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The popliteus muscle in the leg is used for unlocking the knees when walking, by laterally rotating the femur on the tibia during the closed chain portion of the gait cycle (one with the foot in contact with the ground). In open chain movements (when the involved limb is not in contact with the ground), the popliteus muscle medially rotates the tibia on the femur. It is also used when sitting down and standing up. It is the only muscle in the posterior (back) compartment of the lower leg that acts just on the knee and not on the ankle. The gastrocnemius muscle acts on both joints.
Popliteus muscle (shown in red)
Dissection video (2 min 22 s)
|Origin||lateral femoral epicondyle|
|Insertion||posterior surface of tibia proximal to soleus line|
|Actions||Medially rotates tibia on the femur if the femur is fixed (sitting down) or laterally rotates femur on the tibia if tibia is fixed (standing up), unlocks the knee to allow flexion (bending), helps to prevent the forward dislocation of the femur while crouching|
|Latin||Musculus popliteus, poplit=ham (pit) of the knee|
|Anatomical terms of muscle|
The popliteus muscle originates from the lateral surface of the lateral condyle of the femur by a rounded tendon. Its fibers pass downward and medially. It inserts onto the posterior surface of tibia, above the soleal line. The muscle arises within the capsule of knee joint and its tendon separates the lateral meniscus from the lateral ligament of the joint.
Peroneotibialis, 14% of population. Origin is inner side of the head of the fibula, insertion into the upper end of the oblique line of the tibia, it lies beneath the popliteus.
Another variant, the cyamella, is a small sesamoid bone embedded in the tendon of the popliteus muscle. It is rarely seen in humans, but has been described more often in other primates and certain other animals. 
The popliteus assists in flexing the leg upon the thigh; when the leg is flexed, it will rotate the tibia inward.
It is especially called into action at the beginning of the act of bending the knee, in as much as it produces the slight inward rotation of the tibia, which is essential in the early stage of this movement.
When the knee is in full extension, the femur slightly medially rotates on the tibia to lock the knee joint in place. Popliteus is often referred to as the "Key" to unlocking the knee since it begins knee flexion by laterally rotating the femur on the tibia.
Popliteus is also attached to the lateral meniscus in the knee and draws it posteriorly during knee flexion to prevent crushing the meniscus between the tibia and femur as the knee flexes.
Muscles of deep posterior compartment of the right leg
- Strickland, Justin P.; Fester, Eric W.; Noyes, Frank R. (2017-01-01), Noyes, Frank R.; Barber-Westin, Sue D. (eds.), "2 - Lateral and Posterior Knee Anatomy", Noyes' Knee Disorders: Surgery, Rehabilitation, Clinical Outcomes (Second Edition), Elsevier, pp. 23–35, doi:10.1016/b978-0-323-32903-3.00002-0, ISBN 978-0-323-32903-3, retrieved 2021-03-01
- Gray, Henry. 1918. Anatomy of the Human Body. Page 485
- Akansel, Gur; Inan, Nagihan; Sarisoy, H. Tahsin; Anik, Yonca; Akansel, Sertaç (2006). "Popliteus muscle sesamoid bone (Cyamella): Appearance on radiographs, CT and MRI". Surgical and Radiologic Anatomy. 28 (6): 642–645. doi:10.1007/s00276-006-0134-8. PMID 17066262. S2CID 13339926.
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