We have four main abdominal muscles and each can be trained by diffrent exercises.  Doing crunches all day long will not hit them all.  So, I will give you a brief description of each muscle, what it does, and one exercise that will work that particular muscle. 
 
Rectus Abdominus-  This muscle runs vertically and flexes the spine in crunching motions such as getting out of bed.  The typical crunch is good for this muscle.  Try them on a stability ball to eliminate "cheating". 

Internal and External Obliques-  These muscles run diagonally along the side of your torso and help in flexion of the spine, extension of the spine, and spinal rotation.  The best exercises for these muscles are rotational movements.  These can be hard to isolate and care must be given to not tweak the back on twisting motions.  My favorite exercise is actually an isometric standing exercise using exercise tubing.  Place the band, using a door attachment , at shoulder height.  Standing next to the tube pull the band straight out in front of your chest with arms extended and band  to floor.  Hold for 20 to 30 seconds.  You should feel the tension on the side of your torso opposite the band. 

Tranverse Abdominus-  This is your "girdle" .   This muscle runs horizontally and wraps around your body.  It acts to stabilize the spine and protect your low back.  This is a deep muscle and is best activated by "drawing-in" and breathing deeply.  To find this muscles lye on your back and force out a fake cough.  You will feel these muscles contract.  Then put your hands on your hips while in this position and breath deeply watching your belly rise and fall and pulling in this muscle as if zipping up a pair of tight pants. 


Hope this quick anatomy lesson sheds some light on this complicated and hard to train region of your body.  For even more great exercises give me a call or reply to this post with your own thoughts and ideas.  
 

Amy