The semimembranosus muscle is part of the hamstring group, located along the back of the thigh. It contributes to movement and stability, performing various actions involving both the hip and knee joints.
Anatomical Overview
The semimembranosus muscle is located deep within the posterior thigh, beneath the semitendinosus. It is the most medial of the hamstring muscles. This muscle originates from the ischial tuberosity, a bony projection at the base of the pelvis, often called the “sit bone.”
The semimembranosus features a flat, membranous tendon in its upper portion, which gives the muscle its name. This tendon transitions into a fleshy muscle belly midway down the thigh. The muscle inserts onto the medial condyle of the tibia, the inner part of the shin bone just below the knee. Its insertion also connects to the posterior joint capsule and parts of the medial meniscus, contributing to knee stability.
Primary Functions
The semimembranosus muscle performs several actions involving the hip and knee joints. A primary role is knee flexion, which involves bending the knee and drawing the heel towards the buttocks. This action is important for movements like kicking a ball or bringing the foot closer to the body.
Beyond knee flexion, the semimembranosus contributes to hip extension, moving the leg backward at the hip joint. This movement helps propel the body forward during walking or running. The muscle also aids in the internal rotation of the tibia, or lower leg, particularly when the knee is flexed. This rotational capability allows for subtle adjustments in leg positioning.
Role in Everyday Movement
The semimembranosus muscle’s combined actions are important for daily movements, mobility, and stability. During walking, its hip extension helps push the body forward. As the leg swings forward, knee flexion helps clear the foot from the ground, preparing for the next stride.
When running, the semimembranosus engages powerfully, with hip extension generating force for speed and acceleration. Its ability to flex the knee rapidly is important for the recovery phase of the running gait. Climbing stairs relies on hip extension to lift the body and controlled knee flexion to place the foot. During squatting, the semimembranosus controls descent by eccentrically lengthening, then concentrically contracts to extend the hip and flex the knee, aiding return to an upright position. The muscle also helps maintain an upright posture by counteracting any tendency to bend forward at the hip.