hamstring group is the primary muscle group for flexing the knee

The plantaris is the smallest muscle in the human leg. It is used to flex the knee joint and is absent in about 7 percent of the human population.

The main muscle used would be the tibialis anterior

The Gastrocnemius and the Soleus flexes the knee and the foot. The Plantaris is an unimportant muscle. Many people do not have it and doctors tend to use this muscle for tendon graphs.

The plantaris, gastrocnemius and soleus muscles attach (via the Achilles tendon) to the superior aspect of the tuber calcanei.