Muscle · Core

Internal oblique

Musculus obliquus internus abdominis

The middle sheet of the abdominal wall, its fibres fanning upward and forward from the pelvis to the lower ribs — the direction you would take your hands OUT of your front pockets, and the exact opposite of the external oblique lying on top of it. That crossing is the design: the internal oblique of one side and the external oblique of the other pull along the same diagonal, so a single twist is always driven by a pair. The founder's document gives it trunk rotation to the SAME side, plus flexion. It also helps compress the abdomen, so it shares in the bracing job with the deeper transversus abdominis.