I​f you have knee pain you should check this relationship. The Popliteus and Rectus femoris are functional opposites to each other. Meaning they do different actions. The rectus femoris extends the knee joint and the popliteus flexes the knee joint and medially (internally) rotates the bent knee (Unlocks the knee).
The rectus femoris is often weak and inhibited playing an inefficient role of flexing the hip and extending the knee. Why? Well sitting all damn day isn't helping! Your rectus femoris gets shortened at the hip joint and lengthened at the knee joint. It’s a two jointed muscle with a long lever. The longer the lever the more likely for disaster to occur.
As a result the popliteus drives more knee flexion, preventing the lower leg from taking full advantage of the elastic recoil of fascia from the foot to the glute. Torque on the knee from the medial tibial rotation causes pain and discomfort when walking, running, or walking up the stairs. Sometimes you may even feel the knee lock up.

Even if you have quads like the quadfather that doesn't mean they are functional and efficient for movement. That’s like having a Ferrari in the driveway that won’t start. If you have had an ACL injury no matter how long ago you can pretty much bet this relationship exists.

WHAT TO DO ABOUT IT?
Sit and feel for the popliteus just medial and behind the fibular head. It will run diagonal across the knee down towards the tibia. You will know when you hit it because it usually really hurts. Be careful in the posterior knee because there are lots of delicate structure sitting back there. Stay more lateral towards the fibula and you should be ok. To ensure you are on the popliteus resist knee flexion while internally rotating your tibia and it should pop into your fingers. 
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Massage that little guy for 1-2 minutes and then lie in your back and activate your rectus femoris by contract your quad and raising your leg up in the air holding the contraction at the top and bottom for 4 seconds repeat this for 10-20 Reps or until neural fatigue (see below).

OTHER MUSCLES RESPONSIBLE FOR A FACILITATED POPLITEUS
It is important to look at the body holistically as well, by paying attention to the kinetic chain you may be able to find some funky compensation patterns going on.
​In the superficial back line, there is a chain that goes from the bottom of the foot, through the calf muscles, through the popliteus, into 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 (tight). For example, the popliteus may become facilitated due to inhibition of the gluteus maximus.

There is a lot of discussion about strengthening the core if you have low back pain. In this article I will discuss functional relationships that will inhibit the core muscles from being strengthened. Due to dysfunctional movement patterns, you could be wasting your time trying to strengthen your core if these dysfunctional patterns exist and are not reprogrammed. 
NeuroKinetic Therapy (NKT) is used to identify tight (facilitated) muscles that are causing other muscles to go weak (inhibited).

Here is how we identify what muscles are inhibited and which ones are facilitated:
Step 1: Find a muscle that tests weak (e.g. Psoas).
Step 2: Touch a muscle that makes the Psoas test strong (e.g. Quadratus Lumborum/QL).
Step 3: Test the muscle (QL) that made the Psoas test strong then immediately test the Psoas again.
Step 4: If the Psoas is still weak then we know the QL needs to be release and the Psoas needs to be strengthened.

The tight muscles are released using Soft Tissue Release (STR), Fascial Stretch Therapy (FST) or other forms of self myofascial release techniques (stretching, rolling on a lacrosse ball or foam roller etc.) OUCHY!
​After releasing the QL we then need to test the QL and Psoas against each other and if the Psoas now tests strong  then you have successfully reprogrammed the dysfunctional pattern. 
Your homework would be to release the QL and then strengthen the psoas. If you don't do the strengthening component you will be in a vicious cycle of  chronic low back pain and spending all of your hard earned cash to fix the same problem.
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Save your money in your piggy bank and actually do your homework!!! $$$$
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OTHER MUSCLES RESPONSIBLE FOR AN INHIBITED PSOAS MAY INCLUDE:

1. TFL (outer hip/IT band-ish),
2. Rectus femoris (quad),
3. Iliacus (hip flexor),
4. Adductors. 
5. Ipsilateral (same side) scalenes
6. Contralateral (opposite side) pec minor
7. Ipsilateral (same side) tibialis anterior

CORE RELATIONSHIP AND LOW BACK PAIN
An inhibition of the psoas can cause tightness, and pain in the lumbar spine and the sacroiliac joint. Because the psoas attaches to the lumbar intervertebral discs, it is crucial in the management of low back pain to deal properly with these dysfunctional relationships.
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CONCLUSION
Just because the psoas is "tight" doesn't mean it needs to be stretched. The question you need to ask is "why is it tight?" If it is tight and weak then stretching it will make it panic and cause it to rebound and tighten up more. When a muscle is weak it protects itself by shortening. This is why it is very important to determine if the muscle is weak or strong before you go ahead and start yanking away at your hip trying to stretch out your "tight/weak" hip flexors.