What is the primary muscle responsible for flexing the knee?

The primary muscle responsible for flexing the knee is the hamstring group. The hamstrings, which consist of three main muscles—semimembranosus, semitendinosus, and biceps femoris—are located at the back of the thigh. When these muscles contract, they pull the lower leg towards the body, effectively bending the knee.

In contrast, the quadriceps, which are located at the front of the thigh, perform the opposite action by straightening the knee. The sartorius, while it does assist in flexing the knee, also plays a role in hip flexion and does not have the primary responsibility for knee flexion. The calf muscle, primarily involved in movements at the ankle, does not play a direct role in flexing the knee at all. Therefore, the hamstrings are specifically and uniquely suited to this action, making them the primary muscles involved in knee flexion.