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.

Fitness fads may come and go, but there's one thing that's always in style: a strong core. Whether you're a Hyrox addict, a Pilates princess or a running regular, maintaining a stable, functional core ...