Which of the following muscle groups are the prime movers for knee flexion?

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

Answer: (C)

Knee flexion is bending the knee, and the main muscles that pull the shin toward the thigh are the hamstrings on the back of the thigh. The hamstrings—biceps femoris, semitendinosus, and semimembranosus—cross the knee joint and contract to bend it, making them the prime movers for knee flexion.

In contrast, the muscles in front of the thigh—the quadriceps, including vastus lateralis and rectus femoris—primarily straighten the knee (knee extension). Rectus femoris also crosses the hip and helps with hip flexion, but at the knee it acts to extend, not flex. The tibialis anterior acts on the ankle (dorsiflexion) and doesn’t contribute to knee flexion.

So, the hamstrings are the main drivers of knee flexion.