The semimembranosus muscle is one of the three hamstrings muscles.

The semimembranosus reaches from the pelvis to the tibia.
What does the semimembranosus muscle do?
The semimembranosus helps with flexing the knee and also rotating it.
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CLINICAL PUBLICATIONS
Semimembranosus Tendinopathy - One Cause of Chronic Posteromedial Knee Pain. Bylund WE and de Weber K. Sports Health. 2010 Sep; 2(5): 380–384..
Tendinous insertion of semimembranosus muscle into the lateral meniscus. Kim YC, Yoo WK, Chung IH, Seo JS and Tanaka S. Surg Radiol Anat. 1997;19(6):365-9.