Pain Behind the Knee - Popliteus

There are many reasons for pain behind the knee. Your hamstring and gastroc both cross over the knee, so one of these muscles could be irritated. You could have a Baker’s cyst (benign), or have a lymphatic backup. One other possibility is an irritation of the popliteus. 

The popliteus is a small muscle with a big role. It’s a knee stabilizer, but it also can create rotation in both directions depending on the position of the leg. When the leg is planted on the ground (closed chain), it externally rotates the femur. When the leg is off the ground (open chain), it rotates the tibia internally. If you’ve heard of the screw home mechanism of the knee, it’s basically the opposite. The screw home mechanism is like a lock when the knee moves into extension (straightening). The popliteus basically unlocks it from this position and allows flexion (bending).

It has a few other functions. It is intracapsular and attaches to the lateral meniscus. It posteriorly retracts the meniscus so it doesn’t get pinched in the joint. It also prevents knee hyperextension. Because of this role, it can be provoked with excessive sprinting or downhill running. 

Here is a test for the popliteus:

Here is an exercise that may help the popliteus:

Baker’s Cysts

A baker’s cyst, or popliteal cyst, is a collection of fluid behind the knee. It is benign, and often asymptomatic. If it gets severe enough, it can cause pressure on the local nerves and vasculature which will provoke pain and stiffness in the knee. It will usually be difficult to fully flex the knee. 

There is a bursa behind the knee, namely the gastrocnemio-semimembranosus bursa. On the medial side there is a one way valve from the knee into the bursa. When there is any type of inflammatory process in the knee and the swelling builds faster than it can be evacuated, it accrues and pushes into the bursa. Because of this, baker’s cysts are associated with degenerative processes of the knee such as cartilage damage, meniscus tears, or rheumatoid arthritis. 

Sometimes the cyst can rupture which will initially cause some pain and swelling in the calf. It is important to note that a DVT (clot) be ruled out because this can be life threatening. A ruptured cyst can mimic the symptoms of a DVT. 

The bursa can be aspirated (drained) however, if the underlying issue is not addressed, it will just come back. This is not to say that surgery is necessary. The knee is about position and quality of movement. It is thought of as a hinge joint that just bends and straightens, but there is an element of rotation to the knee as well. If the knee is rotated internally (knee caved in) or externally (knee bowed out) it can change the relationship of the femur and tibia. If there is some defect such as a meniscus tear, this rotation can expose it and irritate it further provoking swelling. 

Where does this rotation come from? More often than not, the ankle. Stand barefoot, and slowly roll your ankle in and out. Watch what it does to your knee. If you are flat footed, it will lead to an internally rotated position of the knee and often cause pain when squatting. To fix this, you have to restore your arch and range of motion in dorsiflexion. Here is my favorite exercise for dorsiflexion:

And here is a way to train your arch with squatting:

Notice that the knee tracks slightly outward, this is the position you want. 

A general principle in rehab is that there are symptoms and dysfunctions. The dysfunction causes the symptom. You just have to look at the adjacent joint to find the dysfunction, as this is a prime example. Missing range of motion in the ankle changes how you squat, which provokes swelling and pain. Resolve the range of motion and work on technique, and you just might keep it from coming back.

Want to learn more? Check out https://www.restoreyourarch.com/