If you’re looking to build some serious strength, it might be time to skip the gym and head to the playground instead—at least according to Paola and Pamela Del Hierro, fitness instructors better known as the Iron Twins.

That’s because the schoolyard is where you’ll find the one piece of equipment these self-described “wellness warriors” swear offers the most physically challenging upper-body workout around: the monkey bars.

Making it across correctly requires a ton of power from your back, shoulders, arms, and abs (without a lot of support from your lower body)—which is why the maneuver proves particularly difficult for women to master.

And if anyone knows a thing (or two) about what makes for a truly mettle-testing workout, it’s these two. The dynamic duo are trainers at the Spartan Gym in Miami, FL, where they’re tasked with bringing the muddy endurance challenges’ obstacle-course vibes indoors—from scaling a six-foot wall to crawling on a cargo net, climbing a rope, and, yes, making it across those monkey bars.

But the Del Hierros promises that, if you can get into the swing of things where the latter is concerned, you’ll be building muscle while having fun in no time.

“It brings you back to being a child—and people become very happy,” Paola says. Feeling like a kid again is “really going to come down to strength,” she notes. Work on yours by starting with “dead hangs”—holding on as tightly as possible to one bar for as long as you can. “If you can hang for 60 seconds, then you should be strong enough to work yourself all the way across,” she adds.

When you’re ready to get moving, engage your core so that you don’t exhaust your upper body. And remember that momentum is your friend. “It’s important to [connect] yourself to the animal flow,” Pamela explains.

The twins also emphasize that when it comes to monkey bars—or any other physical challenge—half the battle is psyching yourself up. “Accept that failure is part of the process,” Paola advises. If you do, you’ll get the hang of things in no time.

Other tough moves we challenge you to try: the gallop and boomerang. Once you master the monkey bar, how about trying pull-ups?