Semimembranosus muscle may be reduced or absent, quite large or double in which case arising mainly from the sacrotuberous ligament giving a slip to the femur or adductor magnus.

Ax PD Fat Sat
Incidental finding in 29 yo with knee pain
The semimembranosus muscle originates on the superior lateral quadrant of the ischial tuberosity. It has multiple insertions by way of five tendinous arms or expansions, to the medial tibial condyle (anterior, direct, and inferior arms), the posterior oblique ligament (capsular arm), and the posterior joint capsule and arcuate ligament (oblique popliteal ligament). The first three arms are closely related to the tibial collateral ligament, coursing deep to it. In slightly less than one-half of the population, small slips of the semimembranosus tendon insert onto the posterior horn of the lateral meniscus.
It is inervated by the tibial nerve, division of the sciatic nerve.
It may be reduced or absent, quite large or double in which case arising mainly from the sacrotuberous ligament giving a slip to the femur or adductor magnus.
Suggested Reading:
Beltran J, Matityahu A, Hwang K, et al. The distal semimembranosus complex: normal MR anatomy, variants, biomechanics and pathology. Skeletal Radiol 2003; 32:435–445.
Koulouris G and Connell D. Hamstring Muscle Complex: An Imaging Review. RadioGraphics 2005; 25: 571-586.
