Psoas major

Origin

Transverse processes and intervertebral discs of all lumbar vertebrae and T12

Insertion

top of femur on the inside (shares tendon with iliacus)

Joints crossed

Hip and intervertebral joints of lumbar vertebrae

Joint action

Flexes the hip (origin fixed)

Pulls the trunk towards the legs – sit up action (insertion fixed)

Unilaterally: assists in lateral flexion of the trunk

Stabilises lumbar spine