Psst: it's not another plank.

Core training has always been part of military PT and fitness testing. From crunches to sit-ups, flutter kicks, leg tucks and plank poses, the U.S. military uses many exercises to test core strength.

Don't be misled by the idea that a flatter stomach after 60 requires you to lie on the floor cranking out hundreds of sit-ups. The most effective core-strengthening movements can happen right from ...