Which muscle is responsible for plantar flexing the ankle and assisting with knee flexion?

The gastrocnemius muscle is the correct answer because it plays a dual role in both plantar flexion of the ankle and assistance with knee flexion. This muscle is one of the main calf muscles and originates at the femur, crossing both the knee and ankle joints. When it contracts, it is primarily responsible for pushing the foot downwards (plantar flexion), which is a key movement in activities such as walking, running, and jumping. Additionally, because it crosses the knee joint, it also aids in bending the knee when the foot is planted, thus facilitating knee flexion.

This contrasts with the soleus, which is also a plantar flexor but does not assist with knee flexion as it only crosses the ankle joint. The tibialis anterior has the opposite function, as it is involved in dorsiflexing the ankle, and the quadriceps are primarily responsible for extending the knee, making them unrelated to ankle plantar flexion. The specific functions of these muscles highlight the unique role of the gastrocnemius in lower limb dynamics.