Stephen Benkert of Key Bridge Boathouse in Georgetown leads SUP Fitness classes on the Potomac River, during which participants exercise on stand-up paddleboards. (Nicholas Verrochi)

“I keep my chest upppp! And my butt lowwwww,” Stephen Benkert hollered to the 10 students in his SUP Fitness class with the precise sing-song of a drill sergeant.

The students, myself included, tittered nervously. Which one of us would be the first to follow Benkert’s lead?

We had come to Georgetown on this sweltering Saturday morning for a workout. But there is a vast difference between the comforts of an air-conditioned gym and the Potomac River, on which we happened to be afloat. Waterborne and wobbling on the river on boards similar to the type used by surfers, it was hard not to feel unsettled by the very thought of attempting a burpee.

SUP is the popular acronym for stand-up paddling, a trendy watersport best described as surfing without the wave. Instead, participants use scoop-like paddles to propel the 10-foot-long boards across still waters. It’s a killer workout for the core and the legs, and surfers have been known to turn to the sport to stay in shape in the offseason.

In recent years, fitness instructors have taken yoga and interval training onto the boards, where the precariousness of attempting crunches or a downward dog has the undeniable effect of bringing up the heart rate.

Benkert, 26, had taught group fitness classes before, and then picked up paddling as a hobby while kicking around the West Coast, where the sport has been around for decades.

In April, with Key Bridge Boathouse, Benkert launched SUP Fitness classes, targeted toward paddlers who’ve had at least a few rides on the water. The classes began drawing adventure-seeking athletes, whom Benkert leads onto the Potomac for 50 brisk minutes of burpees, squats, push-ups and sprints al fresco.

The instructor mixes routines but prefers high-intensity interval workouts, which ratchet up your heart rate through fervent exercises punctuated by brief rests. He led the recent class through four sets a little more than 10 minutes each. Together with rests and water breaks, the exercises easily burned through the allotted time, leaving everyone dripping.

With all of the jumping on the boards, many of the students teetered, but only a few fell into the water. (Because there’s little current in the channel, it’s easy to climb back on.) Benkert estimates that by the end of the class, the students burn 600 to 900 calories, typical for an interval workout in the hot sun.

For Key Bridge Boathouse’s classes, split the difference between your usual workout gear and beach wear, and prepare to leave your shoes on the shore. Most of the women wore bathing suits under lightweight shorts and tank tops, while the men mostly wore swim shorts.

Bring a bottle of water in a distinctively colored bottle if you can; Benkert stashes them in a bag and can toss you your bottle during the workout.

It’s best to arrive about 15 minutes early, and if you drive, be sure to pay the parking meters on Water Street NW for at least two hours, to be on the safe side. That gives you time to listen in on Benkert’s brief rundown of the class and strap on small life jackets, not unlike fanny packs; they’ll offer safety but allow you the mobility needed for the exercises.

After that, things move rather quickly. Grab your bright blue board — which, at about 2 feet wide, is bigger than you think — and paddle your way to the relatively still channel where the workout begins. Afterward, most students take a leisurely paddle back to shore.

“I loved it. I do workouts in a gym indoors, but this is a good change. I’m exhausted,” Tanisha Martin of Alexandria said after the class. The burpees, with their body-bending combination of squatting and jumping, she acknowledged, were the hardest.

“But,” she added with a laugh, “I hate those on land.”

SUP Fitness

Saturdays at 10:30 a.m. throughout the summer. Key Bridge Boathouse,
3500 Water St. NW. 202-337-9642. $35.