Which muscles are the primary knee flexors?

• A. Quadriceps femoris
• B. Hamstrings, gastrocnemius
• C. Sartorius
• D. Popliteus

Answer: (B)

Knee flexion is driven mainly by muscles on the back of the thigh that pull the shin toward the thigh. The hamstrings are the primary knee flexors, generating most of the bending force at the knee. The gastrocnemius crosses both the knee and the ankle, so it can assist knee flexion, especially when the ankle is in plantarflexion. The quadriceps femoris extend the knee, so they don’t contribute to bending. Sartorius can assist with knee flexion but is not a primary mover, and the popliteus helps unlock the knee rather than act as a primary knee flexor. Thus, the primary knee flexors are the hamstrings, with gastrocnemius providing additional assistance.