Popliteus

Pronunciation: pop-LIT-ee-us

Structure & function

Location

A small, flat, triangular muscle deep in the back of the knee, beneath the gastrocnemius.

Origin

Lateral condyle of the femur.

Insertion

Posterior surface of the tibia (above the soleal line).

Main action(s)

"Unlocks" the fully extended knee by rotating the femur on the tibia (or the tibia on the femur, depending on which is fixed), allowing flexion to begin; assists knee flexion.

Joint(s) crossed

Knee joint

Movement plane

transverse

Innervation

Tibial nerve (L4–S1)

In daily movement

The small rotation that happens at the very start of bending the knee after standing fully upright, such as beginning to walk or sit down.

In exercise

Not a prime mover in typical strength exercises, but active as a stabilizer, especially in movements that begin from a fully straightened knee.

Commonly confused with

Easily overlooked entirely since it is small, deep, and not a prime mover — but it is a distinct, named muscle with a specific mechanical job.

Common misconceptions

It is not a knee flexor in the same league as the hamstrings — its role is unlocking/rotating, with only a minor flexion contribution.