The semitendinosus muscle is one of three hamstring muscles that are located at the back of the thigh. The other two are the semimembranosus muscle and the biceps femoris. The semitendinosus muscle lies between the other two. These three muscles work collectively to flex the knee and extend the hip.

The semitendinosus muscle begins at the inner surface of the base of the pelvis (known as the tuberosity of the ischium) and the sacrotuberous ligament. It inserts at the medial tibial condyle.

The semitendinosus muscle is comprised primarily of fast twitch muscle fibers. Fast twitch muscle fibers undergo rapid contractions for a short time period and easily wear themselves out.

The inferior gluteal artery and the perforating arteries bring oxygenated blood to the semitendinosus muscle. A segment of the sciatic nerve serves as the sensory and motor nerve for the muscle.

When the semitendinosus muscle becomes overly strained, a pulled hamstring results. There are three grades of pulled hamstring that are defined by how excessive the muscle tear is and the degree of pain and disability.