What is the function of the common extensor?

• A. Flexes the leg
• B. Extends the leg
• C. Extends the knee only
• D. Rotates the leg

Answer: (B)

The common extensor is responsible for straightening the leg. When these muscles contract, they extend the limb at the knee (stifle) and help push off and bear weight, which is the primary action in locomotion. This is the opposite of flexion, which is done by the flexor muscles. The idea that it only extends the knee is too narrow, since the extensor group works to extend the leg as a whole and stabilize the limb during movement. It does not primarily rotate the leg. So, extending the leg is the best description of its function.