Semimembranosus: Muscle Anatomy and Role

The semimembranosus muscle is a crucial component of the posterior thigh, recognized as the most medial of the three hamstring muscles. Its name is derived from its distinctive flat, membranous tendon of origin. This muscle plays a significant role in the movement of both the hip and knee joints, contributing to the overall mobility and stability of the lower limb. Understanding its precise anatomical features, from its starting point to its termination

and the nerves that control it, is essential for comprehending its function within the human body.

Structural Characteristics and Location

The semimembranosus muscle is situated at the back and medial side of the thigh. It lies deep to the semitendinosus muscle, another hamstring, and is notably wider, flatter, and deeper than its superficial counterpart. This positioning means it is not immediately visible beneath the skin but is rather nestled within the musculature of the posterior thigh. A key anatomical relationship is its overlap with the upper part of the popliteal vessels, indicating its proximity to major blood vessels in the region behind the knee.

The muscle's unique structure, particularly its membranous tendon of origin, is what gives it its name. This flat tendon is a defining characteristic, setting it apart from other muscles in the area. The muscle fibers themselves arise from an aponeurosis, which is a sheet-like tendon, covering the upper part of the anterior surface of the muscle. These fibers then converge into another aponeurosis that covers the lower part of the posterior surface, eventually contracting into the tendon of insertion.

Origin and Insertion Points

The journey of the semimembranosus muscle begins with a thick tendon originating from the superolateral aspect of the ischial tuberosity. The ischial tuberosity is a prominent bony projection located at the bottom of the pelvis, often referred to as the "sit bone." This origin point is positioned above and medial to where the biceps femoris muscle and semitendinosus muscle also arise, highlighting the close anatomical relationship among the hamstring group.

From this origin, the muscle extends downwards, eventually inserting onto the medial condyle of the tibia. The medial condyle is a rounded prominence on the inner side of the top of the shin bone. This insertion point is critical for the muscle's actions on the knee joint. The tendon of origin expands into an aponeurosis, which covers the upper part of the anterior surface of the muscle. From this aponeurosis, muscular fibers emerge and converge to another aponeurosis, which covers the lower part of the posterior surface of the muscle and contracts into the tendon of insertion.

Nerve Supply and Actions

The semimembranosus muscle receives its nerve supply from the tibial part of the sciatic nerve. This innervation is crucial for the muscle's ability to contract and perform its functions. The sciatic nerve itself is a large nerve that originates from the sacral plexus, a network of nerves formed by the anterior divisions of ventral nerve roots from L4 through S3. Specifically, the tibial part of the sciatic nerve is responsible for innervating not only the semimembranosus but also the semitendinosus and the long head of the biceps femoris, underscoring its importance to the entire hamstring group.

The primary actions of the semimembranosus muscle are the extension of the hip joint and the flexion of the knee joint. Hip extension involves moving the thigh backward, while knee flexion involves bending the knee. These actions are fundamental for various movements, including walking, running, and jumping. As part of the hamstring group, the semimembranosus works in coordination with other muscles to facilitate these complex movements, making it a vital contributor to lower limb mechanics.