The popliteus is a small muscle that runs diagonally down and across the back of the knee, from outside to inside (anatomically, from the lateral distal femur to the medial proximal tibia).
It lies buried deep behind the fibres of the calf muscle gastrocnemius which comes up from the lower leg and attaches to the bottom of the femur of the upper leg. In fact, it's the deepest of the muscles at this point, lying closest to the bone.
The popliteus muscle is small, with a short belly of diagonal fibres.
Because it crosses the knee joint from behind, popliteus helps to flex (bend) the knee. But it's such a tiny muscle compared to the other muscles that cross the back of the knee (the hamstrings and the gastrocnemius), it really doesn't help that much... we say it's a weak flexor of the knee.
Because it runs diagonally, popliteus also rotates the knee inwards when it's bent.
But the real importance of popliteus is in its role as the key that unlocks the knee - it is responsible for "unlocking" the knee, when the leg is locked straight (fully extended). It's the diagonal nature of popliteus that's important here.... by causing a bit of a twist at the back of the knee when it contracts, it allows the knee to start to bend. Without this, the bigger knee flexors would just pull straight on a locked knee, making it lock tighter.
If you have trouble unlocking your knee from the fully straightened position, then it's the tiny popliteus muscle that's to blame.
Most injuries to popliteus are simple overuse ones. Too much running or walking in a short space of time when you haven't built up your exercise slowly enough.
Overuse of popliteus is pretty much always closely related to tight hamstrings.
Treatment of a troublesome popliteus is usually straightforward:
If you've dislocated your knee, been involved in an accident (or tackle) that's damaged your ligaments, then the damage to popliteus won't be a simple overuse injury, and a doc is more likely to want to scan it to check.
As ever, if you think I can help, give me a call.