Psoas minor: origin, insertion, action and innervation

The psoas minor is a small, thin muscle that lies on the front of the psoas major, running from the T12 and L1 vertebrae down to the brim of the pelvis. It weakly flexes the lumbar spine on the pelvis, is supplied by the anterior ramus of L1, and is missing in many people.

Where it is

The psoas minor sits in the back wall of the abdomen, on the front surface of the much larger psoas major. It has a short, flat belly at the top and a long, thin tendon that makes up most of its length. The tendon runs down over the psoas major and stops at the brim of the pelvis.

That is the key difference from the psoas major: the psoas minor never leaves the abdomen and pelvis. It does not pass under the inguinal ligament and does not reach the femur, so it does not act on the hip.

It is an inconstant muscle. Many people have it on both sides, some on only one side, and a large share of people have none at all. When it is absent, nothing is missing in daily movement: the psoas major and the abdominal muscles do the same work.

Origin and insertion

• Origin: the sides of the bodies of the T12 and L1 vertebrae and the intervertebral disc between them. See the lumbar and thoracic vertebrae on the bones of the spine page.
• Insertion: the pecten pubis (pectineal line) and the iliopubic eminence of the hip bone, the raised area where the ilium meets the pubis. Part of the tendon also blends into the iliac fascia that covers the iliacus and psoas major.

What it does

The psoas minor runs from the spine to the pelvic brim and never reaches the thigh, so its action stays on the trunk:

1. Weak flexion of the lumbar spine on the pelvis. It pulls the front of the lumbar spine and the pelvis toward each other, as at the start of a sit-up.
2. Tensing the iliac fascia. Through its tendon it pulls on the fascia over the iliopsoas. This is often listed as a minor role.

Because it is thin and frequently absent, it is a weak muscle, and no movement depends on it.

Innervation and blood supply

The psoas minor is supplied by a branch of the anterior ramus of L1, the first lumbar spinal nerve, which runs within or close to the psoas major. Some textbooks list L1 and L2. Its blood comes from the lumbar arteries, small paired branches of the abdominal aorta that run beside the lumbar vertebrae.

How to remember it

• Minor stays in the belly; major goes to the thigh. The psoas major reaches the lesser trochanter of the femur. The psoas minor stops at the pelvic brim.
• Top of the stretch only. The psoas major starts on T12 and all five lumbar vertebrae; the psoas minor uses just the top of that stretch, T12 and L1.
• Long tail. Short belly, long tendon: if you see a thin cord on the front of the psoas major, that is the psoas minor.
• Optional muscle. It is one of the muscles that many people simply do not have, together with the palmaris longus in the forearm.

Place it next to the iliopsoas on the muscles of the hip page, then pick it out on the back wall of the abdomen in the abdomen quiz.

Frequently asked questions

What does the psoas minor do?

It weakly flexes the lumbar spine on the pelvis. It does not move the hip, because it does not reach the femur.

Does everyone have a psoas minor?

No. It is absent on one or both sides in a large share of people, with no effect on movement.

What is the difference between the psoas major and psoas minor?

The psoas major runs from T12 to L5 down to the lesser trochanter of the femur and flexes the hip. The psoas minor is a thin muscle on its front, from T12 and L1 to the pelvic brim, and only weakly flexes the lumbar spine.

What nerve supplies the psoas minor?

A branch of the anterior ramus of L1. Some textbooks add L2.

Sources

• OpenStax, Anatomy and Physiology 2e, 11.6 Appendicular muscles of the pelvic girdle and lower limbs
• OpenStax, Anatomy and Physiology 2e, 8.3 The pelvic girdle and pelvis
• Z-Anatomy, the libre 3D atlas of anatomy