1. Jack Knife Crunches
Jack knife crunch is an advance abdominal exercise that engages both the upper and lower abs to build stomach muscles.
How to do it: Lie on your back with your arms extended above your head. Then, lift your hands toward your legs, and your legs toward your chest. (You can either leave your legs extended while doing this, or bend them.) Hold for a few seconds. Keep your back straight as you lower the upper part of your body towards the ground. Repeat for 10 to 15 times.