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BRPsoas major is a long, thick, fusiform muscle located in the lumbar region of the trunk lateral to the lumbar vertebrae and medial to the quadratus lumborum muscle. It belongs to the inner muscles of the hip group. The name ‘psoas’ is of Greek origin meaning ‘muscle of the loin’.
Psoas major is often considered with the iliacus muscle. It merges with iliacus as it passes inferolaterally into the thigh to insert onto the femur. Together, these muscles are known as the iliopsoas muscle.
Psoas major is a long muscle located immediately lateral to the vertebral column. It has an extensive origin from the:
Transverse processes of all the lumbar vertebrae
Vertebral bodies of T12-L4
Intervening intervertebral discs
The fibres of psoas major extend inferolaterally through the lesser pelvis towards the thigh. They pass along the pelvic brim and inferior to the inguinal ligament towards their merged insertion in the anterior thigh. Along its course, the lateral-most fibres of psoas major fuse with the fibres of the iliacus muscle to form the iliopsoas muscle. Iliopsoas passes deep to the inguinal ligament and anterior to the joint capsule of the hip to insert onto the lesser trochanter of the femur.
Psoas major is innervated by branches of the lumbar plexus. These are the anterior branches of spinal nerves L1-L3.
The actions of psoas major are flexion and lateral rotation of the thigh at the hip. It also flexes the trunk at the hip and flexes the trunk laterally. Psoas major is also a postural muscle. During standing, it stabilises the vertebral column by maintaining normal lumbar lordosis and indirect thoracic kyphosis.
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