There is iliacus and there is psoas. There is no iliopsoas.
Let's talk about what they have in common to start with:
- both attach to the lesser trochanter of the femur
- they both flex and externally rotate the hip joint, anteriorly tilt the pelvis and help adduct an abducted hip
There's SO MUCH MORE they don't have in common...
They're innervated differently. Iliacus is innervated by the femoral nerve (L2-4) and the psoas is innervated by the anterior rami of lumbar nerves (L1-3), which is a big deal if there's a pathology or injury.
The whole lumbar plexus is embedded in the posterior aspect of the psoas. This leaves a lot of room for dysfunction if a hypertonic psoas is compressing nerves. It can also be an issue if you're going continually eccentrically load (stretch) the psoas without assessing any underlying issues .
Your psoas massages practically all the organs in your abdomen when it contracts. The iliacus gets the lower intestines, which ain't shabby, but the psoas gets your kidneys, liver, spleen, stomach, pancreas, gallbladder AND your intestines (high and low). Plus all the other glorious stuff in your insides like your lymphatic and blood vessels.
The psoas crosses seven(ish) joints and iliacus crosses one. They both cross the hip joint, but the psoas crosses T12/L1, L1/L2/ L2/L3, L3/L4, and L4/L5. This means there are many more opportunities for movement and lengthening of the psoas.
The psoas has more actions because it crosses more joints. The iliacus and psoas share the same four actions previously mentioned. The psoas also:
- laterally flexes the lumbar spine
- contralaterally rotates the lumber spine
- flexes the lower lumbar spine
- extends the upper lumbar spine
- stabilizes the spine
The psoas has multiple important intimate relationships. It's connected to the intervertebral discs, the diaphragm, the lumbar plexus and shares fascia with the abdominal aorta, the inferior vena cava, the lymphatic system, the quadratus lumborum, the kidneys and the adrenal glands. Don't go jamming hard objects in abdomen with the intention of releasing any muscles. Use a soft object, like the Coregeous ball and find a skilled manual therapist.
Psoas is commonly inhibited, while iliacus is commonly facilitated. Meaning iliacus is doing all the work and psoas is getting all the credit through it's fame. It also means you may not want to excessively stretch the psoas to inhibit it more.
As you can see, these two important hip flexor muscles are quite different, meaning they require different types of attention based on what is going on in the body. Always look at relationships in the body because absolutely nothing stands alone. The psoas and iliacus must be able to lengthen, contract and relax, just like every other skeletal muscle, to function well. Getting a thorough assessment and treatment by a manual therapist is a must if you are suspecting dysfunction (low back pain, excessive stress, hip pain, limited range of motion, breathing dysfunction, digestive problems) in either of the muscles.
Enjoy your iliaci and your psoai ! Much love,