This is an illustration of the back of a right knee.
The main muscles at the back of the knee are the hamstrings muscles at the back of the thigh and the gastrocnemius muscles at the back of the calf.
In the calf you can see the bulky gastrocnemius muscle. Under it lies the soleus muscle (not visible on image). The tendons of the two combine to form the strong Achilles tendon at the back of the ankle.
The long thin tendon of the semitendinosus muscle is frequently 'harvested' by the surgeon for use in cruciate ligament reconstruction. It is commonly referred to as one of -the 'pes' or 'pes anserinus' tendons (semitendinosus, gracilis, sartorius) which insert on the medial (inner) aspect of the tibia in a compound tendon which looks a bit like a goose's foot (hence the latin name 'pes anserinus').
The 'gracilis' muscle is another whose tendon is often used, together with that of semitendinosus, to make a replacement cruciate ligament.The gracilis muscle is, however from a different group of muscles known as the 'adductors' and is not technically a hamstrings muscle.