It’s just as intense as it looks.

Fame and fortune are two obvious benefits of being a celebrity, but the one perk I’m most envious of? How much time celebs get to work out for their jobs—and how they’re able focus on building some serious strength. Case in point: the recent videos showing actors Jessica Biel and Alison Brie knocking out several impressive reps of L-sit chin-ups.

In an Instagram clip posted by Brie, the actor (most known for her work on the TV series Glow and Community) can be seen holding onto gymnastic rings and pulling her entire body up several inches off the ground while maintaining an L-sit posture. In one video, Brie performs the move while squeezing a yoga block between her knees; in another, she’s balancing a hefty medicine ball on her thighs.

 

View this post on Instagram

 

A post shared by Alison Brie (@alisonbrie)

“I see you @jessicabiel,” Brie wrote in her Instagram caption, with the hashstag “#inspo.” Last month, actor Jessica Biel shared an Instagram video of herself doing the same super-human move with her husband, Justin Timberlake.

 

View this post on Instagram

 

A post shared by Justin Timberlake (@justintimberlake)

The move, which Biel’s trainer Ben Bruno calls an “L-sit ring chin-up,” requires some serious strength, both in the upper body and the core. “This is an advanced exercise that requires significant strength, balance, and flexibility,” says Jay Cardiello, CSCS, certified strength and conditioning specialist whose client roster includes Jennifer Lopez and Rami Malek.

Of course, it’s no wonder Brie can complete this move. She’s been training at Rise Movement, home of celebrity trainer Jason Walsh, for years, and she also recently signed on for Masters of the Universe, a live-action He-Man movie, where she’ll play a super-strong villain.

When Bruno first shared a video of Timberlake knocking out 10 reps of this advanced move, he commented, “This is a very, very impressive display of core and upper body strength. Go try it for yourself to see just how difficult this is.”

The mechanics of the move

The L-sit chin-up, much like a regular chin-up, works a lot of upper-body muscles, including the lats, biceps, shoulders, and stabilizing muscles in your back. But “the instability of the rings challenges your upper body more than traditional chin-ups or pull-ups,” says Cardiello.

Plus, being able to pull your entire body weight off of the floor while maintaining that L-sit posture also requires tons of engagement from your core and hip flexors, which are working to hold your legs straight during the move. And you bet your pelvic floor is involved: If you struggle to engage your pelvic floor during a move like this, “consider incorporating pelvic floor–specific exercises, like Kegels or diaphragmatic breathing, to build awareness and strength in that area,” says Cardiello.

That means in order to do this one with good form, you need to already have some full-body strength built up. Here’s the right way to do this expert-level chin-up variation, according to Cardiello.

How to do an L-sit ring chin-up

“In this version, you hold a block between your knees, which increases core engagement and promotes better body alignment,” says Cardiello. “The instability of the rings adds an additional challenge, requiring more focus on balance and coordination.”

1. Sit on the ground under a set of safely secured gymnastics rings. Hold one ring in each hand, palms facing toward you or toward each other.
2. Extend your legs straight out in front of you, flexing your feet. Engage your core and and lift your legs so your thighs are parallel to the floor while maintaining a firm grip on the block.
3. While maintaining the L-sit position with the block held between your knees, pull yourself up toward the rings by bending your elbows and squeezing your shoulder blades together. Aim to bring your chest close to the rings, keeping the block between your knees throughout the movement.
4. Slowly lower yourself back to the starting position in a controlled manner, ensuring that your core stays engaged and the block remains secured between your knees.

Not there yet? Don’t be discouraged: Building strength like this takes a long time. Start with a modified version, says Cardiello. “Try keeping your knees bent at 90 degrees to form a tucked L-shape with your torso and thighs,” he says. Working up to a chin-up or pull-up on a fixed bar is also a great place to begin.

Things to look out for

If you do have the strength to pull off an L-sit chin-up (and, wow, I’m impressed), these are the things to keep in mind.

Core engagement

You need to focus on core engagement to get this move to work. “If your core is not strong enough, you may have trouble keeping the block secure between your knees,” says Cardiello. “This can lead to arching your lower back or losing the L-sit position.” He recommends focusing on keeping your pelvis tucked under and your abs braced.

Leg movement

If you have trouble maintaining the block between your knees without leg movement, the effectiveness of the exercise isn’t going to be great. Modify the move as you need to ensure your thighs remain parallel to the ground.

Shoulder safety

“Like with all pull-up variations, it’s important to retract your shoulder blades before pulling up,” says Cardiello. This will prevent unnecessary strain on your shoulder joints.

Ring instability

Rings naturally move more than a fixed bar, so you need to focus on controlling your movements. “If you’re not used to rings, start slowly to avoid swaying or using momentum,” says Cardiello. Starting on a fixed bar is a good variation for this move, too.

Fatigue

You’re going to feel this move, and fast. Watch your fatigue levels to ensure you don’t get injured. “An L-sit ring [chin-up is] demanding on both the core and upper body, especially with the added instability,” says Cardiello. “Start with fewer reps and focus on form before increasing volume.”