Rectus abdominis

What it is

A long, flat, paired muscle running vertically up the front of the abdomen from the pubis to the lower ribs, enclosed in a sheath formed by the tendons of the flat abdominal muscles either side.

The two halves are separated down the midline by the linea alba, a strip of connective tissue. Crossing each half are three or four tendinous intersections — bands of fibrous tissue running across the muscle.

Those intersections are what create the visible six-pack. The segments are not separate muscles; the divisions between them are tendon. How many you have, whether they are symmetrical, and how far down they extend is anatomy you were born with. Some people have four visible segments, some six, some eight, and asymmetry is common and normal.

What it does

Flexes the trunk — curls the ribs towards the pelvis, or the pelvis towards the ribs. Only the first portion of a sit-up is this muscle; past that, the hip flexors take over.

It also tilts the pelvis backwards, contributes to raising pressure inside the abdomen, and resists the spine being extended.

What loads it

Any resisted trunk flexion, and — more usefully — anything resisting extension of the spine under load: front-loaded carries, overhead positions, and holding a rigid trunk while the limbs move.

The plank and dead bug load it doing the job it actually does — resisting the lower back extending. So, continuously, does a push-up, which is a plank that moves.

What is commonly got wrong about it

That abdominal exercises reveal abdominals. Whether the segments are visible is determined almost entirely by the fat layer over them. This has been tested directly: 24 sedentary adults did seven abdominal exercises five days a week for six weeks on a controlled diet, and there was no significant effect on abdominal fat, abdominal circumference or skinfold measurements — while abdominal endurance improved substantially.¹ Training the muscle makes it stronger; it does not remove what is on top of it.

The honest qualification: the question is not entirely closed. A much smaller study reported that a mixed endurance-and-strength circuit did reduce abdominal subcutaneous fat.² That was fourteen people and a different intervention from isolated abdominal work, and it is why this site says targeted exercise does not preferentially remove fat from the area worked, rather than claiming the matter is settled.

"Upper abs" and "lower abs" as separate muscles. It is one continuous muscle per side with one nerve supply per segment. There is some evidence of regional differences in activity between exercises, but the popular version — that specific movements build specific segments — is far beyond what has been shown.

That everyone can have a visible six-pack. The body fat level required is low, differs between people, and for many is neither comfortable nor healthy to maintain.

That sit-ups are the abdominal exercise. Past the first part of the curl, most of the work is being done by iliopsoas.

Sources

1. Vispute SS, Smith JD, LeCheminant JD, Hurley KS. The effect of abdominal exercise on abdominal fat. Journal of Strength and Conditioning Research 2011; 25(9):2559–2564. doi:10.1519/JSC.0b013e3181fb4a46 PMID 21804427 24 sedentary adults, six weeks of abdominal exercise five days a week on an isocaloric diet. No significant effect on body fat percentage, abdominal fat, abdominal circumference or skinfolds — while abdominal muscular endurance improved substantially. A direct test of the premise.
2. Paoli A, Casolo A, Saoncella M, et al. Effect of an endurance and strength mixed circuit training on regional fat thickness: the quest for the "spot reduction". International Journal of Environmental Research and Public Health 2021; 18(7):3845. doi:10.3390/ijerph18073845 PMID 33917584 Included deliberately because it points the other way: 14 volunteers, and the authors report that a mixed circuit protocol did reduce abdominal subcutaneous fat. Very small, and not the same intervention as isolated abdominal exercise — but it is why this site says targeted exercise does not preferentially remove fat rather than claiming the question is closed.

Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.