Internal ObliqueInternal Abdominal Oblique

coreMusculus obliquus internus abdominis

The internal oblique lies beneath the external oblique with its fibres running the opposite way, upward and forward. It rotates your trunk toward the same side.

The internal oblique is the middle of the three flat muscles of the abdominal wall, lying under the external oblique and over the transversus abdominis. It arises from the hip bone, the inguinal ligament and the sheet of fascia at the back, and its fibres fan upward and forward to the lower ribs and to a broad tendon reaching the midline.

Its fibres run roughly at right angles to those of the external oblique above it, and that crossing is the whole point. Because it runs upward and forward, the left internal oblique turns the trunk to the left — the same side, not the opposite one. So a rotation to the left uses the left internal oblique together with the right external oblique.

In every other respect it does what the external oblique does: side bending toward its own side, trunk flexion when both sides work, and compression of the abdomen. It is only in rotation that the two differ, and only because of fibre direction.

Its lowest fibres join with those of the transversus abdominis to form the conjoint tendon, which reinforces the abdominal wall at the groin. This is a structurally important detail of the inguinal region and one of the reasons the layer is described so carefully in anatomy.

Training notes

It is trained by rotation toward its own side and by side bending toward it — the mirror image of the external oblique, and trained by exactly the same exercises performed in the other direction.

Because it lies underneath the external oblique, nothing isolates it and nothing needs to. Rotational work in both directions covers both layers on both sides.

It works with the transversus abdominis in compressing the abdomen during bracing, so it is loaded isometrically in every heavy compound lift as well as through its range in rotational work.

Anatomy

Origin
  • Thoracolumbar fascia
  • Anterior two-thirds of the iliac crest
  • Lateral half of the inguinal ligament
Insertion
  • Inferior borders of the tenth to twelfth ribs
  • The linea alba, through a broad aponeurosis
  • The pubic crest and pectineal line, through the conjoint tendon
Actions
  • Rotates the trunk to the same side
  • Bends the trunk sideways toward the same side
  • Flexes the trunk when both sides work together
  • Compresses the abdomen and raises the pressure inside it
Innervation
Anterior rami of T7–T12, with contributions from L1

Exercises

Antagonists

Synergists