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Structure
Intermediate
9 min read

Iliopsoas

The iliopsoas is the only muscle that links the lumbar spine directly to the femur. It flexes the hip and acts on the spine, and it is never visible from outside.
3D scene
The iliopsoas in the deep plane
The scene shows the muscle's path from the lumbar vertebrae to the top of the femur, passing in front of the pelvis. Without the 3D, picture a rope stretched from the low back, curving around the rim of the pelvis and hooking onto the inside of the thigh.
Open in the explorer

Where the iliopsoas is

The iliopsoas is really two muscles sharing one insertion. Psoas major runs down alongside the lumbar spine; iliacus lines the inside of the hip bone. They meet at the pelvis, merge into a single tendon, and attach to the femur.
It is the deepest muscle on this page. It passes behind the intestine, the kidney and the great vessels, which makes it impossible to see and very hard to palpate. It can be reached only very indirectly, and examining it belongs to clinical practice.

Origins

  • Psoas major arises from the vertebral bodies and discs from the twelfth thoracic vertebra to the first four lumbar vertebrae, and from their transverse processes.
  • Iliacus arises from the iliac fossa, the broad concave surface on the inner face of the wing of the pelvis.

Insertion

The two muscle bellies converge, pass in front of the hip joint and attach together to the lesser trochanter, a prominence on the inner side of the upper femur. This path, crossing the rim of the pelvis like a rope over a pulley, explains the muscle's effectiveness despite its depth.

Nerve supply and palpable landmarks

Iliacus is supplied by the femoral nerve, from the L2 to L4 roots; psoas major receives direct branches from the lumbar plexus, L1 to L3. There is no reliable surface landmark: the muscle cannot be palpated the way a biceps or a calf can.

What the iliopsoas does

It is the most powerful hip flexor, but its position gives it a second, less obvious role.
  • Hip flexion: it brings the thigh towards the trunk, or the trunk towards the thigh, depending on which end is fixed.
  • Lateral rotation of the thigh: a secondary action, varying with hip angle.
  • Action on the lumbar spine: because it attaches to the vertebrae, it loads the lumbar spine whenever it contracts with the hip fixed.
Fixed end
The end of a muscle that stays still during contraction. The psoas illustrates the idea: with the foot on the ground it draws the trunk towards the thigh; with the trunk still it draws the thigh towards the trunk. The same muscle produces two apparently opposite movements.
reversed origin
This double attachment — spine and femur — is what makes the psoas unusual. No other muscle links the spine directly to the lower limb. As a result a leg movement is transmitted to the lumbar spine, and holding the pelvis depends partly on the balance between this muscle and the glutes, which pull the other way.
Certainty level · Established
The iliopsoas is the main hip flexor and mechanically loads the lumbar spine.
Both points come from descriptive anatomy: the tendon's path in front of the hip joint gives the flexion lever, and the vertebral attachments necessarily transmit force to the spine. What remains debated is the size of that load across postures and weights, and the psoas's share in lumbar stabilisation relative to other deep muscles.
Standring S (2020) · Neumann DA (2016)

Seeing the psoas in movement

The iliopsoas in the deep plane

The scene shows the muscle's path from the lumbar vertebrae to the top of the femur, passing in front of the pelvis. Without the 3D, picture a rope stretched from the low back, curving around the rim of the pelvis and hooking onto the inside of the thigh.
Current step
Deep and medial muscles
Beneath the visible groups work muscles no silhouette shows. The rotator cuff holds the head of the humerus against the shoulder blade, serratus anterior keeps the shoulder blade flat on the ribcage, the iliopsoas links the lumbar spine to the femur, and the adductors fill the inner thigh. Deeper still, the diaphragm closes the trunk as a dome, inside the ribs. They orient and stabilise more than they move.
Scene description
A three-dimensional diagram of a standing human body, arms by the sides, built from simplified paper-coloured volumes. Twelve priority muscle groups are laid over this outline in terracotta: at the front, pectoralis major, the deltoid, biceps brachii, the abdominal wall and the quadriceps; at the back, the trapezius, latissimus dorsi, the erector spinae, triceps brachii, the glutes, the hamstrings and the calves. A simplified skeleton — skull, spine, ribcage, pelvis, humerus, femur and tibia — can be shown beneath the outline to reveal the bony levers these muscles pull on. Each group is selectable and opens its own reference page. This is a teaching diagram with stylised volumes, not an exact anatomical reconstruction.
Visible structures
  • Pectoralis major
    A thick sheet running from the clavicle, the sternum and the upper costal cartilages to the lateral lip of the bicipital groove of the humerus. It draws the arm towards the midline and forwards, and drives horizontal pushing.
  • Deltoids
    Three heads — anterior, lateral and posterior — running from the clavicle, the acromion and the spine of the scapula to the deltoid tuberosity of the humerus. Together they raise the arm; separately they carry it forwards, out to the side or backwards.
  • Biceps brachii
    Two heads arising from the scapula — the supraglenoid tubercle and the coracoid process — joining on the radial tuberosity. It flexes the elbow and supinates the forearm, and contributes to every pulling movement.
  • Abdominal wall
    Rectus abdominis, the external and internal obliques and transversus, running from the ribs and sternum to the pubis and iliac crest. They flex and rotate the trunk and, above all, stabilise the spine by regulating intra-abdominal pressure.
  • Quadriceps
    Four heads — rectus femoris, vastus lateralis, vastus medialis and vastus intermedius — joining through the quadriceps tendon onto the patella and then the tibial tuberosity. They extend the knee, and rectus femoris also flexes the hip.
  • Trapezius
    A diamond-shaped muscle spanning from the occiput and the cervical and thoracic spinous processes to the clavicle, the acromion and the spine of the scapula. Its upper, middle and lower portions elevate, retract and depress the scapula.
  • Latissimus dorsi
    A broad fan arising from the lower thoracic vertebrae, the thoracolumbar fascia, the iliac crest and the lowest ribs, converging on the bicipital groove of the humerus. It pulls the arm down and back, and drives vertical pulling.
  • Erector spinae
    Muscular columns — iliocostalis, longissimus and spinalis — running from the sacrum and iliac crest to the ribs, the vertebrae and the skull, on either side of the spinous processes. They extend the spine and control the trunk as it lowers.
  • Triceps brachii
    Three heads — long, lateral and medial — arising from the scapula and the back of the humerus and joining on the olecranon of the ulna. It extends the elbow, and its long head also assists shoulder extension.
  • Glutes
    Gluteus maximus, medius and minimus, arising from the iliac wing and the sacrum and inserting on the femur and the iliotibial tract. They extend, abduct and rotate the hip, and drive the pelvis upright in the hinge and the squat.
  • Hamstrings
    Biceps femoris, semitendinosus and semimembranosus, running from the ischial tuberosity to the tibia and fibula. They extend the hip and flex the knee, and control the trunk as it lowers in the hip hinge.
  • Calves
    The gastrocnemius and soleus, joining through the Achilles tendon onto the calcaneus. They plantarflex the ankle, and the gastrocnemius, which crosses the knee, also assists knee flexion.
  • Rotator cuff
    Four short muscles running from the shoulder blade to the top of the humerus. Their tendons merge into a sheet that holds the head of the bone against its socket while the deltoid moves the arm.
  • Serratus anterior
    A sheet hooked onto the upper ribs by triangular slips, passing under the shoulder blade and attaching to its inner border. It holds the blade against the ribcage and rotates it.
  • Iliopsoas
    The only muscle linking the lumbar spine directly to the femur. It descends from the vertebrae and the iliac fossa, curves around the rim of the pelvis and attaches to the lesser trochanter.
  • Hip adductors
    Five muscles running from the pubis to the back of the femur in overlapping layers. They draw the thigh towards the midline and control the pelvis in single-leg stance.
  • Forearm muscles
    Masses gathered near the elbow, continued by long tendons to the wrist and fingers. They drive the hand from a distance and carry grip strength.
  • Tibialis anterior
    A muscle pressed against the outer surface of the tibia, its tendon swinging towards the inside of the foot. It lifts the toes on every step and brakes the forefoot after heel strike.
  • Diaphragm
    A muscular dome running from the lower border of the ribs, the sternum and the lumbar vertebrae to a central tendon. As it contracts it descends, the chest gains volume and air enters: it is the engine of quiet breathing, and the ceiling of the abdominal cavity.
Guided steps
  • 1/5
    The front of the body. Pectoralis major covers the upper chest, the deltoid caps the shoulder, the biceps fills the front of the arm, the abdominal wall links the ribs to the pelvis, and the quadriceps fills the front of the thigh. Further out on the limbs, the forearm muscles drive the wrist and hand, and tibialis anterior lifts the foot.
  • 2/5
    The back of the body. The trapezius links the neck to the shoulder blades, the latissimus dorsi fans down to the pelvis, the erector spinae flank the spine, and the triceps fills the back of the arm. Glutes, hamstrings and calves form the posterior chain from hip to ankle.
  • 3/5
    Beneath the visible groups work muscles no silhouette shows. The rotator cuff holds the head of the humerus against the shoulder blade, serratus anterior keeps the shoulder blade flat on the ribcage, the iliopsoas links the lumbar spine to the femur, and the adductors fill the inner thigh. Deeper still, the diaphragm closes the trunk as a dome, inside the ribs. They orient and stabilise more than they move.
  • 4/5
    In a push, force starts at the ground, travels through the quadriceps and the abdominal wall, and then pectoralis major, the anterior deltoid and the triceps drive the load away from the trunk. The skeleton acts as the lever: no group works alone, and the quality of the movement comes from their sequencing.
  • 5/5
    In a pull, the latissimus dorsi and trapezius bring the arm and shoulder blade towards the trunk while the biceps flexes the elbow. Glutes, hamstrings and erector spinae hold the pelvis and spine so that force transfers without the trunk collapsing.
Model licence · Z-Anatomy et BodyParts3D — CC-BY-SA 4.0

The movements it takes part in

The psoas works in every hip flexion. In the hip hinge it lengthens on the way down and brakes the movement; in the squat it accompanies the flexion and helps hold the trunk.
Above all it is the muscle of walking and running, where it swings the leg forward with each stride. Its everyday use is therefore constant, far more than in any exercise.

Synergists

  • Rectus femoris, the head of the quadriceps that crosses the hip, contributes to flexion.
  • Sartorius and tensor fasciae latae accompany the movement, with a rotation component.
  • The upper hip adductors, pectineus in particular, have a flexing action.

Antagonists

What the 3D model simplifies

Limit
A muscle you only see by emptying the abdomen
The scene exposes the psoas as if it were accessible. In an intact body it is covered by the intestine, the kidney, the ureter and the great vessels: seeing it means removing all of that. The model is there to locate the muscle, not to describe what an observer would meet.
Psoas minor, a thin and inconstant muscle, is absent in a notable share of the population; it is not represented. Finally, this muscle is frequently invoked in general discussion about back pain. Describing its anatomy establishes no cause and effect, and persistent low back pain calls for a health professional.

Sources

Main sources

  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • McGill SM (2010). Core training: evidence translating to better performance and injury prevention. Strength and Conditioning Journal.
Put it into practice in Shapier

Work the hip flexors with leg raises

Following the leg raises step by step happens in Shapier; Body Lab only explains why this movement calls on the iliopsoas.
Work the hip flexors with leg raises
Body Lab explains; Shapier lets you act and track.

Check my understanding

What does the term iliopsoas actually refer to?
What happens to the lumbar spine when the psoas contracts with the hip fixed?
Where does the psoas work the most in everyday life?
Choose an answer

Read next

  • Glutes
    The three gluteal muscles cover the back and the side of the pelvis. The gluteus maximus extends the hip, while the gluteus medius and minimus steady the pelvis as soon as the body is supported on one leg.
    With a 3D scene
  • Hip adductors
    Five muscles fill the inner thigh and draw the leg back towards the midline. Most of their work is stabilising, on every step taken on one leg.
    With a 3D scene
  • Erector spinae
    The erector spinae form three muscular columns running along the spine. They straighten it, control its forward bending and hold the trunk during hip hinge movements.
    With a 3D scene

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • The psoas runs behind the abdominal organs: the 3D scene exposes it by removing everything that covers it, a situation that does not exist in an intact body.
  • Psoas minor is absent in part of the population and is not represented.
  • This muscle is often blamed for low back pain in general discussion. This page describes its anatomy: it establishes no cause and effect and does not replace an examination.
Sources
  • Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
  • Neumann DA (2016). Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation, 3rd edition. Elsevier.
  • McGill SM (2010). Core training: evidence translating to better performance and injury prevention. Strength and Conditioning Journal.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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