This is an exercise that works the abdominal muscles. Perform it by lying on your back with your legs together. Your legs should be bent so that the bottoms your feet can be flat on the floor. Interlock your hands behind your head, or cross your arms over your chest. Contract your abdominal muscles to pull your torso up to your knees. Then slowly lower yourself back down. You may want to put your feet under some dumbells or have somebody hold them for you.

Variations
The sit-up is a very versitile exercise that can help you a lot by using variations on it.

Weighted Sit-up
Perform the same motion as above, except hold a weight plate across your chest.

Swiss Ball Sit-up
Lie with your back on a swiss ball, bend at your knees so that your feet can reach the floor and rest flat and spread wide apart. Now perform the same motion stated above, but come up only to a 90 degree level--not until you reach your knees.

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