AbsRectus Abdominis

coreMusculus rectus abdominis

The rectus abdominis is the long strap of muscle down the front of your abdomen — the six-pack. It curls your ribcage toward your hips, and it is visible because of body fat, not because of training.

Abs highlighted on an anatomical figure, from the front.
Where it sits, from the front.

The abs are one flat strap on each side of the midline, running from the pubic bone straight up to the bottom of the breastbone and the lower ribs. They are held in a sheath of connective tissue and crossed by three or four horizontal bands of tendon, which is what divides the muscle into the segments people count.

The action is simple: they pull the ribcage and the pelvis toward each other. Whether the ribs move toward the hips or the hips move toward the ribs depends on which end is anchored, which is the difference between a crunch and a reverse crunch — the same muscle doing the same thing from opposite ends.

Just as often they work without shortening at all. Holding a plank, carrying something heavy, or keeping the ribs down during an overhead press all ask the abs to resist the spine being pulled backward rather than to bend it forward. For most people who lift, that is the majority of what their abs actually do.

The number of visible segments is set by how many tendon bands you have, and how symmetric they look is set by how those bands are arranged. Both are fixed. Training makes the muscle thicker, which makes the segments stand out more at a given body fat, but nobody trains their way from four to six.

Training notes

Visible abs are a body fat outcome, not a training outcome. This is the single most useful thing to know about the muscle, and it is why programs built around high-repetition ab work so reliably disappoint — the muscle underneath gets stronger and the layer over it does not move.

That is not an argument against training them. The abs respond to load like any other muscle, and a thicker rectus abdominis is more visible at the same body fat. Cable crunches and weighted movements build them; endless bodyweight repetitions mostly build endurance.

Both jobs are worth training and they need different exercises. Flexion work — crunches, cable crunches, hanging leg raises — trains the muscle through its range. Anti-extension work — planks, ab rollouts, dead bugs, overhead carries — trains it to hold a position, which is what it does under a heavy bar.

Sit-ups are commonly criticised for using the hip flexors, and the criticism is half right. The iliopsoas does a great deal of the work in a full sit-up. That makes it a worse ab exercise than a crunch, not a useless one — but if the abs are the target, movements that curl the spine without hinging at the hip do the job more directly.

Anatomy

Origin
  • Pubic crest and pubic symphysis
Insertion
  • Xiphoid process of the sternum
  • Costal cartilages of the fifth to seventh ribs
Actions
  • Flexes the lumbar and thoracic spine
  • Tilts the pelvis posteriorly when the thorax is fixed
  • Resists lumbar extension under load
  • Contributes to raising intra-abdominal pressure
Innervation
Anterior rami of T7–T12

Rectus abdominis is a long, flat, paired muscle either side of the linea alba, wider and thinner above, narrower and thicker below. It is enclosed by the rectus sheath, formed from the aponeuroses of the three flat abdominal muscles, whose composition changes at the arcuate line partway down — below that point the posterior wall of the sheath is absent.

It is crossed by three or four tendinous intersections that adhere firmly to the anterior sheath and divide it into segments. Their number and symmetry vary between individuals and are not modifiable; they are the reason the muscle appears segmented and the reason segment counts differ from person to person.

Its action is trunk flexion, approximating the thorax and pelvis. Which end moves depends on which is fixed. It also contributes to raising intra-abdominal pressure with the obliques and transversus abdominis, and works isometrically to resist lumbar extension.

Its segmental innervation by the lower six thoracic anterior rami is characteristic of the abdominal wall and reflects the same dermatomal arrangement as the overlying skin.

Under load

It is loaded through a range by spinal flexion and isometrically by anti-extension work; the two make different demands and both occur in ordinary training.

Its hypertrophy response follows the same rules as any other muscle, and load matters. Its visibility, however, is determined by overlying subcutaneous fat rather than by its own size.

During a full sit-up, hip flexion by the iliopsoas contributes substantially to the movement, so the exercise is a less direct loading of rectus abdominis than a movement restricted to spinal flexion.

Exercises

Antagonists

Synergists