The popliteus is a small triangular muscle at the back of the knee whose most critical function is unlocking the knee from full extension, rotating the tibia internally by approximately 5 degrees to release the screw-home mechanism before flexion can begin. Without this small rotational unlocking, the knee cannot initiate flexion from a fully extended standing position, making the popliteus essential for every step taken from a standing start.
| Origin | Lateral femoral condyle via a tendon that passes within the posterolateral knee joint capsule; Posterior aspect of the lateral meniscus via popliteomeniscal fascicles |
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| Insertion | Posterior surface of the tibia above the soleal line |
| Nerve Supply | Tibial nerve (L4, L5, S1) |
| Blood Supply | Popliteal artery |
| Actions | Internal rotation of the tibia to unlock the fully extended knee; Flexion of the knee; Restrains anterior movement of the lateral femoral condyle to assist the PCL |
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It initiates knee flexion from full extension by internally rotating the tibia to break the close-packed locked position that provides passive stability during upright standing, acting as the key that unlocks the knee joint.
Popliteus tendinopathy is an underrecognised cause of posterolateral knee pain in runners, producing pain that characteristically worsens when running downhill because the popliteus works eccentrically to resist excessive tibial external rotation on the slope. Popliteus injuries are also associated with posterolateral corner knee injuries in contact sport and are assessed with the dial test for posterolateral rotatory instability.
The popliteus tendon is palpable at the posterolateral knee just anterior and inferior to the lateral collateral ligament with the knee flexed to 90 degrees and the tibia internally rotated, a position that brings the tendon forward from behind the lateral meniscus.
Overuse degeneration producing posterolateral knee pain that worsens with downhill running and is reproduced by resisted internal tibial rotation with the knee flexed to 90 degrees.
Acute disruption usually as part of a posterolateral corner injury from a varus contact force, producing posterolateral instability confirmed by a positive dial test at 30 and 90 degrees of knee flexion.
