Semimembranosus
The deepest and broadest hamstring, named for the flat membrane-like tendon it starts from.
- Type
- Muscle
- Region
- Thigh
- Standard name
- m. semimembranosus
- Origin
- Ischial tuberosity, by a broad flat tendon deep to the other hamstrings
- Insertion
- Posterior aspect of the medial condyle of the tibia
- Actions
- knee flexionhip extensionknee internal rotation
- Verify
- TA2 2642 TA98 A04.7.02.036
What it is
The deepest of the three hamstrings, lying underneath semitendinosus on the inner side of the back of the thigh. Its name comes from its origin: a wide, flat, membranous tendon rather than the round cord of its neighbour.
It is the largest of the hamstrings by cross-section, despite being the one nobody can see. Its insertion is short and firm, onto the back of the inner knee, with expansions that reinforce the back of the joint capsule.
What it does
Bends the knee, extends the hip, and rotates the flexed knee inward — the same list as semitendinosus, which lies directly on top of it.
Its attachments into the back of the knee capsule mean it also contributes to the stability of the joint itself, not only to moving it.
What loads it
The same movements as the other hamstrings, and it cannot be separated from them: hip hinging, knee flexion under load, running, and controlling the leg as it decelerates.
What is commonly got wrong about it
Its name. Semimembranosus refers to the flat tendon at its origin, not to anything membrane-like about the muscle itself, and not to any relationship with semitendinosus beyond their being neighbours with similar jobs.
That the biggest hamstring is the one you can see. The visible cord behind the inner knee belongs to semitendinosus. The larger muscle is underneath it.
"Hamstring" as an anatomical term. It is not one — Terminologia Anatomica names the three muscles individually and has no entry for the group. The word is old, useful, and informal, and it comes from the practice of hanging a carcass by these tendons.