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| Psoas major | |
|---|---|
| Name | Psoas major |
| Latin | Musculus psoas major |
| Origin | T12–L5 vertebrae and intervertebral discs |
| Insertion | Lesser trochanter of the femur via iliopsoas tendon |
| Action | Hip flexion, trunk flexion, lateral rotation of thigh |
| Blood supply | Lumbar branches of iliolumbar artery, deep femoral artery contributions |
| Nerve | Branches from lumbar plexus (L1–L3) |
Psoas major The psoas major is a deep-seated hip flexor and lumbar stabilizer located in the posterior abdominal wall between the thoracic cage and pelvis. It spans from the lower thoracic and lumbar vertebrae to the femur and acts with muscles and ligaments to influence posture and gait. Its relationships with lumbar nerves, pelvic organs, and adjacent muscles make it relevant in anatomy, surgery, and rehabilitation.
The muscle originates from the anterior surfaces of the bodies and transverse processes of the lower thoracic and lumbar vertebrae including T12 and L1–L5 and from the intervening intervertebral discs, and it descends inferolaterally to join with the iliacus beneath the inguinal ligament to insert on the lesser trochanter of the femur. Surrounding connective tissue includes the psoas fascia and the iliopsoas tendon, which interact with the iliac fascia, the sacroiliac ligaments, and the pelvic brim adjacent to the sacrum, coccyx, and pelvic inlet. Nearby osseous landmarks and articulations include the thoracic vertebrae, lumbar vertebrae, sacroiliac joint, femoral neck, and acetabulum; neurovascular neighbors comprise branches from the lumbar plexus, the iliolumbar artery arising from the internal iliac system, and tributaries connecting to the deep femoral and external iliac systems. Clinically relevant fascial planes involve relationships with the retroperitoneal space, the psoas compartment, and surgical corridors used by urologists, vascular surgeons, and orthopaedic teams from institutions such as Mayo Clinic, Johns Hopkins Hospital, and Cleveland Clinic for operations influenced by anatomy described in atlases by Netter and curricula at Harvard Medical School.
Primary actions include powerful flexion of the hip at the coxal joint and flexion of the trunk when the femur is fixed, working in concert with the iliacus, rectus femoris, sartorius, and tensor fasciae latae during gait, running, and climbing. It contributes to maintaining lumbar lordosis and resisting posterior pelvic tilt alongside the multifidus, quadratus lumborum, and erector spinae muscles during postural tasks and lifting described in guidelines from World Health Organization and occupational recommendations from Occupational Safety and Health Administration. As part of the kinetic chain it influences lower limb kinematics involved in events such as the Olympic Games and performance protocols used by teams like FC Barcelona and national squads at FIFA competitions; dysfunction can alter biomechanical patterns studied by researchers at Stanford University and University of Oxford.
Innervation is primarily via anterior rami of lumbar nerves L1–L3, contributions commonly documented in anatomical sources used by Royal College of Surgeons and by authors affiliated with University College London. The motor and sensory fibers pass through the psoas sheath and form part of the lumbar plexus elements including the femoral nerve and contributions that relate to branches implicated in procedures performed at centers such as Massachusetts General Hospital. Arterial supply arises from lumbar branch vessels including the iliolumbar artery, segmental lumbar arteries from the abdominal aorta, and collateral contributions from branches of the deep femoral and external iliac arterial systems, important in interventions described in textbooks used at Johns Hopkins University and case series from Mayo Clinic.
Pathologies include psoas abscesses, psoas syndrome, tendonitis, spasm, myofascial trigger points, retroperitoneal hemorrhage, and entrapment-related referred pain with presentations reported in clinical reviews from Cleveland Clinic and case reports in journals affiliated with American Medical Association. Psoas abscess has epidemiological ties to infections such as spinal tuberculosis described historically in reports from World Health Organization archives and in series from hospitals like St Thomas' Hospital and Royal London Hospital. Diagnostic evaluation employs imaging modalities including MRI, CT, and ultrasound as used in protocols at Mayo Clinic and Karolinska Institutet; management options span antibiotics, percutaneous drainage, surgical debridement, physiotherapy regimes developed at University of Toronto rehabilitation clinics, and interventional pain procedures performed in centers such as Cleveland Clinic pain management. Biomechanical implications affect outcomes after hip arthroplasty and lumbar fusion surgeries practiced at Hospital for Special Surgery and discussed by teams at Johns Hopkins Hospital.
Anatomical variations include accessory slips, fusion with the iliacus, anomalous origin from T11 or T10, and rare bifid or doubled bellies; such variants have been catalogued in cadaveric surveys conducted at institutions including University of Cambridge, Yale School of Medicine, and University of Pennsylvania. Variant tendon insertions, aberrant neurovascular passages, and asymmetries can alter surgical approaches in orthopedic, vascular, and urologic procedures performed at Karolinska University Hospital and impact interpretations in comparative anatomy studies by researchers at Smithsonian Institution and Natural History Museum, London.
Embryologically the muscle derives from myotomes of the paraxial mesoderm, specifically the lumbar somites, with molecular patterning influenced by HOX genes and signaling pathways studied in developmental labs at Massachusetts Institute of Technology and Max Planck Institute for Molecular Genetics. Evolutionary comparisons show homologies with iliopsoas components across mammals and adaptations in bipedal hominins that affect pelvic morphology as discussed by paleoanthropologists at Smithsonian Institution, University of Cambridge and in works by Richard Dawkins and researchers at University of California, Berkeley on locomotor evolution. Functional shifts associated with upright posture are addressed in analyses from American Journal of Physical Anthropology and field studies by teams working at sites like Olduvai Gorge and Laetoli.