Which muscles are primarily responsible for knee flexion?

Knee flexion is primarily performed by the muscles located at the back of the thigh, collectively known as the hamstrings, along with a couple of other muscles that assist in this movement. The correct option lists the biceps femoris, semitendinosus, sartorius, and semimembranosus, all of which play a significant role in flexing the knee.

The biceps femoris is one of the key components of the hamstring group, located laterally. The semitendinosus and semimembranosus, which are also part of the hamstrings, lie more medially and help with flexion along with contributing to internal rotation of the knee when flexed. The sartorius, while primarily known for its role in hip flexion and abduction, assists in knee flexion as well.

Together, these muscles ensure effective bending of the knee joint, allowing for activities such as walking, running, and sitting. Understanding the muscle anatomy and their functions is crucial for recognizing how these muscles contribute to knee movements in various contexts, including rehabilitation and exercise.