This weeks continuation of small but important muscles that can create unusual pain is the Popliteus. This small muscle is a triangular shape that is on the back of the knee. From the picture you can see it has attachments on the tibia, the fibula and also onto the lateral meniscus. Fascially it sends strips into the arcuate popliteal ligament.
Functionally, its action is to "unlock" the knee. When the foot is on the ground as in walking and running, it laterally rotates the femur to allow flexion to happen. It's a very weak flexor. With the thigh fixed, leg free to move, it rotates the tibia medially. (think sitting) It's also a big player in keeping the femur from tracking over the tibial plateau when weight bearing.
I have found a lot of knee pain can occur when this muscle is tight that will mimic a lateral meniscus sprain. It is usually more present with running downhill or going downstairs. It's also a player when a patient/athlete can't quite achieve full leg extension. They usually don't even realize they have lost this last 5-10 degrees of motion.
We are born with lots of muscle spindles in this muscle. This means the brain is using this muscle to let it know what the knee joint is doing in space. I believe if it's tight, we lose some of this kinesthesia for the joint.
Females need to get out of high heels when this is tight. Essentially you are walking downhill with each step. This will just aggregate symptoms.
If you back of the knee pain that gets worse with downhill type activities think Popliteus.