What is the best exercise to tone the abdominal muscles?

The abdominal muscles (abs) are just like other muscles in your body. If they are covered by a layer of fatty tissue than no amount of crunching is going to give you a six-pack. In order to get toned abs you have to exercise with your entire body (walking/swimming etc.) and lay off the second helping at dinner.
It is however extremely important to strengthen the abs even if you aren't planning on getting rid of the fatty tissue. Having strong abdominal muscles helps us in every other exercise we do from bending over to lifting to getting out of bed. Strong abs will also help prevent back pain. So here are my favorites in the abdominal/core department.
The Dolphin Plank: http://www.yogajournal.com/poses/2463
The Plank: http://www.yogajournal.com/poses/470
The Side Plank: http://www.yogajournal.com/poses/783
The Basic Crunch: http://exercise.about.com/cs/abs/ht/Crunch.htm
Obviously if there are any others that you love, please tuck in. Remeber to be careful if you are new to ab work and if anything feels funny in your lower back, please stop and consult your doctor. :-)

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