Which muscle group is the primary extensor of the knee?

• A. Quadriceps femoris
• B. Hamstrings
• C. Gastrocnemius
• D. Rectus abdominis

Answer: (A)

Knee extension is produced mainly by the quadriceps femoris, a large group of muscles on the front of the thigh. When it contracts, it straightens the leg by pulling the quadriceps tendon, which continues as the patellar tendon to the tibial tuberosity. The patella acts as a fulcrum, increasing leverage and making the extension movement efficient. The four heads (rectus femoris plus the three vastus muscles) work together to extend the knee through a wide range of motion, which is essential for standing up, walking, and kicking.

The hamstrings lie on the back of the thigh and bend the knee, opposing extension. The gastrocnemius crosses the knee as well as the ankle and can assist knee flexion, particularly when the foot is planted, but it is not the primary extender. The rectus abdominis is a trunk muscle and has no role in extending the knee.