Hip FlexorsIliopsoas

legsIliopsoas

The iliopsoas is the deepest muscle of the trunk and the strongest hip flexor you have. It runs from your lower spine and the inside of your pelvis to the top of your thigh bone.

The iliopsoas is two muscles that join near the hip and share one tendon. Psoas major starts on the sides of the lower spine, running down through the abdomen behind everything else; iliacus starts on the broad inner surface of the pelvis. They meet as they pass over the front of the hip and attach together to a small knob on the inner side of the top of the thigh bone.

Because they arrive at the femur together, they are treated as one muscle for function. But they are not one muscle: they have separate origins, separate nerve supplies, and only psoas major has any connection to the spine.

It is the strongest hip flexor in the body, and it is deep — it lies behind the abdominal organs, which is why you can neither see it nor easily feel it. Raising your knee toward your chest is what it does. So is sitting up from lying down, which is the same movement with the other end anchored.

The psoas major's attachment to every lumbar vertebra means it can also influence the low back, and this is where a great deal of confident and contradictory writing lives. Whether a short or overactive psoas causes an arched lower back, and what to do about it, is a clinical question with no settled answer. What is certain is the anatomy: this muscle connects the lumbar spine to the thigh bone, and nothing else does.

It is often called the hip flexor, singular, when in fact several muscles flex the hip — rectus femoris, sartorius and the adductors all contribute. This one is simply the largest and best positioned.

Training notes

Hanging leg raises and knee raises are the direct movements, and the useful distinction is whether the pelvis moves. A leg raise where the hips stay still is training the hip flexors; one where the pelvis curls upward at the top has added an ab exercise on the end, which is a bonus rather than a problem.

It is heavily involved in sit-ups, which is the basis of the common criticism of them. The criticism is accurate as a statement about the abs and misleading as a criticism of the exercise: hip flexion is real work and this muscle is worth training.

It matters for sprinting and for anything requiring the knee to be driven forward quickly, which is why it gets specific attention in athletic programs and almost none in general fitness programs.

Nothing here is advice about lower back pain, posture correction or stretching the psoas. Those are questions for a qualified professional, and the volume of confident content on them is not a measure of how settled they are.

Anatomy

Origin
  • Psoas major: the vertebral bodies and transverse processes of T12–L5 and the intervening intervertebral discs
  • Iliacus: the superior two-thirds of the iliac fossa, the iliac crest and the sacroiliac and iliolumbar ligaments
Insertion
  • Lesser trochanter of the femur, by a common tendon
Actions
  • Flexes the thigh at the hip
  • Flexes the trunk on the thigh when the lower limb is fixed
  • Assists lateral rotation of the thigh
  • Psoas major additionally acts on the lumbar spine
Innervation
Psoas major: anterior rami of L1–L3. Iliacus: femoral nerve (L2–L4)

Iliopsoas is not a single muscle and is named as one only because its two components share a distal tendon. Psoas major arises from the lumbar vertebrae and T12 and descends through the posterior abdominal wall; iliacus arises from the iliac fossa. They unite as they pass beneath the inguinal ligament and insert together on the lesser trochanter.

The two are distinct in every other respect. Psoas major is supplied directly by the anterior rami of L1–L3; iliacus by the femoral nerve. Only psoas major has vertebral attachments and therefore only psoas major can act on the lumbar spine.

It is the most powerful flexor of the hip, both by cross-sectional area and by its line of pull across the front of the joint. With the trunk fixed it flexes the thigh; with the limb fixed it flexes the trunk, which is its action during a sit-up.

Psoas minor is present in a minority of people as a separate slender muscle in front of psoas major, and is not included in this grouping. A third muscle, psoas major's fascial relationship to the lumbar plexus, makes the region clinically significant beyond its motor function.

Under load

It is loaded through a range by hip flexion against resistance — hanging knee and leg raises, cable hip flexion, and any resisted march.

It is loaded isometrically during trunk flexion movements where the lower limb is fixed, which includes the full sit-up.

Its lumbar attachments mean psoas major is also under load during positions requiring the lumbar spine to be held against a flexing or extending moment, though its contribution there relative to the erector spinae and abdominal wall is not well quantified.

Exercises

No exercise in the library names Hip Flexors yet. Nothing has been invented to fill the gap — a false reference costs more than an absent one.

Antagonists

Synergists