What are the different abdominal muscles and how can they be isolated?

Question

I'm reading about a conditional called Diastasis Recti where the abs separate, leading to a strange 'doming' effect when doing a sit-up/crunch or similar. It's mentioned that someone with this condition might try to remedy it by strengthening their abs doing lots of crunches, but this is the worst thing and will only exacerbate things.

Advice is to exercise the core, but not the abs... this got me thinking about what the various muscle groups are and how one can exercise each group independently? I know core muscles are involved to an extent in a huge number of exercises but the only exercises I know to train them are crunches and leg raises (which I think are very similar)

Answer 1

The "core" is not a particularly well defined group of muscles, but it would commonly include the following muscles.

Rectus abdominis:

• This is the "6 pack" muscle, often just referred to as "the abs".
• Role: Spinal flexion (bending forwards at the lower back)
• Isolation exercises: Most ab exercises, including sit-ups, planks, crunches. It is also worked isometrically (without moving) during most hip flexion exercises (moving at the hip instead of through the lower back), such as reverse crunches and leg raises.

Obliques (internal and external):

• These sit to the sides, with your left and right sides each having an internal and an external oblique. The internal and external obliques cross over each other so that they pull in different directions, with the internal obliques running from the outer edges of the pelvis up to the linea alba, while the external obliques run from the ribs down to the central parts of the pelvis and the lower parts of the linea alba.
• Role: These generally contract in pairs, with one internal oblique and one external oblique contracting at the same time. If the internal and external obliques on one side of the body contract, this causes the body to bend sideways, toward that side. If an internal and an external oblique on opposite sides contract, this causes the trunk to rotate towards the side of the internal oblique.
• Isolation exercises: Rotation exercises such as cable wood choppers, anti-rotation exercises such as the Pallof press, anti-bending exercises such as side-planks and suitcase carries, bending exercises such as side plank hip dips.

Transverse abdominis:

• This is the deepest of the abdominal muscles, running across the abdomen like a belt. It runs from the outer portions of the pelvis and rib cage (as well as from a fascia that runs between the two) inwards, to the linea alba.
• Role: Holding in and compressing the abdominal organs.
• Isolation exercises: The "vacuum" exercise - sucking in the belly to try to create a hollow between the ribs and the pelvis.

In addition to isolation the exercises listed above, the abdominal muscles are also typically all used together, isometrically, when performing a Valsalva manoeuvre (contracting against a held breath) in order to brace the torso when lifting or moving heavy weight.

Other muscles which may be included in the "core" include:

Erector spinae:

• The large and superficial muscle group which runs up either side of the spine.
• Role: Spinal extension
• Exercises: Not normally isolated, but is used in spinal extension (e.g. the Superman exercise) and resisting flexion (heavy lifting such as squats and deadlifts). It also contracts to oppose rectus abdominis during a Valsalva manoeuvre.

Quadratus lumborum:

• A deeper muscle of the low back, running from the back of the pelvis up to the lumbar vertebrae and ribs.
• Role and exercises: Mostly the same as for obliques.

Hip flexors:

• Multiple muscles (primarily iliopsoas) responsible for raising the legs to the front.
• Role: Hip flexion
• Exercises: Hanging leg raises, reverse crunches, sit-ups if the lower back comes up off the ground, kicking, sprinting.

Answer 2

The main purpose of the abs is not to do situps but to stabilize the spine.

The spine is a bit like an antenna mast. The antenna mast have guy-wires to stabilize it against external forces such as wind:

The spine also have guy-wires in the form of muscles running from the base of the spine: the pelvis to the upper part of the spine:

The body can adjust the tension in these muscles thereby bending the spine in different directions.

As an experiment hold a somewhat heavy dumbell with both hands in front of you close to your stomach. Now move the dumbell straight out in front of you:

You should feel your abs tightening up. Perhaps you can also feel your lower back muscles tightening. The situation is analogous to a bending cantilever beam from mechanics:

Here we know that the force F causes a bending moment M=FL at the fixed end, where L is the length of the beam.

Likewise the dumbell causes a spinal flexion (forward bending) moment at the spine: M=FL, where F=mg is the force of the dumbell and L is the length of the arms.

In order to stabilize against this moment the body tenses the erector spinae muscles of the back:

However due to the s shape of the spine the erector spinae pulling on the spine causes extension in the top of the spine but flexion in the lumbar region.

This flexion in the lumbar region must be countered or the intervertebral disks in the spine could be injuried (from Starting Strength):

In order to counter this we instinctively tense the abs to create intra abdominal pressure that counters this flexion of the lumbar region:

The body create this intra abdominal pressure by tensing the innermost ab muscle: the transversus abdominis:

but also the rectus abdominis muscle:

and the obliques:

This co-contraction of the abs and erector spinae is explained further here.

It was measured with EMG in an highly influential experiment from 1996 (1) and (2) where subjects were instructed to rapidly raise and lower their arms, like the alarmist robot in “Lost in Space.” In those with healthy backs, the scientists found that the transversus abdominis tensed several milliseconds before the arms rose. The brain apparently alerted the muscle to brace the spine in advance of movement.

Movements where you hold a weight in front of you such as eg. a goblet squat or a deadlift makes your abs work hard, and trains the abs without them changing length (isometrically).

Other exercises that also work the abs isometrically include the plank and push-ups.

A special case is when you hold something heavy on the side with one arm:

In this case the oblique on the other side have to work really hard to avoid sidewise bending of the spine.

(1) Australian physiotherapy experiment (2) The paper on that experiment