If you’ve ever stepped into a commercial gym or attempted to “get in shape” in the discomfort of your own home, then you’ve almost certainly done a crunch. It’s a movement that’s as ingrained in our fitness culture as bench presses and biceps curls.

But what if you learned that crunches are far from the most effective and efficient way to work your abdominal muscles?

This isn’t a revolutionary concept among fitness professionals. It’s been out there for at least a decade. But you’d never know it by watching what people in health clubs do. Sit-ups may be out of fashion, but the basic crunch is alive and well and performed by almost everyone trying to improve his or her appearance.

“People think the crunch is the equivalent of a biceps curl,” says Lou Schuler, co-author of the book "The New Rules of Lifting For Abs." “You pick up a dumbbell, you bend your elbow, and you feel the biceps working. You know exactly what you’re doing, and why. So when you do a crunch, you feel the abdominal muscles shortening, and you think you’re doing the exact same thing. You’re making the muscles bigger and stronger.”

Your abdominal muscles are unlike your biceps and triceps in both structure and function. Their main job is to protect your spine by helping the other core muscles –- those in your back and hips –- keep your lower back and pelvis in a safe, neutral position.

That’s what we mean when we talk about “core stability.” It’s not what your muscles look like when you flex them in a mirror. What matters is how well they can keep your spine in a stable position during increasingly difficult movements.

There isn’t a single crunch or sit-up in The New Rules of Lifting for Abs, the third book in The New Rules of Lifting Series. Instead, Schuler and Alwyn Cosgrove base their workout around planks and side planks. (A basic plank is just holding a pushup position on your forearms. For a side plank, just rotate 90 degrees onto one arm and the sides of your feet.)

Schuler and Cosgrove argue that the planks and side planks -- and the many variations they show and describe in their book -- are the best entry-level exercises for your core muscles.

If you’ve never tried them before, you’ll be surprised at how hard they can be.

“It’s not hard to get into a plank position and hold it for a few seconds,” Schuler said. “But when you get to 30, 60, or even 90 seconds, you realize just how little strength and endurance you have in those muscles, no matter how many crunches you’ve done in the past.”

You may ask what the point is. It’s hard, but so what?

Ever heard the phrase, “Lift with your legs, not with your back”? That’s a perfect, if simple, way to reinforce the importance of a neutral spine, or keeping your back flat.

If you can’t -- if your core muscles can’t keep your spine in its natural, slightly arched position when you’re lifting weights or playing sports -- you risk serious injury to the discs in your lower back. The better you are at keeping your back in a neutral position, the lower your risk of injury.

Believe it or not, the humble ab wheel offers one of the best examples of how your abdominal muscles function. If you’ve ever used one, you know how hard it is at first to roll the wheel out and extend your arms away from your body. And you also know how sore your abs will be 36 hours later.

The wheel changes your center of gravity. The farther it goes, the harder your core muscles have to work to keep your back from buckling. It’s the hardest thing you can ask your abdominal muscles to do.

“If that’s the hardest thing for your abs to do, then it’s probably the most important thing they’re designed to do,” Schuler says. So rather than flexing those muscles in your workouts, what you really want to do is force them to extend - to lengthen, rather than shorten -– while keeping your back in a safe position.

But simply keeping your back in a safe position through basic stability movements isn’t enough. You have to be able to carry over that stability into basic movements both in everyday life and in the gym. That’s where the “lift with your legs” principle comes back into play. What’s the point of busting your butt if you’re not going to see any applicable carry over?

“Once you've developed those abilities, you can spend less time in the weight room and you can get more done with less risk of injury,” Schuler says. “So you can make your workouts more efficient and productive, which makes them harder. Which really gets back to that there’s no easy way to do this.”

You hear that, The Perfect Sit-up?