Semitendinosus: Origin, Insertion, Nerve, Action & Hamstring Anatomy

The Semitendinosus is a long, superficial muscle located on the medial side of the posterior thigh. It is one of the three "Hamstring" muscles. True to its name, it is remarkable for having a very long, cord-like tendon of insertion that begins roughly halfway down the thigh.

[Image of Semimembranosus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Ischial Tuberosity (Sit bone) - via the Common Hamstring Tendon (shared with Biceps Femoris Long Head).
Insertion (Distal)	Upper part of the Medial Surface of the Tibia (Part of the Pes Anserinus).
Nerve Supply	Tibial Nerve (L5, S1, S2) - Part of the Sciatic Nerve.
Blood Supply	Perforating branches of the Deep Femoral Artery and Inferior Gluteal Artery.
Primary Actions	Hip Extension: Moves the thigh backward. Knee Flexion: Bends the knee. Internal Rotation: Rotates the leg inward when the knee is flexed.

Deep Dive: The "Half-Tendon" Muscle

The Semitendinosus is superficial to the larger, fleshy Semimembranosus muscle.

1. The Pes Anserinus

The Semitendinosus tendon curves around the medial condyle of the tibia to insert on the anterior-medial surface. It joins the tendons of the Sartorius and Gracilis to form the Pes Anserinus (Goose's Foot).
Mnemonic: Say Grace before Tea (Sartorius, Gracilis, Semitendinosus).

2. The Conjoint Origin

At the Ischial Tuberosity, the Semitendinosus shares a common tendon of origin with the Long Head of the Biceps Femoris. If you strain your high hamstring, it often involves this conjoint tendon.

Physio Corner: Clinical Relevance

💪 Functional Fact: Like the other hamstrings, the Semitendinosus acts as a brake. During the late swing phase of running, it contracts eccentrically to slow down the leg before the heel hits the ground. This is when most hamstring tears occur.

Palpation

Have the patient lie prone and flex the knee against resistance. Two distinct tendons will pop out at the back of the knee.
• Lateral side: Biceps Femoris (Thick).
• Medial side: Semitendinosus (Thin and cord-like).
The Semimembranosus lies deep to the Semitendinosus and feels like a thicker mass on either side of the tendon.

⚠️ Clinical Pathology: Pes Anserine Bursitis
Pain on the inner part of the upper shin (tibia) is often due to inflammation of the bursa underlying the Pes Anserinus tendons. This is common in runners, swimmers (breaststroke), and patients with knee osteoarthritis.

Manual Muscle Testing (MMT)

To isolate the Medial Hamstrings (Semitendinosus and Semimembranosus) from the Lateral Hamstring (Biceps Femoris).

Testing Tip: Rotate the leg Internally (turn toes in) before flexing the knee. This lines up the medial hamstrings to do the work.

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 the midline). Action: Patient holds the knee in flexion. 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 the prominent cord-like tendon on the posteromedial knee. Cue: "Bend your knee with your toes turned in." Grade 1: Tendon pops out. Grade 0: No activity.

Frequently Asked Questions

Can the Semitendinosus tendon be used for ACL repair?

Yes! It is one of the most common autografts for ACL reconstruction (often harvested along with the Gracilis tendon). Surprisingly, removing it causes minimal long-term loss of knee flexion strength.

What is the difference between Semitendinosus and Semimembranosus?

Semitendinosus: Superficial, very long cord-like tendon, inserts on anterior medial tibia (Pes Anserinus).
Semimembranosus: Deep, membranous aponeurosis, inserts on posterior medial tibial condyle.

Does it rotate the hip?

Yes, slightly. As a medial hamstring, it assists in Internal Rotation of the extended hip, and extension of the hip.

Test Your Knowledge: Semitendinosus Quiz

1. Where does the Semitendinosus insert?

2. Which nerve supplies the Semitendinosus?

3. The Pes Anserinus is formed by Semitendinosus and which two other muscles?

4. To bias the Semitendinosus during testing, the lower leg should be:

5. What type of rotation does Semitendinosus create at the knee?

6. Where does the Semitendinosus originate?

7. Is the Semitendinosus superficial or deep to the Semimembranosus?

8. Which hamstring muscle is most commonly harvested for ACL repair?

9. The Semitendinosus forms the ________ border of the Popliteal Fossa.

10. True or False: Semitendinosus originates from a conjoint tendon with Biceps Femoris Long Head.

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.