You can do this exercise either at the gym or at home. If you prefer to do them at home, you may be more comfortable performing this exercise on a yoga mat, towel or any other soft surface you can put between you and the floor.c 

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

Start this exercise by lying flat on you back with your legs outstretch along the floor and both hands under your hips/butt for support. From this position, try one of these variations:

• Raise your legs so that they are perpendicular to the ground. Lower them towards the ground without actually touching. Raise them back to the starting position.
• If you have lower back problems, instead of having your legs straight in the air, bend them at your knees and then extend so they are parallel to the floor. Bend back to the starting position.
• To make the move harder, start with you legs at a 45° angle to the floor and lower them so they hover above the ground. Keep raising and lowering them at that half-way point.
• Bring your knees above your hips so your shins are parallel to the ground. Crunch up and tap your heels. Lower back to start.
• Instead of having your hands under your hips, place them out to the side so they are in line with your chest. Bring your knees to your chest as you crunch upward. Tap your heels, then stretch your legs so they hover above the ground to complete one rep.

For each of these exercises, try to complete three sets of 15-20 reps.