Which muscle is NOT a primary knee extensor?

• A. Rectus Femoris
• B. Vastus Lateralis
• C. Biceps Femoris
• D. Vastus Medialis

Answer: (C)

The main knee extenders are the quadriceps femoris group: rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. When these muscles contract, they pull on the patellar tendon, which in turn extends the tibia, straightening the knee. Rectus femoris crosses both the hip and knee, so it can influence hip movement, but it still acts to extend the knee. The vasti are pure knee extensors and contribute to kneeling, rising, and other actions requiring knee straightening.

The muscle that is not a primary knee extensor is the biceps femoris. It belongs to the hamstring group and primarily flexes the knee (and can assist hip extension). Its line of pull on the knee promotes bending rather than straightening, so it does not serve as a primary knee extensor.