Build strong core muscles without weights, just using these three crunch variations from a functional training coach.

Reverse crunches work your oblique muscles and the rectus abdominis, the pair of muscles that run vertically down the core and are responsible for giving your core that “six-pack” look.

Banded reverse crunches, or V-crunches ... which is particularly useful if you suffer from very tight hamstrings. Oblique V-ups help target the side body — as the name suggests, primarily ...