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The semitendinosus is a muscle located on the back of the human thigh. Along with the semimembranosus and biceps femoris, it is one of the three muscles of the hamstrings. A long and narrow muscle that stretches vertically from the underside of the pelvis to just below the knee, the semitendinosus is found to the inside of the larger biceps femoris muscle on the back of the thigh. It is responsible for movements at both the hip and knee joints, as it assists the gluteal muscles in extension of the hip and is also a flexor of the knee. Additionally, it is involved in the minor action of medial or internal rotation of the knee joint.
This muscle originates on the ischial tuberosity, a paired oblique bony protrusion on the underside of the ischium bone on the lower pelvis that makes sitting possible. Specifically, the semitendinosus arises from the upper portion of the ischial tuberosity on its lower inner aspect. It shares this point of origin with the long head of the biceps femoris muscle alongside it, with which it arises from a common tendon.
From this point on the bottom of the pelvis the semitendinosus extends roughly two-thirds of the way down the back of the thigh, where its fibers taper to form a long tendon. This tendon passes to the inside of the posterior knee parallel to the tendons of the sartorius and gracilis muscles of the inner thigh. Crossing the medial condyle of the tibia, one of two rounded prominences on the top of the tibia bone, it attaches to the inner aspect of the shaft of the bone just below the knee.
A portion of the semitendinosus tendon extends beyond this insertion point to merge with sections of the tendons of the sartorius and gracilis. These sections attach to the deep fascia of the leg, the fibrous tissue that separates the muscles of the lower leg into compartments. Collectively, the three are referred to as the pes anserinus for their resemblance to a goose’s foot.
The semitendinosus is more powerful in its action at the knee joint than at the hip, as its inserting tendon crosses the knee directly. It assists the biceps femoris in flexing or bending the knee, particularly when the hip is extended with the thigh behind the body. While it is involved with hip extension, and its action becomes more dominant in individuals with weak glutes, it is the gluteal muscles that perform this movement at the hip joint.