The area behind the knee, the popliteal fossa, is packed with structures vital for movement and sensation. This region houses muscles, tendons, blood vessels, and nerves. The muscles passing through or originating here are responsible for two of the leg’s primary actions: bending the knee and moving the ankle.
The Hamstring Muscle Group
The hamstring muscle group forms the upper boundary of the area behind the knee, originating on the pelvis and running down the back of the thigh. This group consists of three muscles: the semitendinosus, the semimembranosus, and the biceps femoris. They are biarticular, meaning they cross and act upon both the hip and the knee joint. Their primary functions are to flex the knee and extend the hip joint.
The tendons of the semitendinosus and semimembranosus insert on the medial, or inner, side of the tibia. The biceps femoris tendon, the most lateral of the group, attaches to the fibula, the smaller bone of the lower leg. When the knee is bent, the hamstrings also assist in rotating the lower leg, with the medial muscles turning it inward and the biceps femoris turning it outward.
The Gastrocnemius Muscle
The gastrocnemius muscle, commonly known as the calf muscle, makes up the lower boundary of the space behind the knee. It is a two-headed muscle, with its medial and lateral heads originating from the lower end of the femur, just above the knee joint. Because it originates above the knee, the gastrocnemius assists with knee flexion.
Its main function is to plantarflex the foot, which is the action of pointing the toes downward. The two heads merge lower down with the soleus muscle to form the Achilles tendon, which connects to the heel bone.
The Popliteus Muscle
Deep within the popliteal fossa, beneath the hamstring and gastrocnemius muscles, lies the small, triangular popliteus muscle. The popliteus originates on the lateral side of the femur and inserts onto the back surface of the tibia.
The primary action of the popliteus is “unlocking” the knee from a fully extended position. When the leg is straight, the knee is naturally “locked,” and the popliteus initiates the internal rotation of the tibia to allow for flexion. This rotational movement is necessary to begin bending the knee and is a regular part of the walking cycle.
Understanding Pain and Injury
Pain originating in the muscles behind the knee is commonly related to strains or inflammation. Hamstring strains, often called a “pulled hamstring,” frequently occur during sudden acceleration or sprinting. These injuries cause sudden, sharp pain in the back of the thigh, often felt high up near the gluteal fold.
Gastrocnemius strains, sometimes called a calf pull, involve the muscle’s medial head and are common in sports requiring quick push-offs, such as tennis or basketball. A person experiencing this strain may feel a sudden “pop” sensation in the calf. Popliteus tendonitis is less common but causes tenderness and pain at the back and outer area of the knee, often worsening with activity like walking downhill.
This is usually an overuse injury affecting runners or skiers. Pain or swelling in this area can also be caused by a Baker’s cyst, which is a fluid-filled sac, not a muscle injury. This cyst forms when excess joint fluid leaks into the popliteal fossa, often resulting from underlying knee issues like arthritis or a meniscus tear. If the pain is severe, accompanied by significant swelling, or you cannot bear weight, seek medical attention to rule out serious conditions, such as a blood clot.