Which muscle is primarily involved when a person performs flexion of the knee?

The hamstrings are primarily responsible for knee flexion. This group of muscles, located at the back of the thigh, consists of the biceps femoris, semitendinosus, and semimembranosus. When a person bends their knee, these muscles contract to pull the lower leg towards the back of the thigh, effectively causing flexion at the knee joint.

In this movement, the hamstrings work in opposition to the quadriceps, which are primarily responsible for knee extension. Therefore, while the quadriceps are important for straightening the knee, they play no role in its flexion. The gastrocnemius, although involved in knee flexion as well (because it crosses the knee joint), primarily acts as a calf muscle for plantar flexion of the ankle. The gluteus maximus, on the other hand, is mainly involved in hip movements such as extension and external rotation, rather than knee flexion.

Thus, the hamstrings are the key muscles utilized during knee flexion, making this the correct choice.