Wikipedia says about the popliteus: "A popliteus shortened by trigger points can cause pain near its tendinous origin at the lateral knee." Another possibility is a Baker's Cyst: http://www.coolrunning.com/engine/2/2_5/194.shtml#bakers, "also known as a poplitealcyst" (http://en.wikipedia.org/wiki/Baker%27s_cyst).
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