The popliteus is a small muscle at the back of the knee.
It acts to unlock the knees when walking, by rotating the tibia inwards as the foot makes contact with the ground. It also facilitates sitting down and standing up. It also helps to pull the lateral meniscus out of the way when you squat.
The lateral meniscus - the one on the outer aspect of your knee - has an intimate relationship to the popliteus tendon entering the joint from the back of the knee, and this is the region we are referring to in this article. Because the relationship to the waterproof capsule is important in this discussion, I have drawn in the capsule, but you can see through it into the joint cavity.
The popliteus muscle & tendon
The tendon of the popliteus passes through a gap in the joint capsule on the lateral side. It then sweeps around the rounded part of the femur to attach at the side of the femur. There are other ligamentous connections around the lateral meniscus, as you can see, and they all help to stabilise the back of the knee.
Strains of the popliteus muscle may occur in association with tight hamstrings muscles. They tend to be treated with rest and gentle hamstrings stretches. Actual tears of the substance of the popliteus are less common, but do happen, and bleeding may occur into the joint.
Popliteus damage leading to significant incompetence of this structure, is generally in the form of an 'avulsion' (a 'tearing off') in the region where the tendon attaches to the femur. A little bit of femur bone is pulled off with the tendon, although the tendon itself is not actually injured - the fragment of bone can be screwed back on and usually heals well.
Tears of the actual tendon may also occur inside the joint and outside the capsule, and there may also be tears of the muscle itself.
In both tendon tears and avulsions, the tendon may retract back out through the natural hole in the capsule by which it entered the joint originally. It may thus disappear from view during arthroscopy, which is conducted from the front of the knee - and the diagnosis of the damage may be missed by the surgeon altogether!