Semitendinosus

Origin, Insertion, Action & Nerve

Attachments explained

The Semitendinosus is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.

Action & function

When the Semitendinosus contracts, it produces the following movements: Flexion of knee; extension of hip; medial rotation of flexed knee.

As part of the Thigh group of the Lower Limb, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.

Nerve supply & clinical relevance

The Semitendinosus receives its nerve supply from the Tibial division of sciatic nerve (L5, S1, S2). This nerve carries fibres from spinal segment(s) L5, S1, S2.

Because a muscle can only contract when its nerve is intact, injury to the Tibial division of sciatic nerve or to its spinal roots (L5, S1, S2) can weaken or paralyse the Semitendinosus, impairing the movements it normally produces (flexion of knee). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Semitendinosus (the OIANS method)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Semitendinosus, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Semitendinosus fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.

Semitendinosus quick facts

Region

Lower Limb

Group

Thigh

Origin

Ischial tuberosity

Insertion

Superomedial surface of tibia (pes anserinus)

Action

Flexion of knee; extension of hip; medial rotation of flexed knee

Nerve

Tibial division of sciatic nerve (L5, S1, S2)

Spinal roots

L5, S1, S2

Frequently asked questions

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