The rectus abdominis is the pair of long, flat muscles running down the front of the belly, one either side of a midline seam of connective tissue (the linea alba). Bands of tendon cross each one, and those divisions are the bumps of a 'six-pack'.
It curls the trunk forward, as in a sit-up or getting out of bed, and with the other abdominal muscles it squeezes the belly to cough, strain and brace.
The curl-up produced the most rectus activity among stabilisation exercises; roll-outs and hanging knee raises were among the highest of their sets. Hanging and reverse-crunch moves also work the hip flexors hard, which some people with back pain find aggravating.
These are standard low-back stretches from NHS physiotherapy leaflets. They bend the spine the opposite way to this muscle's action; no study has named them as stretches for it.
A widening of the gap between the two sides of the muscle, with a ridge or 'doming' down the middle when you strain. In one cohort of first-time mothers it affected 60% at 6 weeks after birth and a third at 12 months.
Whether it causes back pain or leaking is disputed: a review of 12 studies found no clear link. Exercise narrowed the gap only slightly (about 4 mm) in a meta-analysis, though it may help in other ways. A ridge that worries you is worth showing to a pelvic health physiotherapist.
You can train upper and lower abs separatelyEmerging evidence
In EMG studies the upper and lower parts of the muscle worked together in every exercise tested, including reverse curls. Some exercises may lean slightly toward one end; the effect is small and disputed.
Six weeks of daily ab exercises, with diet held steady, did not change belly fat, waist size or skinfolds in a randomised trial, though endurance improved. A couple of small trials disagree. Visible abs depend mostly on how much fat covers them.
It curls the trunk forward, as in a crunch or sitting up, and with the other ab muscles squeezes the belly to cough, strain and brace.
Are upper and lower abs different muscles?
No. They are one muscle, and EMG studies found both ends working together in every exercise tested.
What is the best exercise for the rectus abdominis?
The curl-up produced the most activity among stabilisation exercises, and ab-wheel roll-outs and hanging knee raises were among the highest of the exercises tested.
Will ab exercises flatten my stomach?
Not on their own: six weeks of daily ab training did not reduce belly fat in a randomised trial. Visible abs depend mostly on overall body fat.
Anatomy and movement education, not a diagnosis or treatment plan. If you have pain that follows an injury, is severe or getting worse, or comes with numbness, weakness or swelling, get it assessed. Health disclaimer.
