Bilateral absence of both quadratus femoris and semimembranosus muscles was identified during a routine dissection of a 73-year-old Caucasian female cadaver. In the gluteal regions, the quadratus femoris was absent bilaterally from the area immediately inferior to the inferior gemellus muscle. The area presented as a shallow depression/space with the obturator externus muscle visible from a posterior view. In the posterior thigh regions, only the semitendinosus and biceps femoris were observed; no semimembranosus was found bilaterally. Only the semitendinosus traveled down to the superomedial border of the popliteal fossa. The absence of these muscles may present clinically as muscle weakness of hip lateral rotation (quadratus femoris action) and knee flexion (semimembranosus action) as well as hypermobility of the posteromedial knee joint (semimembranosus function). Surgeons, radiologists, and rehabilitation professionals should be aware of this anatomical variation.
