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Muscles that act on the knee joint exhibit two actions, flexion and extension of the knee. Knee flexion is the result of the hamstring muscles as the prime movers and the popliteus and sartorius muscles as synergists. More specifically, the sartorius muscle aids in flexion while the popliteus muscle acts to unlock the knee to allow for flexion to occur. Extension of the knee is the result of the quadriceps femoris muscle, the most powerful muscle in the body.