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 semitendinosus is one of the hamstring muscles whose tendon is used for cruciate ligament reconstruction. 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.