semimembranosus

noun

semi·​mem·​bra·​no·​sus ˌsem-ē-ˌmem-brə-ˈnō-səs 

plural semimembranosi -ˌsī 

: a large muscle of the inner part and back of the thigh that arises by a thick tendon from the back part of the tuberosity of the ischium, is inserted into the medial condyle of the tibia, and acts to flex the leg and rotate it medially and to extend the thigh