Which muscle acts as a flexor at the knee joint and also aids in plantarflexion?

The gastrocnemius is the correct choice because it is a two-joint muscle that plays a significant role in both knee flexion and plantarflexion of the foot. When the knee is in a flexed position, the gastrocnemius helps to bend the knee while simultaneously enabling the foot to point downward—a motion known as plantarflexion. This dual function is a key characteristic of the gastrocnemius, making it essential for activities such as walking, running, and jumping where both knee flexion and ankle movement are involved.

While the biceps femoris, semitendinosus, and popliteus are also muscles that influence the knee joint, they do not aid in plantarflexion. The biceps femoris primarily functions as a knee flexor and external rotator of the knee. The semitendinosus, along with being a knee flexor, is primarily focused on internal rotation and does not contribute to plantarflexion. The popliteus assists in unlocking the knee from a fully extended position but does not have a role in plantarflexion of the ankle. Thus, the gastrocnemius uniquely fulfills both functions necessary for the question.