The caud-ilio flexorius (semitendinosus of most authors) arises from the transverse processes of the tail, and from the distal half of the postacetabular ilium, thence passing as a broad ribbon to the popliteal region, where it splits into two portions.
This portion, morphologically the original, was named the " accessory semitendinosus " with the symbol Y; the other portion descends on the hinder aspect of the leg and joins the fascia of the inner femoral head of the gastrocnemius muscle.
Your hamstrings run along the back of your thigh and consist of the biceps femoris, semitendinosus and semimembranosus.