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: Big, Thick, Chunky Ab Training

Related: Abs Are Built in the Gym, Not the Kitchen