Developing abs muscles successfully requires an intimate working knowledge of the function of each of the three abdominal layers - the superficial, middle and deep.
The composite picture of the abdominal musculature looks rather like the British flag, the Union Jack. Rectus Abdominis, the most superficial muscle, runs up and down vertically from the base of the sternum to the top of the pubis. Internal and External Oblique muscles making up the middle layer run diagonally, crossing each other to make a broad 'x' across the front of the belly and Transverse Abdominis, the deepest layer, runs horizontally around the waist like a cummerbund.
The contractile layout of muscle fibres running in four directions is an ingenious design for holding together the soft exposed flank of our underbelly. It also creates a pressure bag that holds us upright, stopping us buckling over either forward or backwards through the middle.
Rectus Abdominis is the least efficient spinal flexor
The rectus abdominis muscle runs vertically straight down your belly from the bottom of the breastbone (sternum) to the pubic bone. This muscle works like a bowstring tipping the upper body forward, as in initiating bending or thrusting the upper body forward as if trudging through a force-9 gale. Strangely enough, the much sought after definition of the six pack is created by strengthening of this least efficient abdominal muscle.
The External Oblique abdominal muscle runs diagonally downwards and inwards across the front of the abdomen in the same direction that your fingers follow when plunged into the pockets in a reefer jacket.
The fibres of Internal Oblique run in the opposite direction to EO, outwards and downwards to the iliac crests. Working together, the obliques twist the torso and gather in the waist at the sides. The most effective exercise for strengthening the obliques is the floor twists. See Best exercises for lower abs for more detail on doing these exercises.
Transversus abdominis fibres run horizontally around your belly, from either side of the midline at the front to the spine at the back. It is most effective at pulling your bellybutton in and pressurising the abdomen, particularly before you go to lift something heavy. Its action gives a combined sucking in action and rounding of the lower back. For more information about pre-emptive timing of transversus abdominis to stabilise the spinal segments before elective trunk movements see Strengthening TrA and the Myths of Core Stability Training.
Interestingly, TrA encapsulates and holds down rectus abdominis at the front of the abdomen, rather like wearing a belt around the waist holds it in. If the transversus belt is weak, the belly wells up like a dome as you attempt to do sit ups. You can deliberately recruit TrA into holding Rectus down more effectively during sit-ups by pulling the bellybutton in hard before rolling up into the sit up.
The purpose of developing abs muscles as a whole should be to create a pocket of high intra-abdominal pressure, rather akin to an airbag in a car, at the front of your spine. This exerts back pressure against the stacked lumbar vertebrae – critically important for when you bend, so they don't all topple off one another. The higher the IAP the more secure you are to bend and the easier you find it to lift. (This is one reason why professional weightlifters wear kidney belts.)
Importantly, the three layers of abdominal muscles cannot generate sufficient pressure if your pelvic floor is weak. If your Pubococcygeusl muscle is flaccid, it is like the bottom of a cardboard box falling out as you pull your tummy in. Your abdominal muscles they will never achieve sufficient intra-abdominal pressure to support the spine - particularly a lifting spine. In other words, the two groups go hand in glove: you will never succeed in developing abs muscles without a strong pelvic floor, and vice versa.
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Refs: O’Sullivan et al 2002
The effect of different standing and sitting postures on trunk muscle activivties in a pain free population
Spine 27 (11):1238
Richardson C A, Hides J A
The rationale of a motor control programme for the treatment of spinal muscle dysfunction, Chpt 31
Grieve’s Modern Manual Therapy
3rd Edition Elsevier Churchill Livingstone
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