It’s a new, if not odd, sight inside some public pools in the District — people kicking submerged punching bags and jumping up and down on floating boards. For about six months, hundreds of D.C. residents have been getting their feet wet with new “HydroSuite” classes, part of Mayor Muriel E. Bowser’s #FitDC initiative. HydroPole, HydroKick and HydroBoard join HydroSpin (the water version of a land favorite, spinning) which has been a Department of Parks and Recreation offering since 2015.

While hydro fitness has been around for decades, its cool factor has shot up in the past few years. Now, you can find hydro classes cropping up in gyms in New York City’s Hell’s Kitchen and college campuses such as the University of Oklahoma. They are extremely popular internationally, as well, in countries such as Canada, the United Kingdom and the Netherlands. The re-brand partly comes thanks to the promotional efforts of the nonprofit Aquatic Exercise Association (AEA). The AEA’s director of education, Julie See, says “getting more people to understand the benefits of water . . . and the wide range of classes now available has helped to promote water exercise as beneficial for all ages and abilities,” meaning aqua classes are no longer primarily viewed as the classes your grandmother takes.

And that’s because so many of the hip new hydro classes are serious calorie burners, at 500 to 800 calories an hour. With a focus on cardiorespiratory fitness, muscle strength and endurance, flexibility, neuromotor training, and body composition, See says, hydro classes can be easily modified to meet different exercise needs. The biggest con is the chlorine or bromine used to keep pools clean, which can be a skin irritant for some participants.

I’ve never strayed very far from lap lane workouts, so this trend had escaped me until a chilly Sunday morning in March, when my attention was diverted from my usual back and forth at Deanwood Pool to a group of people chest-deep in water kicking and punching at punching bags as the teacher called out instructions. The classes had been established in Washington by Rashid Jones, a DPR recreation specialist who is certified in aquatic fitness. Having “seen the end result” of too little exercise while working as an autopsy technician at the D.C. Medical Examiner’s office and George Washington University Hospital, Jones is on a personal mission to help D.C. residents. “The doctors I worked with taught me that your body . . . your health and wellness is what you make of it.”

It looked different and challenging, so I decided to try out some of the classes. First, I joined in for part of a HydroKick class, alternating kicking and punching a bag with Wynonia Harris, 44. Harris, who had been taking the class since October, knew what she was up against: water resistance. According to the AEA, water is 800 times denser — more viscous — than air. And the resistance is bidirectional. So, when kickboxing in water, my quads were working in one direction and my hamstrings in the other. This provides a more balanced workout, See says.

“It was something different and it was in the water, so I figured it would be less pressure on my knees,” Harris says about her decision to try HydroKick. She’s right. Because of buoyancy, impact is reduced by about 50 percent when you’re submerged to the waist and 75 when you’re submerged to the chest.

Along with buoyancy and viscosity, research shows that hydrostatic pressure and thermodynamics play a role in enhancing a water exerciser’s fitness. Hydrostatic pressure — a “multidirectional pressure” that pushes on the chest in water — makes breathing more difficult. Kinetics research suggests this added value forces even fit people who take to the water to develop stronger assistive breathing muscles.

“I never thought that I could sweat so much in the water,” Harris says. We do, indeed, sweat during water exercise, but because of thermodynamics, the water transfers heat away from the body faster than air.

I’ve taken one spin class in my life, and I honestly wasn’t a fan. But I found a sample HydroSpin class intriguing. Joint friendly, it felt like a constant uphill climb as we moved our hands into different positions on the bars as well as in the water and alternated standing and sitting while peddling. After just a few minutes, I was feeling it.

Kimberly Carter, 52, who hit the HydroSpin class with me, found it “very challenging but awesome.” A spin-class newbie and retired police sergeant who lives in Northeast Washington, Carter says she wants to “get back in shape [without] beating on my knee and [without] messing my joints up while healing my body.” The morning I met Carter, she’d already taken the HydroKick class. “I’m a glutton for punishment.”

Apparently, I am, too. I didn’t know real intensity until Jones had me try some HydroBoard exercises. With the surfboard-wide floating mat tethered to the lane ropes by its four corners, I worked my core as I tried to center myself while attempting to jump rope (yes, on the unstable rocking board!), do a burpee or nail a bird-dog plank without rolling into the water. It didn’t help that I couldn’t stop laughing every time I lost my balance and fell in. It was one of the most fun and challenging experiences I’ve had in the water.

Jones says he would like to see more D.C. residents, especially younger ones, take advantage of the aquatics programs. (Residents pay $50 for an eight-class session; nonresidents pay $100.) Although one of their major appeal is their joint-friendliness, he says, the classes are not just for the District’s seniors. “HydroSpinning helps people with their cycling. I’m implementing triathlon camps and I [also] offer prenatal water aerobics. It’s all about what you are actually looking for.”

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