The semimembranosus is a large, elongated muscle situated in the posterior compartment of the thigh. It is a long muscle that spans both the hip and knee joints, making it integral to lower body movement. The muscle is named for its distinctive, flat, and broad tendon near its upper attachment point, which resembles a membrane.
Context: Part of the Hamstring Group
The semimembranosus is one of the three muscles collectively known as the hamstrings, which run down the back of the thigh. The other two muscles in this group are the semitendinosus and the biceps femoris. All three hamstring muscles share common actions and originate from the same bony landmark on the pelvis. This muscle is positioned on the inner, or medial, side of the posterior thigh.
The semimembranosus is situated deep to the semitendinosus muscle, which is located more superficially. This deep placement means the semimembranosus lies closer to the bone and is covered by the narrower, more slender semitendinosus. It is often described as the deepest muscle within the medial hamstring group. The muscle belly is wide and flat, running parallel to the femur.
Exact Location: Origin, Insertion, and Deep Position
The muscle’s starting point, or origin, is a thick tendon attached to the ischial tuberosity, the prominent bony point often referred to as the “sitting bone.” This attachment is located on the superolateral aspect of the ischial tuberosity. The muscle fibers then travel inferiorly and slightly forward as they descend toward the knee joint.
Its ending point, or insertion, is found on the medial condyle of the tibia, the large bony prominence on the inner side of the shin bone just below the knee. The semimembranosus tendon expands at its insertion, contributing to the stability of the posterior knee joint capsule. The membranous tendon is most apparent at its origin, distinguishing it from the more cord-like tendon of the semitendinosus. It lies anterior to the semitendinosus and posterior to the adductor magnus muscle.
Primary Function in Movement
The semimembranosus muscle acts across both the hip and knee joints, giving it a dual role in lower limb movement. Its primary action at the hip is extension, which involves straightening the thigh or pulling the leg backward. This powerful movement assists in standing up from a seated position or propelling the body forward during walking and running.
At the knee joint, the muscle’s main action is flexion, or bending the knee. The semimembranosus also contributes to the internal, or medial, rotation of the tibia relative to the femur when the knee is bent. This rotational ability is important for stabilizing the knee and is particularly noticeable when the knee is flexed past a 90-degree angle.