Which muscle extends the knee and flexes the hip?

Which muscle extends the knee and flexes the hip?

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

Answer: (A)

Extending the knee while flexing the hip is the job of a muscle that crosses both joints. The rectus femoris does cross the hip and the knee, making it biarticular. When it contracts, it helps lift the thigh at the hip (hip flexion) and straighten the leg at the knee (knee extension). The other quadriceps muscles—vastus lateralis, vastus medialis, and vastus intermedius—attach across only the knee and thus act mainly to extend the knee, not to flex the hip. Because rectus femoris is the only one among them that spans both joints, it uniquely performs both actions.

Extending the knee while flexing the hip is the job of a muscle that crosses both joints. The rectus femoris does cross the hip and the knee, making it biarticular. When it contracts, it helps lift the thigh at the hip (hip flexion) and straighten the leg at the knee (knee extension). The other quadriceps muscles—vastus lateralis, vastus medialis, and vastus intermedius—attach across only the knee and thus act mainly to extend the knee, not to flex the hip. Because rectus femoris is the only one among them that spans both joints, it uniquely performs both actions.