When you have shredded external obliques that tie into slashing serratus muscles at the sides of your rib cage, your midsection takes on an entirely new dimension. Unfortunately, most bodybuilders become obsessed with their rectus abdominis, the six-pack on the front of the midsection, and they ignore their external obliques. By neglecting this important muscle group, they miss out on the extra, eye-catching impact of a perfect midsection should have.
The external obliques actually take up more midsection area than the rectus abdominis. They also do a lot of the work that is attributed to the rectus muscle. For example, the primary function of the obliques is to rotate the torso and flex the spine. When you think spinal flexion, you usually think rectus abdominis, and you're right. The external obliques, however, are the prime movers when it comes to pulling your torso from a back-arched position, as in the starting point of an Ab Bench crunch pull or a cable crunch, to a straight position. The rectus helps the obliques during this first part of the range of motion, but it doesn't really take over and dominate until the spine is straight.
Why hasn't anyone made this point before? One reason is that until the Ab Bench came on the scene, no one was concerned with the abs' full range of motion. Everyone was busy doing crunches on the floor, which neglect the back half of the range of motion.
With the Ab Bench crunch pull, you not only train the rectus abdominis through its full range of motion, but you also get at the external obliques. For a more direct hit on your external obliques, try ab twists on the Ab Bench, or simulate this movement with an overhead cable. Start in the fully stretched position, as if you were doing an Ab Bench crunch pull, but instead of pulling straight down, you pull forward and diagonally with your right shoulder until it's directly over your groin. Don't pull with your elbow; focus on trying to crunch the right side of your rib cage down to your navel. Release back to the full stretch and repeat with your left side.
The reason ab twists are such a good external-oblique exercise is that they focus on the specific function of this muscle group, which, as mentioned above, is to flex the spine and rotate the torso. If you notice how the obliques run around the front of the midsection and are interwoven with the serratus, it's obvious that when they pull in a straight line, they rotate the torso.
Most people realize the obliques' function, but they spend too much time spinning and twisting with a stick behind their necks, trying to get an abdominal burn. Instead, they should concentrate on pulling with the external obliques in a straight line using short pulls, with resistance from full extension to full contraction. Remember, contracting the external obliques flexes the spine and rotates the torso, and that's exactly the movement you get with ab twists on the Ab Bench. If you don't have an Ab Bench, you can work these muscles with twisting cable crunches; however, be sure you have lower-back support so you can work through the full range of motion.
One last point is that you shouldn't use the burn as an indicator of your routine's effectiveness. The abdominal muscles burn when they're forced to stabilize the body due to the bouncing that accompanies cheating motions. Use your brain to decide if you're doing the exercise correctly, not the burn. Visualize what the target muscle does, use concentrated, full-range motion to train that specific function, and you'll get faster, more complete development.