Describe the origins (psoas major and iliacus) and insertion of the iliopsoas, and its main action.

• A. Origins: psoas major from T12-L5 vertebrae; iliacus from iliac fossa. Insertion: lesser trochanter of the femur. Action: primary hip flexor.
• B. Origins: gluteal lines; Insertion: greater trochanter. Action: abducts thigh.
• C. Origins: sacrum; Insertion: IT band. Action: extends thigh.
• D. Origins: external surface of ilium; Insertion: lesser trochanter. Action: external rotation.

Answer: (A)

The main idea is that the iliopsoas is the primary hip flexor, formed by the fusion of two muscles and inserting on a single bony landmark. Psoas major originates from the bodies and transverse processes of the lumbar spine (roughly T12 to L5) and the intervertebral discs, while iliacus originates from the iliac fossa and the inner lip of the iliac crest (and the ala of the sacrum). They join to form the iliopsoas, which inserts via a common tendon on the lesser trochanter of the femur. This arrangement makes the iliopsoas the strongest muscle for lifting the thigh toward the trunk, i.e., hip flexion. It can also assist in flexing the trunk when the thigh is fixed, and the psoas major portion can contribute to lateral flexion of the lumbar spine. Other options mix up the origins or insertions with parts of other muscles or assign actions that aren’t the iliopsoas’s primary role.