This is the hardest oblique ab move you’ll ever do, according to a fitness trainer

Any time you walk into a megaformer-inspired pilates class, you know you’re in for a challenging workout. And while all the moves on the machine are enough to leave your body shaking, there’s one muscle group in particular that’s a special kind of hell to target with instability training: your obliques.

These side-core ab muscles are responsible for stabilizing your body, and working them on an unstable platform is a surefire way to burn them out. Fast. While pretty much every oblique move done on a megaformer will leave you sore for at least a day or two, there’s only one that’s earned itself the title of “hardest oblique move of all time.”

“Twisted army crawls”—which are, as you’d imagine, army crawls with a twist—involve holding a plank with your hips and feet rotated and walking forward and backwards on your forearms. In addition to working your internal and external oblique muscles, the plank-forward move also hits what Nat Straub, senior manager at solidcore, calls a “plethora of muscles,” including the rest of your core, lower back, quads, shoulders front hip flexors, inner thighs, triceps, chest, and intercostal muscles (phew!). Full disclosure, they are so challenging that the first time I tried them, I cried a little bit.

So what makes them so difficult? Well, there are a few reasons. “For starters, it’s performed with instability in mind, since the moving carriage or gliders are forcing those stabilizing muscles to work hard,” says Straub. “It’s also an unusual movement pattern to most of us, which adds to the effectiveness—I personally don’t rotate my torso and crawl on the ground in my everyday life that often.”

To try the move on your own,  there’s no need to shell out $40 for a megaformer class. Just grab a slider (or a small towel, which will glide just as easily on top of a wood floor) and follow along with the below.

How to do a twisted army crawl at home

1. Holding a forearm plank, rotate your hips and legs to one side. Stack your feet one on top of the other on the slider (or towel), squeezing your thighs together so that your ankle bones are touching—the foot of your working oblique should be on the bottom—and point all ten of your toes toward the same wall your hips are facing(To break it down for you: If you’re working your right oblique, rotate your hips to the left and stack your left foot on top of the right. If you’re working your left oblique, rotate your hips to the right and stack your right foot on top of the left.) 

2. Keep your hips slightly elevated so that they’re in line with your shoulders, and your belly button twisted toward the wall.

3. Begin to crawl forward using your forearms, keeping your forearms parallel, your hands apart, and your hips twisted and stable. As you move your arms, it will drag your legs and the glider along behind you.

4. Go as far as space allows and then push back to starting position using arms still fighting for stability in the hips.

Can’t complete? Don’t give up yet. There’s an option to modify by dropping to your knees and stacking your knees on top of the slider (with your working oblique’s knee on the bottom).

For an entire pilates-inspired workout that will leave literally every muscle in your body on fire, follow along with the video below:

After you’ve got the best oblique ab exercise down, try your hand at mastering the plank one and for all: Steal these tips from the woman who hold’s the record for world’s longest planks, and be sure to avoid this common modification that could be totally futzing up your form.