The semitendinosus is a significant muscle located in the posterior aspect of the thigh, distinguished by its remarkably long tendon of insertion. This muscle plays a crucial role in the mechanics of the leg and hip, forming part of the group known as the hamstring muscles. Its position is posteromedial within the thigh, where it lies superficially to another important muscle, the semimembranosus. Understanding its precise anatomical features, from
its origin to its insertion points, provides insight into its function and its relationship with surrounding structures.
Origin and Initial Structure
The semitendinosus muscle originates from a specific point on the pelvis: the lower and medial impression on the upper part of the tuberosity of the ischium. This origin is shared with the long head of the biceps femoris, as they arise by a common tendon. Furthermore, the semitendinosus also draws its origin from an aponeurosis, which is a flat sheet of tendon-like material. This aponeurosis connects the adjacent surfaces of the semitendinosus and the biceps femoris for approximately 7.5 centimeters from their shared origin point. This initial structural arrangement highlights the close functional and anatomical relationship between these muscles.
Course and Tendinous Insertion
As the semitendinosus muscle extends down the thigh, it takes on a fusiform, or spindle-like, shape. It terminates a little below the middle of the thigh, transitioning into a long, round tendon. This tendon is a defining characteristic of the muscle, giving it its name, which refers to its long tendinous nature. The tendon then follows a path along the medial side of the popliteal fossa, which is the hollow area at the back of the knee. From there, it curves around the medial condyle of the tibia, which is a prominent bony structure on the inner side of the shin bone.
Insertion at the Pes Anserinus
The semitendinosus tendon continues its course, passing over the medial collateral ligament of the knee-joint. A bursa, a small fluid-filled sac, separates the tendon from this ligament, reducing friction during movement. The tendon then inserts into the upper part of the medial surface of the body of the tibia, extending nearly as far forward as the tibia's anterior crest. At its insertion point, the semitendinosus gives off a prolongation from its lower border to the deep fascia of the leg. Here, it lies behind the tendon of the sartorius muscle and below the tendon of the gracilis muscle, to which it is united. These three tendons—from the semitendinosus, sartorius, and gracilis—collectively form a structure known as the pes anserinus. This term, meaning "goose's foot," describes the distinctive appearance of these conjoined tendons at their insertion on the tibia.
Relationship with the Semimembranosus
While the semitendinosus and semimembranosus muscles are both part of the hamstring group and share close insertion and attachment points, they have distinct characteristics. The semitendinosus is considered more superficial than the semimembranosus. However, the semimembranosus is wider and flatter in shape. This difference in morphology means that despite the semitendinosus being superficial, it is actually possible to palpate, or feel, the semimembranosus directly due to its broader structure. This distinction is important for clinicians and anatomists when identifying and assessing these muscles.
