Answer:

The abdominal crunch is a good exercise for targeting the six-pack abs (the rectus abdominis). This is one sheet of muscle with connective tissue that forms the six-pack blocks.

The best crunches for your lower abs are actually a form of leg raises. The first thing you need is a rolled-up towel. This forms a cylinder.

Lie down on the floor and place the rolled-up towel under the small of your back, just above the waistband.

Start with your arms flat on the floor at your sides, your knees bent 90 degrees and your feet on the floor.

To start, splay your knees out to the side at about 45 degrees. This reduces the involvement of the hip flexors, which will tend to overpower the lower abs. Now, raise your legs up, rolling your hips up and over the towel. Keep your back in contact with the towel throughout the exercise.

Go up until your thighs are vertical then lower yourself back down over the towel. Don't let your feet touch the floor between reps.

Try to keep your stomach sucked in while you are doing these for best results.

----------

The best upper ab exercise I've found is the Abdominal Sit-Up. Use the same rolled-up towel as above in the same position. Don't anchor your feet for this one as you will activate the hip flexors.

This time you will raise your upper body, keeping your lower back against the towel. Use a sit-up movement. It won't hurt your back like regular sit-ups, though, as your back is supported. Come up to about 45 degrees then lower slowly.