Final answer:
The muscles that bend the knee joint at the back of the leg are known as the Hamstrings, which consist of the biceps fumoirs, semitendinosus, and semimembranosus.
Step-by-step explanation:
The muscles at the back of the leg that bend the knee joint are collectively called the Hamstrings. These include three long muscles which are the biceps femoris, semitendinosus, and semimembranosus. These muscles are responsible for flexing or bending the knee, and their tendons form the popliteal fossa, the diamond-shaped space at the back of the knee.
In contrast, the muscle group responsible for extending the knee is called the quadriceps fumoirs. This group is located in the anterior compartment of the thigh and includes the rectus fumoirs, vastus lateralis, vastus Medius, and vastus intermedius. Muscles often function in pairs, such as the hamstrings and quadriceps, which work together to move the legs back and forth during activities like running.