Iliacus: Origin, Insertion, Nerve, Action & The "Hidden" Hip Flexor

The Iliacus is a large, fan-shaped muscle located deep within the pelvis. It lines the "bowl" of the hip bone (iliac fossa). While it is anatomically distinct from the Psoas Major, the two muscles join forces to form the Iliopsoas, the body's strongest hip flexor complex.

[Image of Iliacus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Superior two-thirds of the Iliac Fossa, inner lip of the Iliac Crest, and anterior Sacroiliac ligaments.
Insertion (Distal)	Lesser Trochanter of the Femur (joining the Psoas Major tendon).
Nerve Supply	Femoral Nerve (L2, L3).
Blood Supply	Medial Circumflex Femoral Artery and Iliolumbar Artery.
Primary Actions	Hip Flexion: Powerful flexor of the thigh. Anterior Pelvic Tilt: Pulls the pelvis forward if the legs are fixed. Stabilization: Stabilizes the hip joint.

Deep Dive: The Iliopsoas Complex

The Iliacus and Psoas Major are often referred to as a single unit (Iliopsoas) because they share a common insertion and action, but they are distinct in origin and innervation.

1. The "Bowl" Muscle

While the Psoas originates from the lumbar spine, the Iliacus originates from the pelvic bone itself. This means the Iliacus acts primarily on the hip, whereas the Psoas acts on both the lumbar spine and the hip.

2. The Gap

The Iliacus muscle belly fills the iliac fossa. As it travels downward, it passes under the Inguinal Ligament (through the muscular lacuna) to enter the thigh, where it finally merges with the Psoas tendon.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Iliacus is a "postural troublemaker." In people who sit all day, the Iliacus remains in a shortened position. Over time, it adapts by shortening (contracture), which pulls the pelvis into an Anterior Tilt when standing, causing hyperlordosis (swayback) and lower back pain.

Palpation

Palpating the Iliacus is invasive as it lines the inside of the pelvic bowl.
Technique: Have the patient lie supine with knees bent (to relax the abs). Curl your fingers inside the anterior rim of the Iliac Crest (medial to the ASIS). Press deeply laterally and posteriorly into the fossa. Ask the patient to gently lift their knee (flex hip) to feel the contraction.

⚠️ Clinical Pathology: Iliacus Abscess/Hematoma
Because the muscle is enclosed in a strong fascia (Iliac Fascia), bleeding (e.g., from hemophilia or anticoagulants) or infection can get trapped here. A large hematoma in the iliac fossa can compress the adjacent Femoral Nerve, causing quadriceps weakness and numbness in the anterior thigh.

Manual Muscle Testing (MMT)

Testing the Iliopsoas complex (which includes the Iliacus).

Testing Tip: To differentiate from the Rectus Femoris (which also flexes the hip), flex the knee. This puts the Rectus Femoris in active insufficiency (too short), forcing the Iliopsoas/Iliacus to do the work.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Sitting (legs dangling). Action: Patient lifts the thigh off the table (Hip flexion). Resistance: Applied at the distal anterior thigh (above the knee), pushing down. Grade 3: Lifts thigh. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Side-lying on non-test side. Therapist supports the test leg. Action: Patient flexes the hip (brings knee toward chest). Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate medial to the Sartorius origin, just distal to the inguinal ligament. Cue: "Try to lift your knee."

Frequently Asked Questions

Is Iliacus the same as Psoas?

No. They are separate muscles. Psoas attaches to the spine and is innervated by lumbar plexus branches (L1-L3). Iliacus attaches to the pelvis and is innervated by the Femoral Nerve. They merge only at their insertion.

How do you stretch the Iliacus?

The "Thomas Test" position or a kneeling lunge stretch. The key is to keep the lumbar spine neutral (don't let the back arch) and push the hips forward to extend the hip joint.

Does it rotate the hip?

Yes, slightly. It assists in External (Lateral) Rotation of the femur, especially when the hip is flexed.

Test Your Knowledge: Iliacus Quiz

1. Which nerve supplies the Iliacus?

2. Where does the Iliacus insert?

3. What is the primary action of the Iliacus?

4. The Iliacus originates from the:

5. Tightness in the Iliacus leads to which postural deviation?

6. A hematoma in the iliac fossa can compress which nerve?

7. The Iliacus passes deep to which ligament?

8. Which muscle helps Iliacus flex the hip but also extends the knee?

9. To test Iliacus strength without Rectus Femoris interference, you should:

10. True or False: The Iliacus attaches to the lumbar spine.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.