Semitendinosus

legsMusculus semitendinosus

The semitendinosus is an inner hamstring with an unusually long tendon, running from the sitting bone to the inside of the shin. It is the prominent cord you can feel behind the inside of a bent knee.

Semitendinosus highlighted on an anatomical figure, from behind.
Where it sits, from behind.

Semitendinosus arises from the sitting bone alongside the long head of biceps femoris and runs down the inner back of the thigh. About halfway down, its muscle belly gives way to a long round tendon — which is what the name refers to — and that tendon continues to the inside of the shin bone.

It attaches as part of the pes anserinus, the goose's foot: three tendons from three different muscles, sartorius, gracilis and semitendinosus, arriving side by side on the same small patch of bone below the inside of the knee. The three come from three different muscle groups and three different nerves, and they converge here.

Its actions are the standard hamstring pair — hip extension and knee flexion — plus internal rotation of the lower leg when the knee is bent, which is the opposite of what biceps femoris on the outside does.

Its long tendon is commonly used as graft material in knee ligament reconstruction, which is why the muscle appears in surgical contexts far more often than its size would suggest.

Training notes

Both hamstring jobs train it: leg curls for knee flexion and Romanian deadlifts, good mornings and back extensions for hip extension.

Because it internally rotates the lower leg, pointing the toes slightly inward during a lying leg curl shifts a little more work to it and to the semimembranosus. The effect is small and worth knowing rather than worth building a program around.

The inner hamstrings are commonly less developed than the outer, since biceps femoris takes the larger share of most hamstring work. A little toe-in curl volume is a cheap way to address that.

It contributes to the visible inner cord behind the knee, and its muscle belly to the thickness of the upper inner back of the thigh.

Anatomy

Origin
  • Ischial tuberosity, by a tendon shared with the long head of biceps femoris
Insertion
  • Upper medial surface of the tibia, as part of the pes anserinus
Actions
  • Flexes the knee
  • Extends the hip
  • Internally rotates the lower leg when the knee is bent
Innervation
Tibial division of the sciatic nerve (L5–S2)

Exercises

Antagonists

Synergists