Which muscle is a prime mover for knee extension?

Which muscle is a prime mover for knee extension?

• A. Biceps femoris
• B. Semitendinosus
• C. Rectus femoris
• D. Semimembranosus

Answer: (C)

Knee extension is driven by the quadriceps group on the front of the thigh, which straightens the leg by pulling on the patellar tendon to extend the tibia. The rectus femoris is one of these quadriceps muscles and contributes directly to extending the knee, making it the best choice for a primary knee extensor. The other muscles listed—biceps femoris, semitendinosus, and semimembranosus—are knee flexors located on the back of the thigh, so their primary action is bending the knee rather than straightening it.

Knee extension is driven by the quadriceps group on the front of the thigh, which straightens the leg by pulling on the patellar tendon to extend the tibia. The rectus femoris is one of these quadriceps muscles and contributes directly to extending the knee, making it the best choice for a primary knee extensor. The other muscles listed—biceps femoris, semitendinosus, and semimembranosus—are knee flexors located on the back of the thigh, so their primary action is bending the knee rather than straightening it.