If you do crunches with your feet braced down by a heavy object or under someone's hands, you're strengthening your front stomach muscles to an extent, but you're mostly working your hip flexors (the muscles located on your upper thighs, just below the hipbone), says Thornton. When doing crunches, feet should be in the air (pictured), on the ground, or resting on a bench or couch, and you should lift your torso until your shoulder blades just leave the ground, he says. "A full range of motion is not necessary."
Plus, when training your core muscles, crunches are only part of the equation. You'll have a stronger core and better balance if you do exercises that rotate your torso
, thereby strengthening important muscles in the front and side of the stomach and all along your spine, says Ratliff.
Exercise: Either seated on a bench (or armless chair) or standing with your feet shoulder-width apart, hold a ball or light weight in your hands and twist your torso all the way from one side to the other, moving your arms with it.