Gotta give it up to the ancient Hawaiians. They're credited with being the first to develop strength training in water. Sure, it was a bit makeshift—they'd carry large rocks into the ocean and hold them underwater as they slogged along the seabed. And okay, it was sort of a macho test of strength, but hey, they were onto something!

Since then, ocean lifeguards to elite athletes have used that move in training to kick their fitness up a notch. And now, gym chains across the U.S. have taken notice, adding classes that bring HIIT and weight training—paddles, dumbbells, and kettlebells; no rocks!—into the pool.

Related: Nicole 'Snooki' Polizzi's Been Posting Tons Of Workout Pics On Instagram—And They'll Make Your Jaw Drop

SO WHY WATER + WEIGHTS?

Stronger, leaner muscles are built in H2O. Imagine trying to do a super-explosive move—like a tuck jump—underwater. Not as fast or as powerful as doing it on dry land, right? That's because the harder you push or pull through water, the more resistance you experience. Which makes it an awesome method for building muscular strength and power.

The extra resistance doesn't just make you work harder, it helps you work smarter too. Take the pistol squat, a notoriously tough single-leg exercise. "Even if you can't do it on the ground, you can probably do it in a pool," says Rob Glick, senior director of programming and innovation at Life Time, which now offers an exclusive aquatic boot camp class called WTRX Water Xtreme. "The density of water naturally slows down motion, helping to emphasize proper body position and maintain control through difficult movements."

Find out how to score a water-sculpted body:

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LIGHTER, FASTER, STRONGER

Less gravity to weigh your body down also means less of that pound-after-pound impact on your joints, says Glick. Plus there's water's natural compression qualities, which can boost circulation to help your muscles bounce back faster. "You wouldn't recommend having someone do three intense land workouts in a row because of the risk for injury," Glick says. "But the pool allows athletes to work out harder at a higher intensity several days in a row, without wear and tear on their joints or excessive muscle soreness." Nursing an injury? Switch up your HIIT workouts to twice a week in the water, once on land. (Kick-start your new, healthy routine with Women's Health's 12-Week Total-Body Transformation!)

Or, better yet, use the pool for recovery itself. Just grab a few pool noodles to support your neck, hips, and ankles. Close your eyes, stretch your body as long as possible, and focus on deep breathing through your nose. It's like savasana—only weightless.

This article originally appeared in the July/August 2017 issue of Women's Health. For more great advice, pick up a copy of the issue on newsstands now!