The reverse crunch is a good exercise for your abdominals, and is a good alternative for the standard sit-ups and crunches.

Starting Position: Start by lying flat on the floor. You may want to lie on a padded mat to provide additional comfort. Place hands on floor under your hips/butt and bring knees up toward chest until feet are flat on floor.

Movement: While keeping knees in bent position, contract abdominals and bring knees toward chest. Continue contracting abdominals and bring knees as close as comfortably possible to your chest. Your hips may rise off your hands. Pause at the end of the contraction, and slowly lower yourself back down to the starting position. Be sure to exhale as you bring your knees to your chest, and inhale as you lower yourself back down.