Semimembranosus

legsMusculus semimembranosus

The semimembranosus is the deepest inner hamstring, lying underneath the semitendinosus. It extends the hip and bends the knee, and it is the broadest of the three at the hip.

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

Semimembranosus arises from its own facet on the sitting bone, above and to the outside of the tendon the other two share, by a broad flat tendon — a membrane, which is what the name refers to. It runs down beneath the semitendinosus and attaches to the back of the inner knuckle of the shin bone.

It lies deep to the semitendinosus for most of its length, so it is the less visible of the two inner hamstrings and cannot be felt as a separate cord behind the knee.

Its actions match the semitendinosus exactly: hip extension, knee flexion, and internal rotation of the lower leg when the knee is bent. The two work as a pair in every movement.

Its broad attachment at the hip gives it a good mechanical position for hip extension, and it is often described as contributing more to that end of the hamstring's job than to knee flexion.

Training notes

Hip extension work reaches it well — Romanian deadlifts, good mornings, back extensions and hip thrusts all load it through the hip.

Leg curls train it too, and a slightly toes-in foot position shifts a little more of the work to it and the semitendinosus, matching their internal rotation role.

Because it lies deep, it contributes to the thickness of the inner back of the thigh rather than to any surface feature.

Anatomy

Origin
  • Ischial tuberosity, from a separate facet above and lateral to the shared hamstring tendon
Insertion
  • Posterior part of the medial condyle of the tibia
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