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.
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Note: While this article is geared toward teenagers, the info that it contains will be helpful for anyone who's new to weight training.