Which muscle is primarily responsible for extending the knee?

• A. Rectus Femoris
• B. Sartorius
• C. Gracilis
• D. Vastus Lateralis

Answer: (A)

Knee extension is carried out mainly by the quadriceps femoris group, which straightens the leg by pulling on the patellar tendon. The muscle shown here is part of that group and is the prominent front-of-thigh muscle that directly extends the knee when it contracts. It also crosses the hip, so it can influence hip movement, but its action at the knee is to straighten the leg. The other options don’t extend the knee: the sartorius and gracilis mainly flex the knee and assist hip movements, while the vastus lateralis is another knee-extensor muscle, but the rectus femoris is the representative knee extensor among the choices and is the one most closely associated with straightening the knee.