You can do this exercise either at the gym or at home. If you prefer to do them at home, you will need a chair to perform these exercises. A metal folding chair works best, but whatever you have should work.c 

This exercise works your rectus abdominis muscles, especially the lower part, which are the major muscle group in your core and the muscles responsible for the six-pack look.c

1. Sit down in the edge of the chair, lean back and wrap your arms around the back of the chair.
2. Lift your knees up to your chest, keeping your neck and back aligned.
3. Straighten your legs out in front of you and bring them back in to your chest for one rep.
4. Perform three to four sets of 15-20 reps.

It doesn't matter how many crunches or other abdominal exercises you do, if you have extra belly fat, you'll never be able to see your six-pack abs. The key is to lose weight all over, though cardio and diet, which will eventually reveal the muscle definition you've achieved in the layer beneath your fat.c