The hamstring is composed of three muscles; the biceps femoris (both the long and short head), the semitendinosus, and the semimembranosus.
Origin
The semitendinosus arises from the upper medial part of the ischial tuberosity by a tendon common to the long head of the biceps femoris and the semitendinosus.
In this diagram of the muscles located at the ischium seen below, you can see the semi membranosus, the ischial part of the adductor magnus, and the common tendon origin for the long head of the biceps femoris and semitendinosus.
The semitendinosus and semimembranosus are two different muscles. The semitendinosus starts as a tendon and ends as a tendon- it is a cylinder muscle. On the other hand, the semimembranosus looks like a membrane.
Insertion
The semitendinosus is inserted into the upper part of the medial surface of the tibia. This area of insertion is called the Pes Anserine. The insertion site can be remembered with the acronym SGS. The Sartorius, the Gracilis, and the Semitendinosis all insert at the Pes Anserine, and therefore collectively called the Pes Anserine tendons.
Innervation
The nerve supply to the semitendinosus muscle comes from the sciatic nerve, specifically the tibial branch.
Action
The semitendinosus helps in flexion of the knee, medial rotation of the knee, and extension of the hip (weak function).
