The elbow and the knee are examples of which joints?

• A. Hinge joints
• B. Ball and Socket joints
• C. Pivot joints
• D. Saddle joints

Answer: (A)

The key idea is movement in one main direction. The elbow and knee mainly bend and straighten, like a door hinge, allowing flexion and extension with a single dominant axis of rotation. That simple back-and-forth motion is what defines a hinge joint. The elbow’s primary action is between the upper arm bone and the forearm bone, and the knee primarily between the thigh bone and the shinbone, both designed to swing in one plane and be stabilized to limit other movements. Some extra motion can occur in nearby joints or when the knee is flexed, but the main function is hinge-like.

Other joint types would let you move in multiple directions or rotate around an axis, which isn’t the typical action of the elbow or knee. Ball-and-socket joints (shoulder, hip) spin and rotate freely; pivot joints (like the neck) rotate around a single axis; saddle joints allow a different combination of movements but aren’t the primary action of these two joints.