06/30/2016 04:23 pm ET Updated Jul 01, 2017

The 3 New Rules of Core Training

By: Brian Sabin

An elite performance coach explains how a common posture problem undermines your core workout and makes your lower body muscles tight, too. The good news: A simple breathing change can fix it.

For decades, we all thought that sit-ups and crunches were the way to develop six-pack abs. While exercises that use that bend-forward motion called "trunk flexion" do work the rectus abdominis (the front side of the belly that produces that washboard look), training that muscle through flexion is downright dangerous, according to Stuart McGill, Ph.D., one of the world's foremost experts in spinal biomechanics.

Related: The Intriguing Way Breath Affects Your Workout

McGill's extensive research has led him to conclude that with enough bends, the spinal discs will suffer injury. Not might. Which is why he suggests skipping crunches entirely, and saving those trunk folds for the ones you'll inevitably need in your daily life. Let's call that new core training rule number one.

New Core Training Rule #1: Don't Bend So You Don't Break

Even if we set aside the potential for injury, there's another reason why bends and twists aren't a great way to train the core: Those movements aren't really the core's main function.

"Sure, you can bend and extend and flex and rotate, but at the end of the day the core is probably more of a force transmitter than a force producer," says Mike Robertson, co-owner of Indianapolis Fitness and Sports Training and creator of Robertson Training Systems. "That's a big shift for a lot of people."

Continue reading to learn the two other new core training rules.

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