Which of the following muscles is involved in knee flexion?

Knee flexion primarily involves the hamstrings, which are a group of muscles located at the back of the thigh. The hamstrings consist of three muscles: the biceps femoris, semitendinosus, and semimembranosus. When these muscles contract, they pull the lower leg towards the body, effectively bending the knee.

While other muscles listed, such as the gastrocnemius, also contribute to knee flexion to some extent, they are not the primary movers for this action. The quadriceps, located at the front of the thigh, are actually responsible for knee extension, which is the opposite movement. The sartorius does assist in flexing the knee but is more involved in hip flexion and abduction. Thus, the hamstrings are recognized as the main muscle group responsible for knee flexion in human movement.