The iliopsoas is technically two muscles, the psoas major descending from the lumbar spine and the iliacus from the pelvic bowl, that merge just above the hip joint to become the most powerful hip flexor in the body, attaching together to the lesser trochanter of the femur. It is the only muscle that directly connects the lumbar spine to the lower limb, making it critically important for both spinal stability and hip mobility. In people who sit for prolonged periods, it tends to become adaptively shortened, affecting posture, lumbar curvature, and hip mechanics.
| Origin | Anterior surfaces of T12–L5 vertebral bodies and intervertebral discs, transverse processes of L1–L5; Superior two-thirds of the iliac fossa |
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| Insertion | Lesser trochanter of the femur |
| Nerve Supply | Psoas: lumbar plexus direct branches (L1–L3); Iliacus: femoral nerve (L2, L3) |
| Blood Supply | Iliolumbar artery; Medial circumflex femoral artery |
| Actions | Flexion of the hip, the most powerful hip flexor; External rotation of the femur (at neutral); Psoas major: lateral flexion and stabilisation of the lumbar spine when the femur is fixed |
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During walking and running, it initiates hip flexion to swing the leg forward, while in sitting it becomes the primary muscle keeping the torso upright against gravity, a dual role that explains why it is both chronically tightened in sedentary individuals and prone to fatigue in endurance athletes.
Iliopsoas tightness is among the most common musculoskeletal findings in people with anterior hip pain, low back pain, and lumbar hyperlordosis, the Thomas test is the standard clinical screen for hip flexor restriction. Iliopsoas bursitis (the largest bursa in the body) can cause groin pain with a snapping sensation during hip flexion, a condition known as coxa saltans interna or 'internal snapping hip syndrome.' The psoas is also an important anatomical landmark in lumbar surgery, where its lateral margin borders the approach to the L4–L5 disc.
The iliopsoas is not directly accessible for palpation in most individuals, but the psoas can be approached deep to the rectus abdominis just lateral to the midline in the lower abdomen, though this should be performed with care given the proximity of major vessels.
Tearing of the iliopsoas at the musculotendinous junction or muscle belly, most commonly from explosive kicking or sprinting, producing acute groin and anterior hip pain that is reproduced by resisted hip flexion.
Degenerative changes in the iliopsoas tendon, often accompanied by snapping or clicking as the tendon flicks over the iliopectineal eminence during hip flexion and extension, 'internal snapping hip', producing anterior groin discomfort that worsens with repetitive hip movement.
Inflammation of the iliopsoas bursa between the tendon and the hip joint capsule, causing anterior hip or groin pain that may refer down the anterior thigh and is often associated with rheumatoid arthritis or overuse in running athletes.
