I’ve got a quiz question for all you anatomy nerds: what muscles bend the knee?
If you said hamstrings, you’re mostly correct.
These are for sure the ones you feel during a knee flexion exercise at the gym.
But here’s the catch. The hamstrings are incapable of bending the knee if the knee
is totally locked out straight. They depend on a tiny muscle called the popliteus to unlock the knee
and let them take the joint through flexion.
And the popliteus uses a little trick of kinesiology called the screw home mechanism and it is fascinating to me.
Today, I'm going to show you how it works.
[intro]
Hello and welcome. If you’re new here,
my name is Patrick and this channel is all about anatomy and physiology.
So to understand how this little muscle unlocks our knees,
we need to know some of the more detailed structures of the knee joint.
More specifically, the tibiofemoral joint, which is made of the tibia and femur because anatomists are boring
when it comes to naming. But we’re interested in the bumps at the end of each of these bones.
At the end of the femur, we have two condyles that articulate with the tibia.
This is the right femur, so this is the medial condyle while this is the lateral condyle.
Out on the sides are the epicondyles, and in the middle is the trochlear groove,
or notch, it’s a little groove for your kneecap to glide in.
Those condyles of the femur sit nicely into little discs of connective tissue called meniscus,
or menisci for plural. For those of you who measure liquids in a lab, a meniscus in the
knee is the same shape as the meniscus in a test tube. They sit on top of the tibia, what we call
the tibia plateau, and share a lot of tissue overlap with the big cruciate ligaments — the
ACL and PCL. Inferior to the tibial plateau, the tibia had medial and lateral condyles as well.
That structure — two big condyles sitting in a little groove — creates something called a hinge joint,
which means it only moves in one plane.
It can extend thanks to the quadriceps anteriorly, and it can flex thanks to the hamstring group
in the back. The quads group shares a common insertion on the tibial tuberosity,
that big bump on the front of your tibia, but the hamstrings are more diverse.
The biceps femoris is the most lateral and inserts on the fibular head. Meanwhile the semimembranosus
and semitendinosus insert on the medial condyle of the tibia after they pass behind the medial
condyle of the femur. When the hamstrings work together, they pull the knee into flexion.
Okay, so while we call the knee a hinge joint, it’s quite a bit different to the hinge on a door.
Instead of one static pivot point, the axis for the knee is actually in the middle of the femoral
condyles. So during flexion and extension, the tibia has to slide back and forth on
the condyles of the femur which it does thanks to a little help from the cruciate ligaments.
During extension, the PCL tightens and helps glide the tibia anteriorly.
Likewise during flexion, the ACL tightens and helps glide the tibia posteriorly.
Here’s the problem. The knee isn’t a perfect,
symmetrical hinge and that causes some weirdness at full knee extension.
Specifically, those condyles aren’t perfectly even — the medial condyle of the femur is
about half an inch longer than the lateral condyle. So, while the tibia is sliding on
the femur during the last 20 degrees of knee extension, the tibia externally rotates about
15 degrees — this direction — on the femur. This phenomenon is called the screw home mechanism.
At the same time, this full knee extension is actually slightly hyperextended.
So after we’ve rotated our tibias into position, soft tissues like the cruciate
and collateral ligaments are pulled taut which makes the knee a little more stable.
And when it comes to standing upright that’s a huge advantage.
We can lock out our knees and rely on connective instead of using energy through our muscles.
Unfortunately, getting out of full knee extension presents a little bit of a problem.
The hamstrings pull straight up. They don’t insert at an angle that will
let them untwist a tibia that’s been rotated by the screw home mechanism.
And that’s where the popliteus muscle comes in. This muscle originates on the lateral aspect of the
femur and upper part of the lateral meniscus and inserts on the medial proximal tibia.
See how it runs in this kind of diagonal path? That will allow it to internally rotate the tibia,
unlocking the knee. The muscle has a twisty path of pull because it’s job is to untwist the tibia.
But there’s a big ol’ caveat attached to all this.
Because how the popliteus moves the knee depends on whether the foot is on the ground or not.
If the foot is on the ground, the tibia is stable and it’s what’s called a closed chain
movement. These are things like squats or deadlifts or lunges. But if the foot is off the ground,
the femur is now the more stable part of the joint, and you’re performing what’s
called an open chain movement. This would be something like a hamstring curl on a machine.
In a closed chain movement the popliteus will contract and externally rotate the femur to unlock
the knee. But when you lift your leg off the ground into an open chain movement, the popliteus
will internally rotate the tibia to unlock the knee, then the hamstrings can follow through after that.
So let's do a high level recap: the screw home mechanism is an external rotation of the tibia during terminal
knee extension that happens thanks to the uneven bumps at the end of the human femur.
But what’s the big deal, why does any of that matter?
It matters because the ability to completely extend our knees
is crucial for day to day activities. If we couldn’t get into full knee extension,
we would not be able to stand for a long period of time, and walking would be a lot harder.
So folks in physical therapy and orthopedics spend a lot of time focusing on that specific
range of motion, terminal knee extension. They’re not necessarily trying to rehabilitate
the popliteus muscle, but because they’re working in the last few degrees of knee extension,
they need to know that the tibia is going to twist a little bit. It’s
all part of the puzzle they have to solve in the rehabilitation journey.
I hope you learned something today. If you did, consider liking the video and telling
me more about yourself in the comments. Are you a student who needs a little more help?
A patient who’s learning a little extra outside of the clinic? Who are you people?! Otherwise,
thank you to my patrons on Patreon. Have fun, be good, thanks for watching.