Tibialis posterior

What it is

The deepest muscle at the back of the lower leg, lying between the tibia and fibula on the membrane that spans them. Its tendon passes behind the bony bump on the inside of your ankle, turns sharply forward under it, and fans out into a remarkable number of attachments across the underside of the foot.

That fan is unusual. Most tendons attach in one place; this one spreads across the navicular, all three cuneiforms, the cuboid and several metatarsal bases.

What it does

Inverts the foot — turns the sole inward — and assists plantarflexion.

Its important job is holding up the medial arch. Because its tendon passes under the arch and attaches across the bones forming it, its tension supports the arch dynamically during walking and running. The arch also has passive support from ligaments and the plantar fascia; this muscle is the active contribution.

What loads it

Inversion against resistance, and any weight-bearing on one leg where the arch has to be controlled. Running loads it substantially.

The single-leg Romanian deadlift and step-up load it, because holding an arch up on one leg is exactly its job.

What is commonly got wrong about it

That flat feet are caused by weak tibialis posterior. Dysfunction of this tendon is a real and recognised cause of an arch collapsing in adulthood — that much is well described. But most flat feet are simply the shape a person's foot is, present since childhood, and entirely asymptomatic. The two situations are different and get conflated constantly.

That arch supports weaken it. A widely repeated claim, and the evidence for it is thin in both directions. Orthotics are a clinical question with a genuinely mixed literature.

That you can train the arch back up. Strengthening work has a reasonable rationale and is commonly prescribed. Whether it changes the shape of a foot is a much stronger claim than whether it helps symptoms, and only the second has real support. Either way, a painful or changing arch is a physiotherapy question.