Muscle · Lower leg

Tibialis posterior

Musculus tibialis posterior

The deepest muscle in the calf and the single most important active support of the arch. It sits between the two shin bones, and its tendon curls behind the knobble on the inside of the ankle before fanning out into almost every bone of the midfoot — an attachment pattern that lets it lift and lock the arch from underneath. It turns the foot inward and, more importantly, it stops the arch collapsing every time you take weight. When it fatigues or fails, the arch flattens, the foot rolls in, and the load has to be absorbed somewhere else — which is why it turns up in arch pain, shin pain and gait problems.