Psoas major

Deep muscle — it lies under other muscles. X-ray makes the muscles over it see-through.

Origin

Bodies of T12 to L4 and transverse processes of L1 to L5

Insertion

Lesser trochanter of the femur (with the iliacus)

Action

Flexes the hip; helps flex the trunk

Nerve supply

Anterior rami of the lumbar spinal nerves (L1–L3)

The long part of the iliopsoas, running from the lumbar spine through the pelvis to the thigh. Because it starts on the spine, it can flex the trunk as well as the hip, as in sit-ups and leg raises.