Subtalar

What it is

The joint between the talus above and the calcaneus — the heel bone — below. Its name is simply its position: below the talus.

It is a joint with an oblique axis, angled through the foot rather than aligned with any of the usual anatomical planes. That oblique axis is why the movement it produces is a combination rather than a simple one.

What it does

Inversion and eversion — turning the sole of the foot inward and outward. The ankle joint above it does none of this; it is a pure hinge.

Its practical importance is adapting the foot to the ground. When you walk on a slope or an uneven surface, this joint is what lets the sole stay in contact while the leg above stays vertical.

It also couples foot and leg rotation: because of its oblique axis, turning the foot in and out produces rotation up the leg, and vice versa. Movement here is transmitted upward towards the knee.

What loads it

Any weight-bearing on uneven ground, single-leg balance, and running — particularly on cambered or irregular surfaces.

What is commonly got wrong about it

That "ankle rolling" happens at the ankle. The rolling motion of an inversion sprain is largely subtalar. What tears are the ankle ligaments, which is why the injury is named for the wrong joint.

"Pronation" as a running-shoe category. The pronation that shoe marketing refers to is a combined movement involving this joint and the midfoot. The claim that classifying runners by pronation and matching them to shoe categories reduces injury has been tested and has generally not held up. It remains the organising idea of shoe retail.

That flat feet or high arches predict injury. The relationship is far weaker than commonly assumed, and foot shape varies enormously among people with no problems at all.