Popliteus and the locked knee
Popliteus is a small muscle behind the knee that releases the lock of the straight, weight-bearing knee by rotating the femur laterally on the tibia, so flexion can begin.
Remember
With the foot planted, popliteus turns the femur laterally on the tibia to release the locked, fully extended knee, so that flexion can start.
Memory aid
the popliteus is the key that turns in the lock of the straight knee
Think of popliteus as a key that turns in the lock of the straight knee, so that the knee is released and can bend.
Popliteus is a small muscle lying behind the knee joint, forming part of the floor of the popliteal fossa (the hollow at the back of the knee). It arises from the lateral condyle of the femur (the rounded end of the thigh bone on the outer side) and from the lateral meniscus (a C-shaped pad of cartilage on the outer side of the knee), and it attaches to the upper tibia.
When you stand with the knee fully straight, the knee is held in a locked position. By the time the knee reaches full extension, the tibia has turned slightly outwards relative to the femur (the screw-home movement). With the foot planted, that means the femur has turned medially on the tibia. A locked knee can support the standing weight of the body, but the lock has to be released before the knee can bend.
With the foot planted, popliteus rotates the femur laterally on the tibia, which reverses the locking turn so that flexion can begin. With the foot off the ground, the same pull instead turns the tibia medially on the femur. The quadriceps femoris (the large muscle group at the front of the thigh) straightens the knee, and the hamstrings (the muscles at the back of the thigh) are the main flexors that bend it. Popliteus has a different job from both.
Popliteus also stands out within the deep posterior compartment (the layer of muscles behind the tibia and fibula, beneath the calf muscles). It is the only muscle of the four whose tendon does not pass behind the medial malleolus (the bony bump on the inner side of the ankle). All four are supplied by the tibial nerve.
Key facts
- Position
- popliteus lies behind the knee joint and forms part of the floor of the popliteal fossa.
- The lock
- as the knee reaches full extension the tibia turns slightly outwards relative to the femur (the screw-home movement), and the knee can then support standing weight.
- Foot planted
- popliteus rotates the femur laterally on the tibia, releasing the lock so that flexion can begin.
- Foot off the ground
- the same pull turns the tibia medially on the femur.
- Only the knee
- popliteus acts only on the knee, whereas its neighbours in the deep posterior compartment move the ankle and foot.
- Nerve supply
- all four deep posterior compartment muscles are supplied by the tibial nerve.
Exam traps
- Lateral rotation of the tibia on the femur does not release the knee, because it is the screw-home movement that locks the joint at full extension.
- With the foot planted, the femur does turn medially on the tibia as the knee reaches its locked position, but that is the locking direction, and popliteus does the reverse by turning the femur laterally.
- Popliteus does not extend the knee, because extension is produced by the quadriceps femoris and popliteus sits behind the joint.
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