Next, let’s look at the muscles of the posterior
abdominal wall individually. Our first one
will be the psoas major. So, we want to understand
its attachments, its innervation and actions
or functions. The origin of the psoas major
and we can see several points of origin in
the illustration. Here is the right psoas
major, the left one is shown over in through
here. The points of origin of the psoas major
will be the vertebral bodies, the lateral
aspects of T12, L1, L2, L3, L4 and L5, the
intervening intervertebral disc and then it
also will originate from the transverse processes
of your lumbar vertebrae, all five of them.
The insertion will extend underneath the inguinal
ligament to then insert on the lesser trochanter
of the femur. The innervation of your psoas
major muscle will be the anterior rami of
lumbar spinal nerves L1, 2 and 3. And the
primary function of the psoas major is to
flex the femur at the hip. It is a very powerful
flexor of the hip.
The psoas minor is a small muscle that runs
on the superficial aspect of the psoas major.
This muscle may be absent in some individuals.
Its points of attachment would be the lateral
surfaces of the bodies of T12 through L1 and
the intervening intervertebral disc. Its insertion
is more inferior and will be to the pectineal
outline region of the pubic bone. It’s innervated
by the anterior ramus of the first lumbar
spinal nerve and its action is to produce
a weak flexion of the vertebral column.
The next muscle to take a look at is our quadratus
lumborum. It’s a quadrate shaped muscle
in the lumbar area and that is visualized
in the illustration right in through here.
The attachment of this would be to the transverse
process of L5, the ileal lumbar ligament and
then the iliac crest. It also attaches for
its points of insertion to the transverse
processes of L1 down through L4 and also has
an attachment for its insertion to the 12th
This muscle is innervated by the anterior
rami at T12, L1, L2, L3 as well as L4 and
its main function is most likely to stabilize
the 12th rib during respiratory movements.
Diaphragm also has an attachment to the
12th rib. And so, when it’s contracting
during inspiration, it wants to pull that
12th rib superiorly. What will happen then
to counteract that is that your quadratus
lumborum will contract and stabilize that
12th rib to keep it from being pulled more
superiorly by the action of the diaphragm.
The last muscle to point out with respect
to the posterior abdominal wall is your iliacus
muscle. It is shown right in through here.
It is originating from the internal surface
of the iliac ala or the wing of the ilium.
Also has a partial attachment to the sacrum
and some associated ligaments in this region.
It has the same point of insertion as does
the psoas major. It inserts onto the lesser
trochanter of the femur. Collectively, the
psoas major and the iliacus are known as the
iliopsoas and since they both contract to
the same point, the function of the iliacus
is the same as the psoas major, a powerful
flexor of the femur at the hip. The nerve
that innervates your iliacus will be the femoral
nerve and the femoral nerve would run on the
lateral margin of the psoas major.