Here is a brief description from the Merck Manual. http://www.merckmanuals.com/professional/injuries-poisoning/sports-injury/popliteus-tendinitis.
This is what the muscle does. “The popliteal muscle in the leg is used for unlocking the knees while walking by medially rotating the tibia during the closed chain portion of the gait cycle (one with the foot in contact with the ground). It is also used when sitting down and standing up. It is the only muscle in the posterior (back) compartment of the lower leg that acts on the knee instead of the ankle (other than the gastrocnemius, which acts on both).” http://en.wikipedia.org/wiki/Popliteus_muscle
About 15 years ago I was primarily an ultra runner. I was training in California for the Kettle Moraine 100 mile endurance run in Wisconsin and began having a sharp pain behind my knee. It hurt as much to walk as it did to run but I began to reduce mileage because of the discomfort. Needless to say, I was not properly prepared and was a dnf at mile 90. The discomfort was so great I couldn’t straighten my knee. My calf was tight and extremely sore. It felt like my lower leg was being twisted.
It is a horror story of a lengthy period of misdiagnosis and treatments. Meanwhile the pain was so great, I could barely walk. I intentionally had a bee sting me behind the knee, went to an acupuncturist, physical therapist, Chiropractor, had arthroscopic knee surgery, continually stretched with no relief and used Ace bandages on my knee.
Here is what I believe helped when the surgeon said I had arthritis in my knee and would never be able to straighten it. To this point the correct diagnosis had not been made. Additionally, I have a leg length discrepancy because of a spiral femur fracture from skiing. It is my longer leg, which had the problem.
1. The Physical Therapist noticed biomechanical problems when I did knee bends. My knee turned in as I lowered myself. He fitted me for orthotics to eliminate my foot rolling so far inward which kept my knee straight.
2. I began cycling again as my primary exercise. It didn’t hurt and it strengthened my quadriceps, which help stabilize my knee. I also continued weight training for my core and upper body. I also swam three times a week. I still walked a couple of miles a day but mostly on relatively flat surfaces.
3. I went to a trusted massage therapist weekly who worked the knee, hamstring, IT band and calf.
4. I stopped using ice and started using heat and a spa jet on my knee. I continued to stretch the calf and hamstrings.
5. I stopped the daily use of Ibuprofen.
6. My wife pressed daily on the knee of my outstretched leg. I had a rolled up towel under my ankle, which allowed my knee to flex back. It was painful but effective and got my leg closer to being straight.
This problem did not go away overnight. Most folks who offer advice say that rest is the answer. It is impossible to rest if you walk at all. I still think ice and ibuprofen may help initially but both became counterproductive for me.
If you have a sharp pain that begins behind the knee and later extends down into the calf. Consider the possibility of tendinitis of the popliteal tendon. See a doctor and a physical therapist FIRST. Don't let the need to be fit override your need to be well and pain free. I think it was Hal Higdon who once said, “Runners are the fittest group of injured people I know.”
I hope someone out there is helped, especially the ones who are yet to identify the problem with the correct diagnosis.