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Exploring The Iliopsoas Muscle

Iliopsoas: Iliacus and Psoas Major Muscle

The core of our body isn't necessarily what we might think it is. While there are many different ways to understand our core, one is considering the layers of muscles and fascia that make up our structure. In that sense we're talking about superficial versus deep. Superficial muscles are those closest to our outer surface, like the rectus femoris in our quadriceps for example. Underneath that muscle, or deep to it in anatomical language, we find other muscles. Iliopsoas is among the deepest layer of our muscles, with psoas major running along the lumbar spine. Muscles that are found in this deepest layer are a key part of functional movement in our body. Read on for an introduction to one of these core muscles.

Some key facts

  • The iliopsoas is a key postural and structural muscle.
  • It is the primary muscle that connects our torso to our legs.
  • The iliopsoas is the strongest hip flexor.
  • It is the primary muscle responsible for an important movement: walking.
  • It is intimately related to the convergence of tissues and structures often referred to as “the core.”
  • It spans more joints in the body than any other muscle.

What is the iliopsoas muscle?

Iliopsoas could be associated with two different muscles: iliacus and psoas major. Iliacus and psoas major are frequently grouped together as the “iliopsoas” because they share a common attachment and they work together to do the same primary movement.

Some people also have a psoas minor muscle. However, the psoas minor is a fairly insignificant muscle that is absent in approximately half of the population. When it is present, it contributes to different actions than psoas major and iliacus. Additionally, psoas minor is not particularly relevant to movement because it is a small, thin muscle. So, psoas minor is not usually included as part of the iliopsoas. Instead when we say that, we are referring to just iliacus and psoas major. 

Where are these muscles located?

The proximal end of iliacus originates on the fossa of the anterior ilium. This is the broad area on the inside of the pelvic bowl. The proximal end of psoas major originates on the body of the vertebrae from T12 to L5.

Psoas major has a long path from its proximal end to its distal end where it meets up with iliacus. It runs down the sides of the spine, anteriorly over the front of the pubic bone, and then  heads posteriorly to meet up with the iliacus. Along its path, psoas major crosses nine joints: T12–L1, L1–L2, L2–L3, L3–L4, L4–L5, L5–sacrum, sacrum–ilium (SI joint), pubic symphysis, and the hip joint.

When the iliacus and psoas major join fibers at their distal ends, they form a common tendon and insert on the lesser trochanter of the femur.

For those who have a psoas minor, it's found anterior to (in front of) the psoas major. It's a thin muscle even in those who have one, unlike the iliacus and psoas major. The psoas minor attaches at its proximal end onto the bodies of T12 and L1. At its distal end, it attaches onto the pubis.

What actions does the iliopsoas do?

Iliacus flexes and externally rotates the femur. Both iliacus and psoas major flex the hip joint. This is their primary action. Psoas major can also contribute to external rotation of the hip joint and adduction of the hip joint.

Remember, actions can go origin to insertion or insertion to origin. Iliopsoas is the strongest hip flexor, whether this means we are standing and bringing the leg to the torso or we are sitting on the floor and bringing the torso towards the legs.

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