Which muscle assists in knee flexion?

Knee flexion primarily occurs when the bend at the knee joint decreases, and the hamstring muscles play a crucial role in this movement. The biceps femoris, one of the hamstring muscles, attaches to the ischial tuberosity of the pelvis and runs down the back of the thigh, inserting into the fibula. When it contracts, it pulls the lower leg towards the thigh, effectively resulting in knee flexion.

While the quadriceps group is primarily responsible for knee extension, and muscles like the gracilis and sartorius also assist in flexion to some extent, the biceps femoris is the most significant contributor to this action, especially in activities such as running and jumping. It not only flexes the knee but also helps in externally rotating the leg when the knee is bent. Thus, its strong involvement in the knee flexion movement makes it the correct answer.