Which muscle group primarily extends the knee?

• A. Quadriceps femoris
• B. Hamstrings
• C. Gastrocnemius
• D. Biceps femoris

Answer: (A)

Extending the knee is carried out mainly by the quadriceps femoris, a group of four muscles on the front of the thigh. When they contract, they pull on the quadriceps tendon, straighten the leg by pulling the patella, and transmit force through the patellar ligament to the tibia. The other muscles listed have different primary roles: the hamstrings, including the biceps femoris, bend the knee; the gastrocnemius crosses both the knee and ankle but is primarily involved in plantarflexing the foot, with only a secondary, non-primary role in knee motion. So the quadriceps femoris is the main knee extender.