Which muscle is primarily responsible for flexing the knee?

The hamstrings are primarily responsible for flexing the knee because they are a group of three muscles located at the back of the thigh: the biceps femoris, semitendinosus, and semimembranosus. When these muscles contract, they flex the lower leg at the knee joint, bringing the heel towards the buttock. This action is critical in activities such as running, walking, and cycling, where bending of the knee is necessary for movement and propulsion.

The quadriceps, while also significant in leg movement, are mainly responsible for extending the knee rather than flexing it. The gastrocnemius, located in the calf, also aids in knee flexion but is not the primary muscle for this action since its main function is to plantarflex the ankle. The soleus, which lies beneath the gastrocnemius, also assists in plantarflexion of the foot and does not play a role in knee flexion. Thus, the hamstrings are indeed the correct choice for their primary role in flexing the knee.