The abs are composed of mainly fast-twitch (FT) muscle fibers. That means they're best suited for lower reps using heavier weight rather than tons of reps with just bodyweight. It's easy enough to add weight to "upper ab" exercises like crunches, but what "lower ab" movements? Try this:

The Low-Pulley Knee-In

Knee-Ins 1
Knee-Ins 2

In most gyms you'll see a medicine ball or dumbbell held between the feet for resistance during knee-ins, but since the weight is acting downward, this tends to over-activate the hip flexor muscles to keep the weight up off the ground. A much better option is to use horizontal loading using a cable, which provides resistance in the direction of the movement.

  1. Hook your feet into two handles or secure both feet onto a weight belt attached to a low pulley.
  2. Place your arms at your sides with palms down. Keep your chin on your chest. This will help flatten your lower back against the ground to encourage greater lower ab activity over the strong pull of the iliopsoas muscle.
  3. Pull the knees toward the chest, then extend your legs back to the original position.

Related:  The 4 Dumbest Ab Exercises

Related:  Big, Thick, Chunky Ab Training