Internal oblique
The middle layer of the abdominal wall, with fibres running at right angles to the muscle above it.
- Type
- Muscle
- Region
- Trunk
- Standard name
- m. obliquus internus abdominis
- Origin
- Thoracolumbar fascia, iliac crest and the lateral half of the inguinal ligament
- Insertion
- Lower three or four ribs, the linea alba, and the pubic crest via an aponeurosis
- Actions
- trunk rotationtrunk sidebendingtrunk flexion
- Verify
- TA2 2373 TA98 A04.5.01.017
What it is
The middle of the three flat abdominal muscles, lying beneath external oblique and above transversus abdominis.
Its fibres run upward and forward — roughly perpendicular to those of external oblique. Stack three sheets with fibres at different angles and you get a wall much stronger than three parallel sheets would be, which is the same principle as plywood.
Like external oblique it becomes an aponeurosis towards the midline, contributing to the sheath around rectus abdominis.
What it does
Rotates the trunk to its own side — the right internal oblique turns you to the right — which is the opposite of external oblique.
Rotating your torso to the right therefore uses the right internal oblique and the left external oblique together. Their fibre directions line up across the front of the abdomen when they work as a pair, forming a continuous diagonal sling.
It also side-bends the trunk, assists flexion, and contributes to intra-abdominal pressure.
What loads it
Resisted and resisted-against rotation, side bending, and anti-rotation holds.
What is commonly got wrong about it
That it rotates the same way as the external oblique on the same side. It rotates the opposite way. This is the single most commonly muddled fact about the abdominal wall.
"Twisting exercises for the obliques." They do train them. They do not change the shape of the waist, for the same reason nothing else does.
That the layers can be trained separately. They are loaded together in every trunk movement.