Sorry, leg day loathers, squats aren’t going anywhere. This classic move is just that important. “Whether you want to shore up your strength, build muscle, or lose fat, squats are one of the foundational movements that you have to be doing,” says Sean De Wispelaere, master trainer at MBSC Thrive and owner of Sean D. Thrive.

Since they work some of the biggest muscles in your body—your glutes, quads, hamstrings, your abs, and back—squats provide some of the biggest return on your reps. Since big muscles burn more calories in order to power through and recover from exercise than small muscles, squatting is crucial whether you’re looking to build strength or slim down. “And you don’t have to load up barbells to make squatting worth your while,” says De Wispelaere, “cranking out a few sets of bodyweight squats will have your muscles burning in no time.”

Whether you’re squatting heavy loads or sticking to bodyweight squats, proper form is key. “Allowing your knees to cave in or your back to round, for instance, can potentially lead to injury,” says De Wispelaere. “And if you load weight on top of improper movement pattern, you’re likely in for a world of hurt at some point.” That’s why De Wispelaere recommends establishing a solid foundation with the bodyweight squat before progressing to weighted versions.

And even though the squat isn’t a terribly difficult-looking move, it can be tricky to master perfect form. Look out for these common mistakes—and take De Wispelaere’s advice for a more effective burn the next time you drop it low.

Mistake #1: You’re Rounding Your Back

What it means: Your core is weak. (Your “core” is more than just your abs. It’s all the muscles on your front and back between your hips and shoulders.)

While your legs are working to bend and extend through the reps of your squats, you want your core in what’s called an isometric hold, a.k.a. braced as it would be when you do a plank. If your back rounds while you squat, it indicates that your core isn’t strong enough to maintain the tension needed to keep you upright throughout the whole movement, says De Wispelaere.

Why it’s a problem: Repeating a movement over and over with your spine in flexion (meaning your spine aggressively rounds forward, causing your vertebrae to run into each other) can do damage to your back, even if you’re squatting without any weight.

The quick fix: Before lowering yourself down into the squat position, tense your core as though someone is about to punch you in the gut. Keep your abs clenched and pull your chest up in order to keep your back as straight as possible. “No matter how strong you are, only squat as far as you can without your back rounding,” De Wispelaere warns.

The long-term solution: To build lasting strength, performing other isometric holds—like planks or hollow holds—can help build your core strength. You can also practice tightening up your torso during the movement by practicing wall slides, De Wispelaere recommends.

To perform wall slides, face a wall and stand at arm’s-length away from it with your feet slightly wider than hip-distance apart. Raise your arms over your head until you form a “Y” shape. Keeping your chest up and your arms high, squat down until your thighs are parallel with the floor. If your hands do not touch the wall at any point, inch yourself a little closer and perform another rep. Repeat until you can’t perform a squat without your hands brushing into the wall. This is a great load-free way to make sure you’re practicing good form.

Related: How To Lift Heavy For Maximum Strength Gains

Mistake #2: You’re Lifting Your Heels

What it means: Your ankles are tight.

This sounds silly, right? Unfortunately, tight ankles are more common than you think, because pretty much everything you do contributes to the struggle. Walking, playing a pickup game, and even sitting on the couch with your feet relaxed can shorten and tighten the tendon (your Achilles) that connects your heel to your calf. Since your ankles are so close to your feet—which are your physical foundation—having tight tendons can affect everything up the chain from there, even leading to knee and hip pain, warns De Wispelaere.

Why it’s a problem: While squatting, you want all parts of your feet—heels, ball, and toes—firmly planted on the floor for maximum stability. This stability will not only keep you from tipping forward or back, but will give more power to your push as you stand, helping you to lift more weight or perform more reps.

“If your heels come off the ground, your base of support is less stable, which means you won’t be able to perform the full range of motion of your squat,” according to De Wispelaere. This undercuts the benefits of the move and sabotages your ability to progress. Think about it this way: You can’t build a solid house on a shaky foundation. You can’t build well-balanced strength and muscle with and unstable base either.

The quick fix: Put a weight plate under each of your heels so they’re elevated while you perform your reps. While this doesn’t fix the problem of inflexible ankles, it does allow you to fully access the squat without lining yourself up for injury, according to De Wispelaere. If you feel unstable with your heels elevated, hold a light five to 10-pound dumbbell in front of your chest for counterbalance.

The long-term solution: To gain more range of motion in your ankles, De Wispelaere recommends performing a super-simple ankle mobility drill.

Related: I Stretched For 30 Days With The Goal Of Touching My Toes—See How It Went

Here’s how it works: Face a wall and plant your palms on it about shoulder-width apart. Assume a split stance, like you would if you were about to lower into a lunge, with your front foot about six inches away from the wall. Keep your feet hip-width apart. Without taking your feet off the ground, try to touch your front knee to the wall by driving it forward until your heel lifts off of the floor. Hold your position wherever your heel pops up for five deep breaths. Repeat five times on each side.

Mistake #3: Your Knees Are Caving

What it means: Your hips are weak.

Shakira might be the only person whose hips don’t lie. For the rest of us hip weakness is really common, according to De Wispelaere. From prolonged periods of sitting at your desk, driving in your car, and relaxing on the couch, your hip muscles shorten and weaken because, in those seated positions, they hardly have to work.

Most people fall into one of two categories: Those who found out they had weak hips and fixed them, and those who don’t know they have weak hips, he says.

Why it’s a problem: When your knees cave in towards each other instead of staying out over your toes while you squat, it can make you more susceptible to knee injuries over time, says De Wispelaere. Why? Your knees are meant to travel front to back (unlike a joint like your wrist, which can rotate around in circles), so any time you force them to go diagonally or sideways, you’re making demands that your tendons and ligaments just can’t keep up with.

The quick fix: Focus on pushing your knees away from each other and lowering yourself down between them as you squat. If you can’t do this on your own, De Wispelaere recommends squatting with a light-resistance miniband and looping it around your legs just below your knees. “The feedback from the miniband will naturally force you to push against it, driving your knees out as you squat,” he says. And, bonus perk: Squatting with the miniband will also help you build strength in your outer hips.

The long-term solution: De Wispelaere recommends doing banded walks to strengthen your hips for the future. To perform a band walk, loop a miniband around your legs just below the knee and stand with your feet a little further than hip-width apart. Keeping your legs the same distance away from each other the whole time, walk yourself in a box pattern using short, choppy steps. Perform 10 steps to the left, then 10 to the front, 10 to the right, and 10 to the back to return to your starting position. Repeat three times.