This tutorial is on the muscles of the posterior abdominal wall. You’ve got five muscles that you need to know about here. You’ve got the quadratus lumborum. You’ve got the psoas major and minor. You’ve got the iliacus. And you’ve got the diaphragm.
The psoas major and minor sit medially, the iliacus is inferiorly. The diaphragm sits at the top, superiorly. And then you’ve got the quadratus lumborum sitting posteriorly and laterally.
This view that we’re looking at here, I’ve removed the anterior abdominal muscles and I’ve removed the muscles of the back and the spine, so we can view these posterior abdominal walls quite nicely. I’ll start off with these medial muscles, the psoas major and minor.
This muscle here, this large muscle is the psoas major. You can see its attachment here on the vertical bodies. It attaches from the vertical bodies from T12 down to L5. And if I fade away the other muscle, you can see its insertion in the lesser trochanter of the femur. You can see its origins here. It sits anterolaterally on the vertical body. It sits in front of the transverse processes on the bodies of vertebra T12 down to L5.
It’s a hip flexor. It flexes the thigh at the hip joint. It brings the thigh up like this. And also in the lying position, if you imagine this model in the supine position, the muscle can sort of act in the reverse way. It can flex the vertebra. If it’s in the supine position, the psoas major can contract to bring the vertebra up (so flexion of the vertebral column).
The psoas minor sits on top of it. And this muscle isn’t always present in everybody, but if it is present, it originates on the bodies of T12 and L1 and it inserts on the pectineal line and the iliopubic eminence. You can see that here. I’ve just isolated the muscle so you can see its origin on T12 and L1 and it inserts on the iliopubic eminence and pectineal line.
The psoas major is innervated by the anterior rami of spinal nerves L1-L3. The psoas minor is innervated by the anterior ramus of L1.
The psoas minor acts as a weak flexor of the vertebral column.
If I just get rid of these two muscles now, we can take a look at the iliacus, which sits just a little bit inferior to it. The iliacus muscle sits in the iliac fossa. It joins the psoas muscle to insert into the lesser trochanter. You can see its insertion on the lesser trochanter here. You can see how it runs through and combines with this psoas major muscle. Collectively, the iliacus and the psoas major are referred to as the iliopsoas.
This serves the same function as the psoas major muscle. It flexes the thigh at the hip joint. It’s innervated by the femoral nerve.
You can see how both the distal ends of the iliacus and psoas major pass underneath this ligament, the inguinal ligament to insert onto the lesser trochanter of the femur.
If we just rotate around to the back, we can see the quadratus lumborum muscle, which is this muscle here. If I just fade away the other muscles, we’ll take a look at its origin and insertion.
You can see here that it has this origin on the iliac crest and it also has an origin on the transverse process of lumbar vertebra L5. Also, if I rotate anteriorly, you can see this ligament connecting the lumbar vertebra to the inside of the ilium. This is called the iliolumbar ligament and it also originates on this ligament.
It’s not shown very clearly on this model, but the quadratus lumborum muscle actually inserts onto the transverse processes of lumbar vertebra L1-L4. It inserts here, here, here and here as well as the inferior margin of the 12th rib.
This muscle can laterally flex the spine and it can also depress the rib. This muscle is innervated by the anterior rami of T12 and also of L1-L4 (spinal nerves T12 and L1-L4).
Right at the top, we’ve got the diaphragm. The posterior parts of the diaphragm contribute to the posterior abdominal wall, the muscular parts of the posterior abdominal wall.
We’ll just take a quick look at the diaphragm. You can see it’s got this domed shape. It separates the thorax from the abdomen (the thoracic cavity from the abdominal cavity). It forms the superior aspect of the abdominal cavity, so it lines the inferior thoracic aperture.
It’s attached to the lumbar vertebra by these two legs. These two legs are called crura (plural) or crus (singular). This is Latin for ‘leg’ and you’ve got a right and a left crus.
And if I rotate the model around, you can see that there’s this central tendinous section and there’s a few holes. Various structures pass from the thorax into the abdominal cavity via these holes. There’s a lot to talk about the diaphragm, but I won’t go through all that. I’ll do a separate tutorial. It’s the posterior parts of the diaphragm that contribute to the posterior abdominal wall, the muscular part of the posterior abdominal wall. The diaphragm is innervated by the phrenic nerve.
Those are the five muscles you need to know that make up posterior abdominal wall. You’ve got the diaphragm at the top (the posterior parts of the diaphragm), you’ve got the psoas major and minor, you’ve got the iliacus and you’ve got the quadratus lumborum.
- psoas major
- psoas minor
- quadratus lumborum
- vertebral body
- transverse process of vertebrae
- lesser trochanter of femur
- pectineal line
- iliopubic eminence
- iliac fossa
- iliolumbar ligament
- inguinal ligament
Now for a couple of questions…
1. Where does the psoas major originate?
2. What is the function of the quadratus lumborum?
3. What nerve supplies the iliacus?
4. Where does the psoas minor insert?
5. Does the iliopsoas muscle pass above or underneath the inguinal ligament to insert onto the femur?