Which of the following lists the prime movers for knee extension?

• A. Biceps femoris, Semitendinosus, Semimembranosus
• B. Rectus femoris, Vastus intermedius, Vastus medialis, Vastus lateralis
• C. Tibialis anterior
• D. Gastrocnemius

Answer: (B)

Knee extension is produced mainly by the quadriceps femoris group. This four-muscle group—rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius—pulls on the patella via the quadriceps tendon, converting knee flexion into full straightening of the leg. Rectus femoris crosses the hip too, so it can influence hip movement, but its crucial role at the knee is extending it. The vastus muscles act as pure knee extensors and work together to produce the primary extension torque.

In contrast, the hamstrings (biceps femoris, semitendinosus, semimembranosus) bend the knee, opposing extension. The tibialis anterior mainly dorsiflexes the ankle. The gastrocnemius crosses both the knee and ankle and primarily plantarflexes the foot; its knee action is not that of a primary knee extensor. So the four quadriceps muscles are the prime movers for knee extension.