What are pistol squats, the intense-looking exercise Jessica Biel just did on Instagram?

Celebrities are flooding Instagram with their prepping routines for Monday night’s Emmy Awards — and Jessica Biel is no exception. She posted a video of herself on Sunday doing “pistol squats” — and they definitely look challenging. In it, the star of The Sinner stands on a box and slowly squats on one leg while holding weights in front of her. “Hey, Emmy, my butt is coming for you. (Assuming I can walk after these pistol squats),” she wrote.

Most people are familiar with squats, but … what are pistol squats, exactly?

“Pistol squats are basically a single-leg squat,” Albert Matheny, CSCS, of SoHo Strength Lab and Promix Nutrition, tells Yahoo Lifestyle. Like “regular” squats, there are plenty of variations you can do on a pistol squat, and Biel’s version, while challenging, is a little simpler than it looks.

“This is actually easier than a standard pistol squat because her inactive leg is able to go straight down,” Matheny says. “If you were doing that on the ground, you’d have to keep your other leg straight in front of you — and that’s harder.”

It’s possible to do a pistol squat without weights, but lighter ones like Biel appears to be using can actually help with your balance, Matheny says. “Arm weights counterbalance your weight and help with stability,” he says.

That doesn’t mean pistol squats are easy, though. “Pistol squats require significantly greater balance, flexibility, ankle mobility, and strength, since you’re lifting your body weight with only one leg instead of two,” Doug Sklar, a certified personal trainer and founder of New York City fitness training studio PhilanthroFIT, tells Yahoo Lifestyle.

Pistol squats work your glutes, quads, abs, and obliques “as they act to stabilize your body against the imbalance caused by lifting one foot off the ground,” Sklar says.

If you want to try pistol squats and have never done them before, Matheny recommends doing less than a full squat and seeing how that feels. “This is a new stress on your connective tissues, and your knee joint might not be used to that,” he says. “You may have to build into it.”

Then, if you get comfortable with it, you can incorporate pistol squats into your workout routine like any other exercise, Sklar says. “Two to three days a week is plenty,” he adds.

Read more from Yahoo Lifestyle:

• Brie Larson works out with Alison Brie and it’s (Captain) Marvelous
• ‘Cycle of abuse’: Model Katie Willcox on what it’s like to be neither plus-size or straight-size
• WWE star Nia Jax on how sports gave her confidence: ‘My body had its purpose’

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