The area directly behind the knee, known anatomically as the popliteal fossa, contains a complex arrangement of muscles, nerves, and blood vessels. These muscles are responsible for nearly all movements involving bending the knee and play a significant role in hip and ankle movement. They provide the primary power for activities like walking, running, and climbing stairs, contributing greatly to the stability and mobility of the lower limb.
The Hamstring Group: The Primary Muscles of the Posterior Thigh
The hamstring muscle group is the largest muscular structure forming the upper boundary of the popliteal fossa, running down the back of the thigh. This group consists of three distinct muscles: the biceps femoris, the semitendinosus, and the semimembranosus. Most of these muscles originate high on the pelvis, specifically at a bony prominence called the ischial tuberosity, often referred to as the sit bone.
The hamstrings cross both the hip and knee joints, giving them dual functions in movement. Their collective primary action is to flex the knee, pulling the lower leg backward toward the thigh. They also work to extend the hip.
The biceps femoris muscle lies on the lateral side of the thigh and inserts onto the head of the fibula near the knee. The semitendinosus and semimembranosus muscles are positioned on the medial side of the thigh, inserting onto the tibia. This arrangement allows the hamstrings not only to bend the knee but also to rotate the lower leg when the knee is already bent.
The Upper Calf and Deep Knee Muscles
The lower half of the popliteal region is dominated by the upper portions of the calf muscles, particularly the large, two-headed gastrocnemius muscle. Its two heads originate from the back surfaces of the femoral condyles, the rounded ends of the femur just above the knee joint. Because it originates above the knee, the gastrocnemius assists the hamstrings in flexing the knee, though this is a secondary function.
The main role of the gastrocnemius is to plantarflex the foot, pointing the toes and foot downward. Its muscle fibers converge to form the Achilles tendon, which attaches to the heel bone. The size of this muscle means it generates substantial force for propulsion during running and jumping.
Deep to the gastrocnemius lies a small, flat muscle called the popliteus, important for smooth knee function. This muscle originates on the lateral femoral condyle and inserts into the back of the tibia. The popliteus has a unique action of internally rotating the tibia to “unlock” the knee from a fully straightened position, allowing the leg to bend.
Without the popliteus, initiating the bending motion from a locked, straight-leg position would be difficult and unstable. The popliteus also helps to prevent the forward sliding of the femur on the tibia. A smaller accessory muscle, the plantaris, is also found in this deep region, recognized by its long, thin tendon.
Common Causes of Posterior Knee Pain
Pain in the back of the knee often stems from issues within these frequently used muscle and tendon structures. One common muscular cause is a hamstring strain, or “pulled hamstring,” which occurs when muscle fibers tear due to sudden, forceful movements. This pain is typically felt high up in the thigh but can radiate down toward the knee where the tendons insert.
Tendinopathy, the degeneration of a tendon, can affect the tendons of the hamstrings, the gastrocnemius, or the popliteus, causing localized pain and tenderness. Repetitive strain from activities like sprinting or running frequently contributes to these tendon issues. Popliteus tendinopathy often causes pain on the outer, back side of the knee.
Tightness in the gastrocnemius and hamstrings can also lead to secondary problems, including the formation of a Baker’s cyst. This cyst is a fluid-filled sac that bulges into the popliteal fossa, usually resulting from excess joint fluid pushed out of the knee joint. While the cyst itself is not a muscle issue, tightness and strain can exacerbate underlying knee conditions that lead to its development.