A neurosurgery resident is reviewing myotome testing for lower limb nerve root injuries. All of the following myotomes are correctly matched with their primary motor function EXCEPT:

Explanation

## Myotome-Function Correlation in the Lower Limb **Key Point:** A myotome is a group of muscles supplied by a single spinal nerve root. Accurate myotome testing is critical for localizing nerve root lesions in the lower limb. ### Lower Limb Myotome Map | Myotome | Primary Motor Function | Key Muscle(s) | Testing Position | | --- | --- | --- | --- | | L2 | Hip flexion | Iliopsoas | Supine, lift thigh | | L3 | Knee extension | Quadriceps femoris | Supine, straighten knee | | L4 | Ankle dorsiflexion | Tibialis anterior | Sitting, dorsiflex foot | | L5 | Great toe extension (hallux) | Extensor hallucis longus (EHL) | Sitting, extend big toe | | S1 | Ankle plantarflexion | Gastrocnemius, soleus | Standing, rise on toes | | S2 | Knee flexion | Hamstrings | Prone, flex knee | **High-Yield:** **L5 myotome is ankle dorsiflexion is INCORRECT.** L5 primarily tests **great toe extension (hallux extension)** via the extensor hallucis longus (EHL). Ankle dorsiflexion is actually an **L4 function**, supplied by the tibialis anterior muscle. ### Common Confusion: L4 vs L5 Dorsiflexion - **L4 myotome** → Ankle dorsiflexion (tibialis anterior) — this is the primary dorsiflexor - **L5 myotome** → Great toe extension (extensor hallucis longus) — this is the hallmark L5 test **Clinical Pearl:** In L5 nerve root compression (e.g., from L5–S1 disc herniation), patients lose the ability to extend the great toe and may develop a **foot drop** due to weakness of the extensor digitorum longus. However, ankle dorsiflexion may be relatively preserved because tibialis anterior (L4) is still functional. **Mnemonic:** **L-QUEUE** — L2 (hip flexion), L3 (Knee extension), L4 (dorsiflexion), L5 (great toe extension), S1 (plantarflexion). L5 is about the **toe**, not the ankle. ### Why This Matters Clinically Misidentifying L5 as ankle dorsiflexion can lead to: - Missed L5 radiculopathy diagnosis - Incorrect localization of disc herniation - Delayed surgical intervention