Here's something you probably never thought you'd hear out of the mouth of a fitness pro: Stop doing sit-ups. But according to Jay Cardiello, celebrity strength and conditioning coach, not only do sit-ups and crunches isolate very few muscles in your abdomen, they also aggravate an existing problem for most Americans: the anterior tilt.

"Most people sit at a desk all day, staring at their computers or talking on the phone, which creates a hunched-over position or an anterior tilt in your body," Cardiello says. "So you're already crunching forward at work all day – why would you want to do that at the gym?"

To train your abs, do planks instead, says Cardiello. Planks and side planks work your abs, in addition to your posterior side – the underdeveloped, underutilized backside of your body that includes your back, butt, and hamstrings. "A full-body movement like a plank or side plank works more joints and activates more muscles than doing crunches, and they train your posterior and anterior at the same time," Cardiello concludes.

When doing planks and side planks, remember to keep your spine as straight as possible, without hunching your upper back, raising your hips, or letting your hips or knees drop. Hold for 15 to 30 seconds, or until you lose form, and repeat if you don't feel any strain or pain. When regular planks become easy, try doing them on an unstable surface, like a Swiss ball or even a bed, to create a sense of imbalance and force your body to recruit even more muscles.

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