The psoas major is the only muscle connecting the lumbar spine to the lower limb, running from the lumbar vertebral bodies to the lesser trochanter. It is both a hip flexor and a lumbar stabiliser. Psoas abscess from Crohn's disease, vertebral osteomyelitis (Pott's disease), or appendicitis can present as hip flexion contracture — the patient lies with the hip flexed to relieve the psoas spasm (positive iliopsoas sign in appendicitis).
| Origin | Anterolateral surfaces of T12-L5 vertebral bodies and discs; Transverse processes of L1-L5 |
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| Insertion | Lesser trochanter of the femur (with the iliacus tendon) |
| Nerve Supply | Branches directly from L1-L4 anterior rami (not via the femoral nerve) |
| Blood Supply | Iliolumbar and lumbar arteries |
| Actions | Hip flexion (most powerful in the supine position); Lumbar spine lateral flexion (unilateral); Lumbar stabilisation (deep postural role) |
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The psoas acts as a hip flexor when the hip is beyond 45 degrees of flexion; at less than 45 degrees the primary flexors are the rectus femoris and tensor fasciae latae. The psoas also acts as a lumbar compressor when the hip is fixed, increasing disc loading — this is why supine straight-leg exercises with both legs elevate lumbar disc pressure.
Psoas abscess presents as hip pain, fever, and inability to extend the hip (psoas sign). Primary psoas abscess (haematogenous) occurs in IV drug users and immunocompromised patients. Secondary psoas abscess from vertebral TB (Pott's disease) tracks along the psoas sheath to present as a fluctuant mass in the groin at the lesser trochanter level (psoas pointing into the femoral triangle). CT-guided drainage is the primary treatment.
The psoas is not palpable directly from the surface but deep palpation medial to the iliac crest and lateral to the lumbar spinous processes in a relaxed patient may reproduce psoas trigger point pain.
Psoas sheath infection from vertebral osteomyelitis or Crohn's disease producing hip flexion contracture managed with CT-guided drainage and antibiotic treatment.
