurfSet regular. The Vienna resident is originally from Miami, so she especially appreciates the talk of sun and sand. “It helps you forget about how cold it is outside.”

First-timer Mike McCabe, also from Vienna, got the hang of the RipSurfer quickly at a recent class. “It’s just enough to keep you off balance,” says the 50-year-old, who’s glad that the workout is easier than actual surfing. “My kids surfed, but I never did it successfully.”

Although the class might not prepare you to hang 10, it can help you hang in there. Those balance skills are going to be useful the next time you find yourself out on the water — the kind that’s frozen all over your front steps.

@postmisfits on Twitter

Hallett edits the Fit section of Express.

Also at Read past columns by Hallett and Lenny Bernstein at . There, you can subscribe to the Lean & Fit newsletter to get health news e-mailed to you every Wednesday.