The psoas major is the primary hip flexor for everyday activities such as walking and stair climbing, arising from the lumbar vertebrae and crossing the hip joint to insert on the lesser trochanter. Its origin from the lumbar spine means that when it contracts it simultaneously compresses the lumbar spine — a clinically important fact that explains why psoas stretching and hip flexor length optimisation are components of lumbar spine rehabilitation.
| Origin | Transverse processes of all lumbar vertebrae and lateral surfaces of T12 through L5 vertebral bodies and intervening discs |
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| Insertion | Lesser trochanter of the femur via the iliopsoas tendon |
| Nerve Supply | Ventral rami of L1, L2, L3, and L4 |
| Blood Supply | Lumbar arteries |
| Actions | Hip flexion — the primary hip flexor when arising from a neutral or extended position; Lateral flexion of the lumbar spine when acting unilaterally; Flexion of the lumbar spine when the hip is fixed; Compression of the lumbar intervertebral discs and facet joints when active |
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During walking the psoas decelerates hip extension in late stance and then accelerates hip flexion in early swing, providing the initial propulsive impulse that begins the swing phase. Its vertebral origin also creates the lumbar compression force that increases disc loading during hip flexion activities.
Psoas major tightness from prolonged sitting produces anterior pelvic tilt and increased lumbar lordosis that loads the posterior lumbar structures and facet joints. The Thomas test specifically tests psoas length by showing whether the tested hip can fully extend when the opposite hip is maximally flexed. Psoas abscess from vertebral osteomyelitis or Crohn's disease tracks down the psoas sheath to present as a groin mass.
The psoas major is not accessible to external palpation due to its deep retroperitoneal position. It is palpated indirectly by pressing deeply in the iliac fossa medial to the ASIS in the femoral triangle region, where the iliopsoas tendon becomes accessible just before it inserts on the lesser trochanter.
Bacterial infection of the psoas sheath from vertebral osteomyelitis or Crohn's disease producing fever, groin pain and a hip flexion contracture, managed with CT-guided drainage and antibiotics.
Iliopsoas tightness and overuse producing anterior hip and lower abdominal pain in runners and cyclists, managed with targeted hip flexor stretching and strengthening.
