Unlock Cristiano Ronaldo’s secret abs routine with this high-intensity, no-equipment workout inspired by the football ...

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.

Case in point: You rely on your core muscles to carry you through the miles. And research shows that core strength training can improve the force you put into your steps, your overall energy transfer, ...