I have very strong abs (too bad you can't see them)x(
I think the strength came from the Tamilee Web workouts I did on Exercise TV on Demand. I don't know if they are still on there, but there were 3 that I used to do pretty frequently.
She does a lot of moves that works the obliques, which is great and they really hurt! Anway, I did these for a few months. When I started doing cathe dvds, I had no problem with the ab sections. The jack knives are challenging, but definitely doable.
I would recommend substituting some other ab workouts for cathe's for a while, hers are pretty advanced.