Popliteus

What it is

A short, flat, triangular muscle deep in the back of the knee, running diagonally from the outer side of the femur down and inward to the tibia. It lies in the floor of the popliteal fossa, the hollow behind your knee.

Unusually, part of its tendon runs inside the knee joint capsule.

What it does

Its famous job is unlocking the knee, and it is genuinely interesting.

When your knee straightens fully in standing, the femur rotates slightly on the tibia and the joint reaches a close-packed position — the so-called screw-home mechanism — where it is mechanically locked and can be held straight with very little muscular effort. That is efficient for standing but it means you cannot simply bend from there.

Popliteus reverses that rotation. It fires first, unlocks the joint, and only then can the hamstrings flex the knee. It is the key turning in the lock.

It also helps stabilise the knee from behind and draws the lateral meniscus backwards out of harm's way as the joint bends.

What loads it

Nothing loads it deliberately and nothing trains it as a target. It works at the beginning of every knee bend from full extension, and it stabilises against rotation continuously.

What is commonly got wrong about it

That the screw-home mechanism is disputed folklore. It is not — the rotation is measurable. What is less settled is exactly how much of it occurs, how much varies between individuals, and how much it matters clinically.

That it can be strengthened usefully. Its role is a timing and stabilising one at a joint angle you pass through constantly. There is no meaningful training target here.

Pain behind the knee as "popliteus tendinopathy". It exists as a described condition and it is uncommon. Pain behind the knee has several more likely causes, and distinguishing them needs an examination rather than a search engine.