There are three muscular layers of the abdominal wall,
with a fourth layer in the middle anterior region.
The fourth layer in the midregion is the rectus abdominis,
which has vertically running muscle fibres that flex the
trunk and stabilize the pelvis. To either side of the rectus abdominis
are the other three layers of abdominal muscles. The deepest
of those layers is the transversus abdominis, which has fibres t
hat run perpendicular to the rectus abdominus; the transversus
abdominis acts to compress and support the abdomen
and provides static core stabilization. The internal oblique
layers run upward and forward from the sides of the abdomen,
and the external oblique layers, which form the outermost muscle
layers of the abdomen, run downward and forward. The internal
oblique layers act in conjunction with the external oblique
on the opposite side of the body to flex and rotate the trunk toward
the side of the contracting internal oblique (“same-side rotator”).