Are planks and sit-ups sufficient for getting a Six-pack?!

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You probably would want to vary your abdominal exercises a little bit. Sit ups aren't that great for your back, but the plank I have found to be quite useful. Check out this website for some more ideas on the different kinds of exercises that you can do.
http://easiestwaytogetasixpack.org/

Both are good but don't forget the lower Abs and the Obliques. You can mix them up and do combinations of all of them. There are also variations including weights, exercise balls, and other exercises. To get the maximum benefits follow good techniques and work the whole body.

Fitness expert Tom Venuto says he only spends about 15 minutes a day working on abs, and that fat loss/cutting is the key to great abs. I have included his article in the source section. It's very useful.

http://www.muscleandstrength.com/article…

Crunches with a weight behind the head. Do drop sets then bicycles.