semitendinosus A fusiform muscle of the posterior and inner part of the thigh that arises from the ischial tuberosity along with the biceps femoris, that is inserted by a long round tendon which forms part of the inner hamstring into the inner surface of the upper part of the shaft of the tibia, and that acts to flex the leg and rotate it medially and to extend the thigh. BTO:0001236 BrendaTissueOBO musculus semitendinosus semitendinosus muscle hamstring muscle