Which joint is the primary hinge joint of the lower limb?

• A. Shoulder
• B. Knee
• C. Elbow
• D. Ankle

Answer: (B)

Movement in a hinge joint is mainly in one plane, bending and straightening. In the lower limb, the knee is the main hinge because its primary action is flexion and extension of the leg, which are essential for standing, walking, and running. It bears the body's weight and coordinates with ligaments and cartilage to provide stability during movement. Other joints listed perform different roles: the elbow is a hinge in the upper limb, the ankle does hinge-like movements for foot motion, and the shoulder is a ball-and-socket joint that allows a wide range of directions. The knee’s role as the primary hinge joint of the lower limb comes from its key function in locomotion and weight-bearing through flexion and extension.