This post has been updated. It was originally published on January 4, 2019.

Whether or not you’ve resolved to get into shape this January, Muscle Month is here to teach you a thing or two about stretching, contracting, lifting, tearing, gaining, and so much more.

Unbeknownst to the bros who skip leg day, squats are an incredible exercise for many reasons. They help build your major leg muscles plus all the tiny stabilizing muscles that you need to do basic daily tasks like getting up from a chair. You may not yet be of an age where standing up is challenging, but when you are you will be better off if you built up those muscles earlier in your life. Plus, squats sculpt your butt—and who doesn’t want a nice-looking butt?

All of this is true if you’re doing a real squat. You may have heard gym-goers talk about getting “below parallel”—that means that when you squat, your butt drops below your knees. They’re not just being pedantic. Getting down to that full depth squat is part of what makes it such an effective, useful exercise. “With a squat, you’re basically trying to extend your hips and your knees,” explains Greg Nuckols, who holds both degrees in exercise science and three all-time world records in powerlifting (his site, Stronger by Science, is a nerdy weightlifter’s goldmine). To accomplish that you need to engage your quads, your glutes, and your adductor magnus. (Don’t worry, we’ll explain exactly what those actually are.)

To be more precise about it, you have four quadricep muscles (that’s why they’re called quads), and squats primarily work three of them: your vastus lateralis, your vastus medialis, and your vastus intermedius. The fourth, the rectus femoris, doesn’t do as much work. Those three muscles all cross your knee joint, so they help with the knee extension portion of the move.

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To flex and extend your hips, you need the help of your glutes and your adductor magnus, which roughly translates to your butt and your inner thighs. Lots of people think squats also work your hamstrings (that’s the muscle group on the back of your thighs), but Nuckols says they don’t do much. “When your hamstrings contract they do help extend your hips, but they also kind of fight against what your quads are doing by imposing a knee flexion moment,” he says. Your body is trying to find the most efficient pattern of muscle activation to accomplish any given motion, and your hammies aren’t part of that equation in the squat. Other hip flexor muscles will help you far more.

So all of that is how you manage to do the lower body portion of a squat—above parallel, it’s a lot of quad work, and below parallel, you engage your glutes.

But you’ve got one more task to accomplish, especially if you’ve got a barbell on your shoulders: keeping your torso upright. A rounded back is a recipe for an injury, so maintaining rigidity through your trunk is crucial. Your chest should be upright and your shoulders should be back, which involves engages all those back muscles and abs to maintain a straight spine. “Squats generally work all of the muscles in your torso,” Nuckols says, “so that includes your spinal erectors, your abs, your obliques, and probably even your lats to some degree.”

The end result is a single lift that works most of your body in one fell swoop. If you’re not able to maintain perfect posture or get below parallel, ease up on the weight until your form is flawless, and then build the resistance back up slowly. An unweighted squat will still engage all those muscles if it’s done properly, but a sloppy squat under a loaded barbell could leave some of them out—which is inefficient at best, and potentially dangerous at worst.