What are the main flexors of the knee joint?

The main flexors of the knee joint are primarily the hamstring muscles: Semimembranosus, Semitendinosus, and Biceps Femoris. These muscles primarily function to bend the knee and are located at the back of the thigh. The Popliteus is also involved in flexing the knee, specifically helping to unlock the knee joint from a fully extended position, making it relevant in the context of knee flexion.

Each of these muscles plays a significant role in the action of knee flexion. The Semimembranosus and Semitendinosus provide powerful flexion and internal rotation of the tibia when the knee is flexed. The Biceps Femoris contributes to external rotation, and its longer head assists in flexion. The Popliteus, while not a primary flexor, is critical for initiating flexion when the knee is straight, which adds to its importance in this context.

In contrast, the other options include muscles that are either primarily knee extensors (like the quadriceps in option A and C) or target other actions rather than flexion. Therefore, the distinct role of the specified muscles in option B makes it the correct choice as they directly contribute to knee flexion.