Your abdomen actually has 5 major muscle groups.
The "6 pac" muscles that everyone likes to see are all one muscle. The rectus abdominis. (I think that's spelled right) But that does not make up the entire musculature of your abs.
This is not true. Even if the abs were all one muscle group (which they are not) you would still need to vary the exercises due to the fact that your body will adapt to the motor pattern of only one exercise, causing stagnation.so it doesn't matter what exercises you do, they'll all work the same thing?