The Semimembranosus is the most medial and deep of the three hamstring muscles. It gets its name from the flat, membrane-like tendon of origin that makes up the upper half of the muscle. It is a powerful extensor of the hip and flexor of the knee, and plays a unique role in posterior knee stability.
[Image of Semimembranosus muscle anatomy]Quick Anatomy Snapshot
| Origin (Proximal) | Upper lateral facet of the Ischial Tuberosity (Sit bone). |
|---|---|
| Insertion (Distal) | Posterior surface of the Medial Condyle of the Tibia. (Also sends expansions to the Oblique Popliteal Ligament and popliteal fascia). |
| Nerve Supply | Tibial Nerve (L5, S1, S2) - Part of the Sciatic Nerve. |
| Blood Supply | Perforating branches of Deep Femoral Artery and Popliteal Artery. |
| Primary Actions |
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Deep Dive: The Membranous Muscle
The Semimembranosus is often confused with the Semitendinosus, but they are structurally distinct.
1. The "Membrane"
The muscle arises as a long, flat, membranous sheet from the Ischial Tuberosity. This sheet extends almost halfway down the thigh before turning into a fleshy muscle belly. It lies deep to the Semitendinosus muscle.
2. The Complex Insertion
The insertion isn't just a simple attachment to the tibia. It splits into three parts:
• Direct Arm: Attaches to the posterior medial tibial condyle.
• Anterior Arm: Blends with the MCL (Medial Collateral Ligament).
• Oblique Popliteal Ligament: Reflects upwards across the back of the knee joint to reinforce the posterior capsule.
Physio Corner: Clinical Relevance
Palpation
Have the patient lie prone. Locate the Semitendinosus tendon (the very prominent cord on the medial side of the back of the knee). The Semimembranosus is the thicker, fleshy mass felt deep to and on either side of the Semitendinosus tendon. It extends lower down the leg than the Semitendinosus.
A Baker's Cyst (Popliteal Cyst) is a fluid-filled swelling behind the knee. It most commonly forms in the bursa located between the Semimembranosus tendon and the Medial Head of the Gastrocnemius. Friction here causes the bursa to swell and communicate with the knee joint.
Manual Muscle Testing (MMT)
To isolate the medial hamstrings (Semimembranosus/Semitendinosus) from the lateral hamstring (Biceps Femoris).
Step-by-Step Procedure (Oxford Scale)
| Grade | Patient Action & Resistance |
|---|---|
| Grade 3, 4, 5 (Against Gravity) |
Position: Prone (face down). Knee flexed to 45°. Setup: Leg is Internally Rotated (toes pointing toward midline). Action: Patient holds the knee flexion while keeping toes turned in. Resistance: Applied at the ankle, pushing the leg down into extension (and slightly trying to de-rotate). |
| Grade 2 (Gravity Eliminated) |
Position: Side-lying on the test side. Leg supported. Action: Patient flexes the knee with internal rotation. Result: Full range of motion. |
| Grade 0, 1 (Palpation) |
Action: Palpate deep to the Semitendinosus tendon on the medial aspect of the posterior knee. Cue: "Try to bend your knee with your toes turned in." |
Frequently Asked Questions
Is it part of the Pes Anserinus?
No. The Pes Anserinus is formed by Sartorius, Gracilis, and Semitendinosus. The Semimembranosus inserts separately on the posterior aspect of the medial tibial condyle, deep to the Pes Anserinus.
Why is it called "Membranosus"?
Because its proximal origin is a long, flat, membrane-like aponeurosis, unlike the cord-like tendon of the Semitendinosus.
Does it help lock or unlock the knee?
It assists the Popliteus in unlocking the knee by internally rotating the tibia (or externally rotating the femur) to initiate flexion from a fully extended position.
Test Your Knowledge: Semimembranosus Quiz
1. Where does the Semimembranosus insert?
2. Which nerve supplies the Semimembranosus?
3. A Baker's Cyst is found between the Semimembranosus and which other muscle?
4. To bias the Semimembranosus during testing, the lower leg should be:
5. Which ligament is formed by an expansion of the Semimembranosus tendon?
6. Is the Semimembranosus deep or superficial to the Semitendinosus?
7. What is the action of Semimembranosus on the hip?
8. The Semimembranosus forms which border of the Popliteal Fossa?
9. What happens to the meniscus when Semimembranosus contracts?
10. True or False: Semimembranosus is part of the Pes Anserinus.
References
- Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
- Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
- Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.
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