I have a questions about what abdominal muscles I actually have, and what they do.

On one hand, I often hear about the distinction between outer and inner abdominals: The outer abs are mostly worked by crunches, the inner ones by exercises like the plank or lying on the back and slowly lifting ones stretched legs of the ground. I feel a distinctive difference between those exercises (meaning I can do a reasonable amount of crunches, but can't plank for long) and from what my body tells me this distinction seems to make sense - but it may also be a matter of strength qualities, like power vs. endurance.

On the other hand, sources like exrx.net list both types of exercise as targeting the same muscle and I've never read or heard about the distinction in the english speaking internet or literature.

I want to understand what I read and what I'm told. So, what's happening the truth behind the inner and outer abs?

1 Answer 1


There are basically four muscles (In reality there are a lot of smaller muscles involved) that comprise the "abdominals" or "core" muscles.

  • Rectus abdominus - These are the abdominals that produce the "6 pack" effect. They are responsible for flexion of the lumbar spine. In reality, you may have a 4, 5, 6 or 8 pack, or other variation. This effect is produced by the connective fascia in the muscle, and is genetically determined. This is also the muscle that is probably referred to as the "outer" abdominal.
  • Transverse abdominus - This is the muscle that is responsible for compressing ribs and viscera (intestines), and providing thoracic and pelvic stability. It's the deepest, and you won't see it. Best exercise for this is the vacuum exercise. This is what is probably referred to as the "inner" abdominal.
  • External obliques - These are the most superficial, and located mostly along the lateral portions of the abdomen. They assist in flexion, and lateral flexion if only one is enervated. They also assist in rotation.
  • Internal obliques - Assist in breathing by acting in opposition to the diaphragm, and it rotates and side bends the trunk.

Situps, crunches, abdominal raises will all hit most of the groups in varying intensities. You will want to use a variety of abdominal exercises to ensure that you are hitting the various muscles equally.

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