What is the hardest crunch?

Like situps, crunches help you build muscle. But unlike situps, they work only the abdominal muscles . This intense muscle isolation makes them a popular exercise for people trying to get six-pack abs. This also makes them ideal for strengthening your core, which includes your lower back muscles and obliques.

The 7 Hardest Ab Exercises

While crunches exclusively workout your abdominal muscles, a sit up is a slightly more intense exercise that strengthens your hip flexors, abdominal muscles, chest, neck, lower-back, as well as the muscles in your lower-legs.