There's nothing like a real pain in the ass: that feeling from a workout that results in a firm derriere. While there are a number of ways to work your backside, or the muscle group known as your glutes, arguably the most popular is the squat. Adding the movement to your routine makes you a better runner, and when done properly, squats can also help you stave off injury by strengthening your knees and hamstrings. That's critical, considering researchers estimate nearly 6.5 million people between the ages of 35 and 84 will be diagnosed with knee osteoarthritis by 2020.

"Squats are an unbelievable functional exercise that strengthens and tones the lower body, like your glutes, quads, and hamstrings, and also works your core," says Keoni Hudoba, trainer at Barry's Bootcamp. "Plus, they're an amazing way to shed fat, helping you torch calories well after you are finished—unlike traditional cardio activities."

That's thanks to the "afterburn effect," or excess post-exercise oxygen consumption (EPOC). Simply put: Since squats can be so intense, your body is forced to consume oxygen even after you're done. So much so, in fact, that bouts of weight training have even been shown to be more effective than cardio in certain instances.

The issue is—and we hate to be the barer of bad news—grunting at high volumes while channeling The Rock doesn't make for flawless squat form. And like other exercises, squats can be more detrimental than beneficial when done improperly. We caught up with top experts to uncover the four biggest mistakes you're making at the squat rack, and how to fix them:

The Mistake: You're rounding your back.

"If you're rounding your back, you may be putting too much stress on your lumbar spine," says Dan Giordano, CSCS and co-owner of Bespoke Treatments Physical Therapy in New York City. "Some may refer to this as a 'butt wink' because as you descend down into a squat your butt will tuck under and your lower back will round." (Read: a little too much crack for comfort.)

The fix: Before putting weight onto the barbell, try practicing your squat with just a kettlebell and a 24-inch plyobox. Hold the kettlebell racked close to your chest, and then squat down maintaining a flat back until your glutes touch the top of the box. Press through your heels to return to start.

The Mistake: You're lifting too heavy.

"You don't have to lift extremely heavy to reap the benefits that squats offer," says Hudoba. "By throwing on too much weight, your form can suffer and ultimately lead to injury. The last thing you want is to be out of the game completely."

The fix: It may feel light, but start with the empty bar. At 45 pounds, this is just enough load to work the basics of the movement. Then, Hudoba suggests you add on in 10- to 25-pound increments. Try starting with 25-pound plates until you get the bar up to 95 pounds, then move forward with 10-pound plates from there.

The issue is grunting at high volumes while channeling The Rock doesn't make for flawless squat form.

The Mistake: Your heels are coming off the floor.

Your heels should be the primary pressure point on the floor during your squat. "It's easy to lower your chest toward the floor, which causes you to roll onto the balls of your feel," says Alex Isaly, head trainer for KettleWorX, a training system available in gyms nationwide and on DVD. "This will put way too much stress on your lower back."

The fix: Emphasize sending your glutes back, like you're sitting into a chair, during the downward movement of the squat while simultaneously keeping your chest lifted. This will automatically cause you to place weight in your heels.

The Mistake: Your knees are collapsing inward.

"If your knees collapse, or turn in toward the midline of your body, it's likely because you have weak hips," says Giordano. "The stronger your hips are, the less pressure there will be on your knees. Strong hips also help stabilize the pelvis, which can take pressure off of your lower back."

The fix: Grab yourself a resistance band. Place it right below your knees with your feet turned slightly outward, and perform a few bodyweight squats while maintaining tension on the band. This will activate the area, and help you gain strength and execute the movement with better form.

From: Esquire US