Popliteus muscle

Origin and insertion

The popliteus muscle originates from the lateral condyle of femur by a strong tendon called popliteus tendon. The popliteus tendon courses in the groove for popliteus muscle on the lateral aspect of the lateral femoral condyle. This tendon is joined medially by tendinous fibers arising from the posterior horn of the lateral meniscus. In addition, the popliteofibular ligament connects the popliteus tendon to the medial aspect of the head of the fibula. 

The popliteus tendon then expands into a triangular muscle belly that passes inferomedially over the posterior joint capsule of the knee joint. The popliteus muscle inserts on the posterior surface of the proximal tibia (medial two-thirds) above the soleal line, which is the origin point of the soleus muscle.

Relations

The popliteus muscle is situated deep in the popliteal fossa, forming its floor. The popliteus tendon arises within the capsule of the knee joint, hence it is intracapsular. On its posterior aspect, the popliteus tendon is crossed by the arcuate popliteal ligament, the fibular collateral ligament and the tendon of biceps femoris.

Upon passing underneath the arcuate ligament, the popliteus tendon is replaced by the fleshy portion of the popliteus muscle. Posteriorly, the popliteus muscle is covered by an investing fascia that is reinforced by a dense aponeurotic expansion, largely derived from the tendon of semimembranosus muscle. Posterior to the investing fascia of popliteus are the gastrocnemius, plantaris, tibial nerve and popliteal artery and vein.

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Innervation

The popliteus muscle receives nervous supply from the tibial nerve (L4-S1), a branch of the sciatic nerve.

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Blood supply

The popliteus receives arterial blood supply mainly from branches of the popliteal artery, namely the inferior medial and lateral genicular arteries. In addition, it receives smaller contributions from the posterior tibial recurrent artery, the proximal part of the posterior tibial artery and the nutrient artery of the tibia.

Function

Despite its anatomical classification, the role of the popliteus muscle as a flexor of the knee is quite insignificant. However, the popliteus muscle plays an important role in initiating flexion of the fully extended (“locked”) knee. Thus, the popliteus is referred to as “the key to unlock the knee”, making it essential for the processes of walking, standing up, and sitting down.

The popliteus muscle can produce two types of actions; lateral rotation of the femur or medial rotation of the tibia. These actions are dictated by the phase of the gait cycle:

• During the closed chain phase of the gait cycle, the foot is in contact with the ground. The tibia is laterally rotated (about 5º) on the femur, which “locks” the knee joint. With the tibia fixed in position, the popliteus muscle acts on its origin and laterally rotates the femur on the tibia. This movement “unlocks” the knee and allows flexion to occur.  
• During the open chain (swing) phase of the gait cycle, the foot is above the ground and the tibia is free to move. Therefore, the popliteus muscle can act on its insertion to medially rotate the tibia on the femur. This action provides stability to the tibia during knee flexion.

The popliteus muscle is also the main stabilizer of the posterior knee region via its connections to the posterior joint capsule and popliteofibular ligament. In addition, the popliteus muscle pulls the lateral meniscus posteriorly during knee flexion and femoral lateral rotation, thus preventing its entrapment.