How many sit ups to get a six pack?

This is like asking how many pounds do 8-year-olds weigh. If you are looking to slim your waist line, doing sit-ups will not necessarily help. There is no such thing as "spot fat" reduction. If you wanted the back of your upper arm to be thinner and more cut, doing arm extension all day will not help. You must lose total body fat in order to slim your waist line.

In fact, doing crunches, over and over, will cause your upper thighs to become stronger and tighter causing there to be a pull that draws you forward. Because the natural occurrence for your body is to be up-right, so now your back has to work over time to fight against the tightness in your thighs which will lead to low Back pain.

So, the answer is none. If you want to strengthen your abdominal muscles with one exercise and get the most out of it, I recommend doing lying bicycles. See Related Links.