Extensor

Extensor surface refers to the body surface which is on the outer aspect of a joint. For example, the dorsal body surface situated on the back of the elbow or that which is located in front of the knee joint.

The extensor surfaces cover the extensor muscles, which overlay the dorsal aspects of joints. Upon contraction, the extensor muscles straighten up the joint and pull the flexor surfaces away from each other, thereby increasing the angle between the bones.