Learning how to do a hack squat is essential leg day info.

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Love 'em or hate 'em, squats are one of the most famous leg exercises out there for a reason. But which variations of the foundational booty-building move are worth your time? Well, it depends. Here, the experts break down one popular variation—the hack squat—and whether it deserves a spot in your next leg day workout line-up.

What Are Hack Squats?

The simple answer: Hack squats are essentially just squats that you do on a certain machine.

The hack squat machine looks kind of like a reverse leg press machine, explains Pete McCall, M.S., C.S.C.S., host of the All About Fitness Podcast. To use a leg press machine, you sit on a fixed seat and push your feet against a weighted, moving platform that's above you. To use the hack squat machine, you stand on a fixed platform (facing away from the machine) beneath shoulder pads that bear however much weight you select.

Typically, that platform is angled so that your toes are slightly below your heels, positioning your body as if you are doing traditional squats with weight plates under your heels. Why? "This reduces your need to have lots of ankle mobility in order to correctly perform the movement," adds Jacob Wilson, Ph.D., C.S.C.S.*D., CEO of the Applied Science & Performance Institute and member of The Vitamin Shoppe's Wellness Council.

Hack Squats vs. Traditional Squats

Though the hack squat may seem like it's just a traditional barbell squat without a barbell, there are actually a few key differences between the regular squats and this machine variation.

First, unlike squats performed with free weights—which require lots of core stability on your part—hack squats keep your upper back and hips in a stable position, explains McCall. (Translation: You don't have to worry about balancing a barbell or holding a kettlebell.)

However, this stable positioning means hack squats involve a different movement pattern than traditional squats. "In a standard squat, the hips move back while the knees bend," he says. In hack squats, since your hips are in a fixed position, your knees do most of the work.

The result: Your quads bear much more of the burden in hack squats than in other squat variations, which distribute the work among your quads, hamstrings, glutes, and lower back.

What About Reverse Hack Squats?

If you're curious about hack squats because you've seen them all over your Instagram, you've probably seen people doing reverse hack squats as well. To do reverse hack squats, you stand facing the machine instead of away from it. In this position, your hips aren't locked into a specific motion path, since your shoulders and feet are the only points of contact with the machine.

"This switches the emphasis from your quads to your hamstrings and glutes," says Wilson. Because most hack squat machines have angled platforms, this may be a bit trickier to pull off if you have tight ankles; you'll need decent ankle mobility to squat with your feet already at an angle where your toes are above your heels.

Hack Squats: The Good

If you're new to the weight room, "hack squats are likely safer than barbell squats since you don't have to balance free weights and can't fall backward or forward," says Wilson.

Plus, hack squats are a great move if you want to work on your quads. "They isolate the quad muscles for optimal growth," says McCall. In fact, hack squats are a go-to for bodybuilders and figure athletes who want to build strong, powerful-looking thighs. (Curious? Here's a guide to bodybuilding for beginners.)

Hack Squats: The Bad

Hack squats aren't all rainbows and glorious quads, though: "They're not the best exercise for much beyond aesthetics," says Wilson. "They don't work the hamstrings, glutes, or lower back very well."

Not to mention, all that extra emphasis on your knees isn't necessarily a good thing. "If the hips and knees don't move in-sync, you could risk knee injuries from overuse or over-flexing the joint," explains McCall.

So, Should You Try Hack Squats?

If building solid quads is one of your fitness goals, go on ahead and incorporate hack squats into your regularly-scheduled programming.

Just know that they "don't necessarily improve the function of your hips and knees working together," says McCall. "Fixed-path machines aren't necessarily bad, but sometimes they restrict normal or optimal joint function—and this is one of those cases." (Experts generally agree that most workout machines can be risky or ineffective for the same reason.)

Translation: Your body reaps a more well-rounded benefit from other, unrestricted types of squats, like bodyweight squats, goblet squats, or barbell squats (whether with the barbell positioned across the front of your shoulders or your upper back). (P.S. Check your squat form with a little thing called 'squat therapy.')

Want to give your quads a little extra oomph? Try doing your usual squats (whether that's with a barbell or dumbbells or just your bodyweight) with 10-pound weight plates beneath your heels to mimic the hack squat position.

If you do want to get the most bang for your buck on the hack squat machine, though, incorporate both typical hack squats and reverse hack squats into your routine so you give your hamstrings and glutes a little love too.