If someone tells you to drop down and knock out some ab work, nine times out of ten a person is going to lie down on the floor, bend their knees with their feet flat, put their hands behind their head, and start crunching. It’s the quintessential, most basic ab-busting move that exists, and the hands-behind-the-head aspect has been given as a modification for ages—but, when done incorrectly, this position can wreck your whole ab workout.

“Trainers—myself included—often cue supine spinal flexion because it gives such a satisfying burn and you can really feel like you’re doing something—and I find that it’s actually helpful with posture correction if someone’s locked in spinal extension,” says Helen Phelan, trainer and creator of Pilates Rebel.  But there are a number of ways that the most fundamental ab move can be sabotaged—which can result in all kinds of unwanted pain and injuries.

“First, you aren’t supposed to pull your head up,” says Phelan. “The classical Pilates terminology for a crunch is ‘chest lift,’ which I think is a really helpful visual.” She notes that if you’re using your arm strength to yank your head off of the ground, you probably aren’t actually working to activate your rectus abdominis—AKA your six-pack muscles—so likely aren’t doing a whole lot to make them stronger. If you still need some assistance to get your head off of the ground, she recommends reaching your hands out in front of you to get the support minus the temptation to cheat, or to instead let the head feel heavy in your hands, “almost pressing the back of your head into your hands, and letting your hands hold the weight of your head to make sure the curl happens from the abs.”

With crunches, the idea is to have your shoulder blades lifting off of the floor at the top of the motion—not just your head and neck. “Just nodding your chin back and forth is going to strain your neck, and potentially reinforce forward head posture,” says Phelan. “I like to cue bringing the bottom rib to the top hip to help people visualize that curling in motion.”

Crunching with your hands behind your head can also lead to the use of momentum in your ab work—which can actually lead to spine issues. “Any sort of momentum or rushing puts you at risk of strain through that cervical spine area, and probably means you’re cheating your ab work anyway,” she says.  “I like to fully set the head on the floor between reps so you’re not working in tension the whole time, rather than only coming down to a hover above the floor.” Good to know so that nothing stands between you and your strong AF core.

While working on that core, try this 6-minute at home abs workout from our Trainer of the Month Club. But don’t overdo it—here’s how to tell if you’re overworking your core.