Popliteus Exercises and Stretches Popliteus – The Key of the Knee Due to this unlocking function, the Popliteus is often referred to as the key of the knee joint. However, it is not only this analogy that gives rise to this nickname. The Popliteus is a very important and often overlooked muscle when it comes to injury prevention . This is why it is VERY important to use these stretches.