The abs are composed of primarily type-II or fast-twitch (FT) muscle fibers. That means they're best suited for heavier weight and lower-rep sets rather than excessive numbers with bodyweight alone. The Sicilian crunch is a great exercise to hit them.
This exercise really taps into those FT fibers under load. During this movement, the lower body is secured as the upper body moves.
related posts
Fix it or wreck your shoulders.
Ramp up your conditioning with only one kettlebell and your bodyweight. Here's the workout.
Varying your lifting speed is a great way to promote growth. Here's how to choose the right tempo to reach your goals.
Chris Shugart recently wrote an article about balance. Basically he said, "Balance good, karate good, everything good. Balance bad? Better pack up, go home."