Here's the heads up on the abdominal muscles: what they are, where they are, how to train them, which exercises work best.
The Abdominals Muscles: Three Main Groups
The 'abs' are the groups of muscles in the abdomen or the stomach area -- generally below the rib cage toward the pelvis and at the sides. The three main groups and their function are:
- Rectus abdominis (RA)
The RA flexes the lumbar spine and tilts the pelvis and helps maintain curvature of the lower spine. It is commonly known as the six-pack because with strong muscle development and low body fat these muscles stand out. In some well developed bodies eight distinct segments are visible.
- Transversus abdominis (TA)
This is a deep layer of muscle supporting the internal structure and organs of the abdomen. It helps flatten the stomach, is used in forced breathing out, and in urination, defecation and childbirth. The TA also assists in supporting the spine in some exercises.
- The internal (IO) and external obliques (EO)
The internal and external obliques run down the side of the abdomen and help you bend and rotate at the sides and assist in trunk flexion. The external oblique muscles can be made to show through to some extent with exercise development and a low body fat percentage. Consider three layers of muscle from the external obliques on the outside and to the sides, then the internal obliques followed by the transversus abdominis at the deepest level wrapping the abdominal cavity from back to front.
Consider this list of exercises for the abs muscle groups. The abbreviations, as included above, are for each muscle group.
- Standard crunch (shoulders up) -- RA, IO, EO
- Reverse crunch (legs up) -- RA, IO, EO
- Seated fitball crunch -- RA, IO, EO
- Bicycle maneuver -- RA, IO, EO
- Hollow out or suck in stomach -- TA
- Situps, bent knees -- RA, EO
- Incline bench situps -- RA, EO
- Incline leg raises -- RA, EO
- Machine crunches -- RA, EO
- Leg raises (Captain's chair) -- RA, EO
- Broomstick twists -- RA, EO, IO
- Dumbbell side bends -- RA, EO, IO
- Low pulley side bends -- RA, EO, IO
- Roman chair side bends -- RA, EO, IO
Which Exercise Works Best for Which Muscles?
Controversies. Now we're getting into controversial territory. Much has been written about how best to train the abdominal muscles and opinions are fast and furious. Sucking in the stomach, situps or not, ab rockers, ball exercises and so on. Bear in mind that exercises for strength and conditioning in healthy people may be somewhat different to that which could be prescribed for back rehabilitation purposes.
Obliques. One of the more interesting conclusions from a few recent studies of abdominal muscle training is that you probably don't need to bother too much with side crunches, twist crunches or similar exercises to train the obliques. In these versions you twist the body to the side to, theoretically, recruit the obliques to a greater extent. However, it seems that they are quite well activated with the best exercises that activate the rectus abdominis and that also require stabilization, although additional work will do no harm. Instability is the key. If the obliques need to work to keep your body stable, that's a good result. A fitball crunch with feet closer together is a good example. You will get the wobbles and the obliques get a good workout as they automatically try to stabilize your position. Any ab exercise where the legs are raised will also tend to invoke the obliques to do work -- as you can see from the list below.
Situps. Another key factor in abs training is to ensure that you aren't exclusively using the hip flexors, the iliopsoas muscles that run down to the groin and are used to flex the hip, lift the legs and pull the spine into a curve. You want the abs to work and not the hip flexors. The standard sit-up in which the trunk flexes to almost vertical position is an example of this. The military situp where the outstretched hands only reach to the knees at 45 degree flexion is much better.
Upper versus lower abdominals. Can you work different sections of the RA independently? Probably not. The rectus abdominis is a single sheath of muscle, and although exercises like the captain's chair leg raise make you feel as though the lower part of your abdomen is going to burst, this does not necessarily mean that the lower RA is being recruited exclusively.
Do we need to exercise the deep abdominals? Orthopedic and biomechanical experts have been telling us for years that exercising the transversus abdominis (TA) is crucial to the support of the spine in exercise. Now that may have all changed. Read my article on the deep abdominals for more information. In the light of this I won't spend much more time on the TA except to say that utilizing a suite of abdominal muscle exercises, such as those below, in conjunction with abdominal bracing should provide sufficient work for the TA. (See Grenier and McGill in the Sources.)