Which muscle is responsible for flexing the knee?

The biceps femoris is indeed a key muscle responsible for flexing the knee. It is one of the hamstring muscles located at the back of the thigh. During activities such as walking, running, or squatting, the biceps femoris contracts to bend the knee, allowing for motion and stability.

The quadriceps femoris, while a powerful knee extensor, plays no role in flexing the knee and instead works to straighten the leg at the knee joint. Similarly, the sartorius, although it assists in knee flexion, has a more significant role in flexing the hip and does not primarily act on the knee. Lastly, the gluteus medius primarily facilitates hip abduction and stabilization rather than flexion at the knee joint. Understanding the specific roles of these muscles provides clarity on how knee movement is orchestrated through muscle contractions.