Popliteus
| Popliteus | |
|---|---|
| Muscle Type | |
| Origin | Lateral condyle of femur and lateral meniscus |
| Insertion | Posterior surface of tibia above soleal line |
| Action | Unlocks the knee by medially rotating tibia (or laterally rotating femur), weak knee flexion, stabilizes knee |
| Synergists | |
| Antagonists | Biceps Femoris, Tensor Fascia Lata, Quadriceps Femoris |
| Spinal innervation | L4, L5, S1 |
| Peripheral Innervation | Tibial nerve |
| Vasculature | Popliteal artery (genicular branches) |
The popliteus is a small, short muscle at the back of the knee that unlocks the knee from full extension by internally rotating the tibia (or externally rotating the femur) and also stabilizes the knee by resisting lateral rotation of the tibia.
Muscle Type
Popliteus is a triangular muscle of the deep posterior knee region (forming the floor of the popliteal fossa). It is relatively small and lies behind the knee joint capsule. Unlike the long leg muscles, popliteus acts primarily on the knee joint itself rather than the foot or ankle. It is sometimes described as an “unlocking” muscle for the knee.
Origin
Popliteus originates from the lateral surface of the lateral femoral condyle, specifically from a pit/depression on the condyle, and also from the lateral meniscus of the knee (via the popliteomeniscal fiber). In essence, it arises inside the knee joint capsule, attaching to the lateral femur near the epicondyle.
Insertion
Its tendon passes inferomedially, under the lateral collateral ligament. Popliteus inserts on the posterior surface of the tibia, above the soleal line (in the proximal tibia). This broad insertion on the tibia’s proximal posterior aspect is just distal to the knee joint. The muscle fibers run obliquely from lateral (femur) to medial (tibia) across the back of the knee.
Action
Popliteus’ primary action is to “unlock” the knee from full extension. It does this by internally rotating the tibia relative to the femur when the foot is off the ground (open chain). If the foot is planted (closed chain), it externally rotates the femur on the fixed tibia (since the tibia can’t move, the femur rotates laterally) – this achieves the same relative motion needed to initiate knee flexion. By undoing the “screw-home” mechanism (the locking mechanism of the knee in full extension where the tibia is externally rotated), popliteus allows the knee to flex. Additionally, popliteus weakly flexes the knee (assisting hamstrings in initiating knee bend). It also helps stabilize the knee by resisting excessive external rotation of the tibia and pulling the lateral meniscus posteriorly during knee flexion (protecting the meniscus from being pinched).
Synergists
Popliteus works with the hamstring muscles (especially the medial hamstrings: semitendinosus and semimembranosus) to medially rotate the tibia and flex the knee. At the very beginning of knee flexion, popliteus is active to unlock the joint, then the hamstrings take over the bulk of knee flexion. The pes anserinus muscles (gracilis, sartorius, semitendinosus) also contribute to internal rotation of the tibia at the flexed knee, synergizing with popliteus’ action of tibial internal rotation. During weight-bearing, popliteus synergizes with the quadriceps to maintain knee stability by fine-tuning the rotational alignment of the tibia and femur.
Antagonists
The main antagonists to popliteus in terms of rotation are the biceps femoris (which externally rotates the tibia, opposite to popliteus’ internal rotation) and the tensor fasciae latae (via iliotibial band, which tends to externally rotate the tibia or internally rotate the femur when the knee is slightly flexed). In full knee extension, the locked position (tibial external rotation) is essentially the opposite state of what popliteus creates, and that locked position is maintained by the shape of joint surfaces and taut ligaments rather than by a specific muscle acting as an antagonist. For knee flexion, the antagonists are the knee extensors (quadriceps femoris) which extend/lock the knee.
Spinal Innervation
Popliteus is innervated by L4, L5, S1 nerve fibers (primarily L5–S1) via the tibial nerve. These segments align with the innervation of the other deep posterior compartment muscles.
Peripheral Innervation
The tibial nerve (a branch of the sciatic nerve) innervates popliteus. In fact, the nerve to popliteus branches off the tibial nerve high in the popliteal fossa, often near the fibular neck area, and runs into the muscle’s proximal aspect. This branch also often supplies the tibialis posterior and FDL (as part of a common trunk) before reaching popliteus.
Vasculature
Popliteus is supplied by the popliteal artery via its genicular branches. The medial inferior genicular artery and lateral inferior genicular artery (branches of the popliteal artery encircling the knee) provide blood to the popliteus region. Additionally, muscular branches directly from the popliteal artery can feed popliteus. Venous drainage is through the popliteal vein.
Clinical Application
Injury to the popliteus is relatively uncommon compared to other knee structures, but strain can occur in trauma where the knee is forcefully rotated (as in certain twisting injuries). Popliteus tendinopathy or strain may cause posterolateral knee pain. A dysfunctional popliteus can contribute to slight instability in the knee, especially in controlling the terminal phase of extension. Popliteus helps retract the lateral meniscus during knee flexion; thus, weakness or injury could theoretically increase risk of lateral meniscus injury (the lateral meniscus may not move enough and get caught). In posterolateral corner injuries of the knee (involving structures that resist external rotation and varus stress), popliteus and its tendon are often involved – such injuries can lead to difficulty “unlocking” the knee and a feeling of instability. Rehabilitation of popliteus often overlaps with general knee rehab: strengthening internal rotators of the tibia and knee flexors (e.g., through resisted knee flexion exercises) will engage popliteus. Clinically, the popliteus is tested by the patient seated with legs dangling: ask them to rotate the foot medially (internally rotate tibia) – difficulty or pain in this motion might indicate popliteus injury. Popliteus also plays a role in the screw-home mechanism: in patients with extension lag or inability to fully extend or flex the knee, popliteal muscle tightness or weakness could be a factor. It is a relatively small muscle, but its function is essential for fluid knee motion from locked to unlocked positions.
