Perhaps no bodypart inspires as much interest, consumer marketing and pure myth regarding training and results as the abs, which is actually a single muscle called the rectus abdominis. It originates, or attaches, on the bottom of the ribs and on a bony prominence called the xyphoid process that's on the bottom of the breastbone and inserts, or attaches, onto the front of the pelvis. Its function is to flex the spine, or bend it forward, as you do when you round your back. The rectus also compresses the rib cage, as when you force air out of your lungs. The rectus abdominis does not cross the hip joint. That's an important point.

There are two sets of muscles on the sides of the waist, the internal and external oblique muscles. The obliques originate on the outer lower ribs and insert at the top of the pelvis on the lower side of the waist. Their function is to flex the spine if both sides contract equally and simultaneously. If the external oblique on one side and the internal oblique on the other side contract, the result will be rotation, or a slight twisting, of the spine. The oblique muscles also don't cross the hip joint.

The transverse abdominis muscle has been receiving a great deal of attention the past few years. A very thin muscle that wraps around the front and sides of the abdomen, it compresses, or tightens, the abdominal wall. Some research supports the idea that if the transverse abdominis fires, or contracts, there's greater abdominal activity and core stability and less back pain. It's difficult to believe that a very thin muscle can do all of that; however, the activity of the transverse abdominis may simply be an indicator that the other abdominal and postural muscles are working well'sort of a cascade of events. The transverse abdominis doesn't cross the hip joint either.

Gym myth has it that you can train the upper and lower abdominal muscles separately, but biomechanical research contradicts that idea. If two people play tug-of-war, the force won't be felt only on half of the rope. The entire rectus abdominis is under tension during a crunch. The so-called lower abs'meaning the lower portion of the rectus'can't be under tension and not respond. Under normal conditions muscle can't remain relaxed or inactive when you place a load on it.

The problem with doing leg raises for the abdominals has to do with the fact that the abdominal muscles don't cross the hip. The abdominal muscles cross over the spine, producing spinal flexion. The leg raise involves an action called hip flexion. The prime hip flexors are the psoas major, which attaches to the lumbar vertebrae (lower back) and the disks in between, and iliacus muscles, which originate on the floor of the pelvis. The flexors share one attachment to a bony point on the upper thighbone, or femur. The action of these muscles'known together as the iliopsoas'varies with position.

If you're standing and raise one knee toward your chest, that's hip flexion. If you're lying on your back and raise both legs up, that's also hip flexion. The abdominal muscles don't raise the legs; however, they do perform an isometric contraction to hold the pelvis steady so the hip flexors can work, which is why you can feel a burn in your abs. The muscles that are lengthening and contracting, however, are the hip flexors.

If the leg can't move, the other end of the muscle pulls on the attachment. That means the lower back is pulled forward, creating more of an arch. If you're standing, the back looks more arched (think of the lower back of a gymnast or ballet dancer). When you lie on your stomach, your thighs can't move, so the tight hip flexors pull on the lower back, increasing the curve. That increases compression force on the small joints of the spine, called facet joints. It can also irritate a nerve if a disk has protruded directly into the space where the nerve is. In addition, the iliopsoas is deep in the abdomen and can't be seen, which means that working it won't enhance your physique. So while the crunch is an effective exercise, the leg raise is not so effective and can actually cause or contribute to lower-back pain.

Let's get back to the upper-ab, lower-ab argument. A recent issue of the Journal of Strength and Conditioning had several very interesting articles about abdominal training. The most compelling of the lot presented research on the true function of the rectus abdominis.1 It compared six exercises: crunches, crunches across a Swiss ball (Sissel ball), Ab Trainer crunches, leg lowerings, facedown rollouts with a Swiss ball and reverse crunches. There was no statistical difference in upper and lower abdominal activity, though some exercises were better than others. The crunch on the Swiss ball was most effective, followed by the crunch, Ab Trainer crunch, leg lowering, Swiss ball rollout and reverse crunch.

The review of other studies in this article cited some research indicating that upper and lower rectus abdominis activity could differ, and other research showing no difference between the two during positive, or concentric, activity. Bottom line: More research is needed to reach a sound conclusion.

1 Clark, K.M.; Holt, L.E.; and Sinyard, J. (2003). Electromyographic comparison of the upper and lower rectus abdominis during abdominal exercises. J Strength Cond. 17:475-483.

Editor's note: Visit www.softtissuecenter.com for reprints of Horrigan's past Sportsmedicine columns that have appeared in IRONMAN. You can order the book Strength, Conditioning and Injury Prevention for Hockey by Joseph Horrigan, D.C., and E.J. 'Doc' Kreis, D.A., from Home Gym Warehouse, 1-800-447-0008 or at www.home-gym.com.