Knee flexors.

• A. Hamstring muscles (biceps femoris, semimembranosus, semitendinosus, sartorius, gracilis, popliteus)
• B. Quadriceps femoris
• C. Gastrocnemius and soleus
• D. Tensor fasciae latae

Answer: (A)

Knee flexion happens when the leg bends at the knee, pulling the heel toward the back of the thigh. The main drivers are the hamstrings—the muscles at the back of the thigh (biceps femoris, semitendinosus, semimembranosus)—which cross the knee and shorten to bend it. In addition, a few muscles crossing the knee assist this action: sartorius and gracilis help by contributing to knee bending as synergists, and popliteus helps unlock the knee and aids flexion through tibial rotation.

Gastrocnemius crosses the knee and can contribute to knee flexion when the knee is bending, but its primary role is as a plantarflexor of the ankle; soleus does not cross the knee and doesn’t flex it. The quadriceps femoris, on the other hand, extends the knee, not flexes it, and tensor fasciae latae acts on the hip and stabilizes the knee rather than bending it.

So the muscles listed that actively produce knee flexion best match the action, making that option the correct one.