**Core Concept:** Locking of the knee joint is a clinical sign that occurs due to mechanical or neurological causes, causing a temporary inability to extend or flex the knee. Locking can be caused by various factors such as muscle spasm, ligamentous instability, meniscal or chondral lesions, and nerve entrapment. Understanding the anatomy and physiology of the knee joint is essential to identify the cause of locking.

**Why the Correct Answer is Right:** The correct answer, **B**: **Flexor Hallucis Brevis (FHB)** is a muscle in the foot that plays a crucial role in maintaining the stability of the knee joint and preventing flexion contracture. When FHB contracts, it helps to extend the knee joint by pulling the medial malleolus towards the midline. This action prevents excessive flexion of the knee, which is essential for maintaining joint stability.

**Why Each Wrong Option is Incorrect:**

A. **Gastrocnemius:** Although the gastrocnemius muscle is involved in knee flexion, it is not considered a locking muscle for the knee. Its primary function is to flex the ankle and knee joint, not to prevent excessive flexion or locking of the knee.

C. **Soleus:** Similar to gastrocnemius, soleus is also involved in knee flexion but is not a primary locking muscle for the knee joint. Its primary function is to flex the ankle and knee joint, which does not contribute to preventing excessive flexion or locking of the knee.

D. **Extensor Hallucis Brevis (EHB):** EHB is a muscle in the foot that primarily contributes to plantarflexion of the ankle joint, not locking of the knee joint. Its role in preventing excessive flexion of the knee is minimal, making it an incorrect answer for this question.

**Clinical Pearl:** Understanding the role of the muscles in the lower limb is essential for recognizing and treating patients with knee locking symptoms. In clinical practice, identifying the cause of knee locking is crucial for appropriate management and rehabilitation strategies.