You may like the look of the glorious "6-pack" of the abdominal wall but if you're serious about being a competitive athlete or pain-free you'll focus more on the obliques and transverse abdominals. These muscles together are responsible for maintaining inter-abdominal pressure once the diaphragm does its job. More importantly, the muscles are not fine-tuned with crunches or twisting exercises. Yes, They are strengthened this way but strength does not make for stability, directly. Stability requires proper posture followed by dynamic movements while maintaining this posture. In order to achieve this posture these muscles are engaged at a high level which translates into functional stability with time and repetition. So for those of you constantly in the training room rehabbing low back, shoulder, elbow or knee pain you might consider having a look at your abdominal stability!
The Obliques..... The oblique abdominals (internal and external) are mentioned all the time from your personal trainer types. My questions to them: "do they know what they actually do?" Most folks would simply say they perform rotary movements, twisting of the trunk. And they would be right, partially. What they're leaving out or perhaps unaware of, is the Obliques do more than just twisting. If you look at the bigger picture you'll see the obliques perform rib stability during respiration. With proper activation of the Internal and external obliques the lower rib cage becomes more rigid, providing a solid anchor from which the diaphragm can effectively anchor when contracting and descending into the abdomen. Without this activation the diaphragm cannot properly descend equally which results in decreased Intra-Abdominal Pressure (IAP) and decreased lumbar stability
A good indicator of insufficient oblique activation is a flaring of the ribs (laterally) when ab bracing, think bracing before getting punched in the stomach. This happens because the rectus abdominus is the primary source of the brace. This is also a good indicator of bad posture, improper neurological stabilization patterns. Overactivity of the Rectus can contribute to an increased kyphotic curve of the Thoracic spine, much like you see in senior citizens that can no longer stand up straight to hold their head high. The "six-pack" look is not stable and can be causing bad posture! If you want to know what a good, stable core looks like, have a look at Roger Federer. There's a reason he's he's still on top at 35+ years of age! So if you have back pain it's not due to a weak core but a core with improper stabilization from a neurological dysfunction.