The popliteus tendon originates on the lateral surface of the lateral femoral condyle (in front of and below the lateral collateral ligament origin) and also from the fibular head. It also has an origin stemming from the posterior horn of the lateral meniscus. The tendon then courses under the lateral collateral ligament, descends into the ‘popliteal hiatus’, and becomes extra-articular (outside the knee joint) before joining its muscle belly. It inserts into the tibia above the popliteal line. It is therefore a relatively horizontal muscle lying deep in the back part of the knee.
The popliteus is believed to have a number of functions, made possible by its unique ability to reverse its origin and insertion, depending on whether the femur or the tibia is fixed.
1.Internal rotation of the tibia in an already extended knee. Due to the contour of the femoral condyles, this internal rotation of the tibia ‘unlocks’ an extended knee. In essence it initiates knee flexion.
2.External rotation of the femur on a tibia that is fixed, as in the stance phase of gait. It is an important controller of knee rotation during the stance phase of locomotion.
3.Helps to bring the knee out of a position of full extension.
4.Helps the PCL (posterior cruciate ligament) maintain stability by preventing excessive posterior translation of the tibia.
5.Helps to withdraw the lateral meniscus during knee flexion.
6.Provides some rotary stability of the femur on the tibia.
7.Prevents excessive external rotation and varus rotation of the tibia during knee flexion.
I have heard it said that the popliteus can contract to up to 300% after knee surgeries such as meniscal repair, and ACL and PCL reconstruction. Inability to fully extend the knee after such surgeries may be a result of a hypertonic popliteus. Let’s now look at some of the functions to see where and why an overactive popliteus can become such a problem.
It’s ability to initiate knee flexion makes it an antagonist for the quadriceps and a synergist for the hamstrings and gastrocnemius. Inhibition of the quadriceps, either of all four or one in particular, may cause facilitation of the popliteus. Inhibition of the vastus medialis is very common. Inhibition of the hamstring group, especially the biceps femoris, may also cause facilitation of the popliteus.
The hamstrings group is the main torque producer for knee flexion. It helps control tibial rotation during the stance phase of gait. The lateral hamstrings (biceps femoris) actively externally rotate the tibia on the femur but also control internal rotation of the tibia on the femur in stance phase of gait. The medial hamstrings (semitendinosus and semimembranosus) control external rotation of the tibia on the femur in stance phase of gait.A poorly functioning hamstring due to weakness or pathology may result in a compensatory overuse of the popliteus to control tibial rotation in stance phase.
KINETIC CHAIN ANALYSIS
In the superficial back line, there is a chain from the bottom of the foot, through the calf muscles, to the popliteus, to the hamstrings, and the gluteus maximus. In the extension phase of gait, these muscles work together. Inhibition of one or more of these muscles may cause another muscle in this chain to become facilitated. The popliteus may become facilitated due to inhibition of the gluteus maximus. The popliteus can also become facilitated due to inhibition in the superficial front line. Inhibition of the extensor hallucis longus, rectus femoris, and psoas commonly cause facilitation of the popliteus. In internal rotation of the tibia, inhibition of the medial head of the gastrocnemius may cause facilitation of the popliteus. In external rotation of the femur, inhibition of the biceps femoris may cause facilitation of the popliteus. I have also seen compensation patterns involving the popliteus due to inhibition of the neck extensors, flexors, and rotators.
It is very important to consider the popliteus not only when dealing with knee pain, but also when dealing with complex kinetic chain compensations. Never underestimate this tiny but critical muscle.
Thanks to Chris Mallac for his excellent article. http://www.sportsinjurybulletin.com/archive/popliteus.html