The Quick Guide to Understanding How Your Knee Works

The knee isn’t super complex. It bends, it straightens, and allows for a bit of rotation. Yet even simple things sometimes need a user manual.

That’s because a pair of healthy knees is essential to active living. Try getting off the floor or landing with your legs locked out to understand why.

Without the knees walking and running would be awkward and much slower, jumping wouldn’t be much of anything, and our ability to raise and lower ourselves to a chair or ground level would be extremely limited. They also dissipate a ton of force which lessens the loads on our hips and low back. 

So thankfully we have them, even if they are prone to pain and injury. 

This is partly due to restrictions on how the knee is designed to move. Apply enough force the wrong way and you will quickly be out of commission. 

It also plays the middle man between the ankle and the hip. Often taking the brunt of any movement issues coming from these areas above and below the joint even with everything around the knee working just fine.

We’re here to help guide you through these issues, but the first step is to develop an understanding of your knees. From there it should make more sense as to why they hurt and the plan to fix them.  

This article is the 101 to the knee joint. We’ll walk through the major structures around the knee, laying the groundwork to get deeper into the issues that affect it and ways to fix it.

Bones of the Knee- “The Framework”

Let’s start by reviewing the bones that make up the knee. There are only 3 (and kind of a 4th) that lay the groundwork.

Femur

(aka- Thigh Bone)

The femur is the only bone of the upper leg. It is both the longest and strongest bone in your body and is excellent as at absorbing force.

It’s not pictured above, but as you’re probably aware, the upper end of the femur connects to your hip as a ball-and-socket joint. Thus, movement at the hip joint directly affects the position and alignment of the knee.

 

Tibia/ Fibula

(aka- Shin Bone)

The tibia is your shinbone and creates the lower part of your knee. On top, the tibia is a shallow surface where the cushioning for the knee sits. This cushion is called the meniscus and will get further discussion later.

Some people say the fibula is a part of the knee joint too; however, most don’t agree since it doesn’t directly connect to the knee. It’s still worth noting since it lives so close and is the site for an important ligament attachment for the knee.

 

Patella

(aka- Knee Cap)

The patella is best known as the kneecap and sits over the top of the knee in a groove in the femur simply called the patellofemoral groove.  It acts as a hard hat to protect the inner workings of the knee joint, but most importantly it acts as pulley system to help straighten the leg.

 

The Patella Pulley System

To further explain the pulley system of the knee joint, think of the muscles on the front side of the leg as a rope, which pulls over the patella. The patella provides mechanical advantage to help increase the amount of power that the quadriceps can generate.

You can see from the picture that without the kneecap, the quad tendon would essentially pulling on the lower leg from directly above it. This would make it really tough to extend the leg, especially from positions where the joint was extremely flexed, like crouching.

This also highlights the issues that come up if the knee cap isn’t moving correctly. If the patella isn’t tracking the right way in its grooves, it will affect how knee moves and limit the legs ability to generate force.

Ligaments, Tendons, & Cartilage- “Connecting the Bones”

The next piece of the puzzle is what brings these bones together. Just like our other joints, it’s a connection made of ligaments, tendons, and cartilage.

Many of these structures have infamous names for their association with major knee issues. Things like the ACL, meniscus, and patellar tendon probably sound familiar, but ever wonder what they are?

 

Ligaments- Connecting Bone to Bone

Starting with the ligaments, which are the the things that connect the bones together.  Remember, ligaments connect bone to bone. Each of the ligaments has 1-2 specific jobs, for example, stopping the lower leg from sliding too far forward.

There are four main ligaments in the knee:

ACL- anterior cruciate ligament
PCL- posterior cruciate ligament
LCL – lateral collateral ligament
MCL- medial collateral ligament

The ACL and PCL are termed the “cruciate” ligaments for the cross, or x-shape, that they form inside the knee joint.

  • Patellar and Quad Tendon
    The quad tendon is located on the top of the knee, attaches the quad to the patella.    Go below the knee cap and you’ll find the patellar tendon which isn’t actually named correctly.  It’s actually a ligament (remember that means bone to bone) because it attaches the kneecap to the shin bone. It’s common for both of these areas to become irritated, resulting in forms of tendonitis, usually due to strain and overuse.
  • Hamstring Tendon
    The hamstring tendon crosses the back of the knee and attaches to the tibia and fibula. Some will have irritation here for various reasons, but it’s more common to strain a hamstring near the hip rather than the knee. You’ve probably heard of an athlete having a high-hamsting strain. For another fun knee fact, one of the hamstring tendons is a popular replacement for then ACL  during surgical reconstructions.

Cartilage- “The Cushion”

  • Meniscus
    The largest piece of cartilage in the knee is called the meniscus. The meniscus is a C shaped ring that lives on top of the tibia (shin bone). It helps cushion a lot of force coming from the ground and reduces friction.You can see why the meniscus is easy to injure. It is the cartilage that helps take the force that is coming from the ground and disperse it.The meniscus has been likened to a sponge in that it needs some water to be healthy (we keep water in the meniscus with the right level of impact). But, too much impact (or too much water in the sponge) can leave the sponge brittle.
  • The Other Cartilage
    There is lesser-known cartilage to the knee that’s equally as important called articular cartilage.Articular cartilage covers the bone ends to reduce friction when bones rub against each other. Most importantly it covers the areas between the femur and tibia, and underneath the kneecap.Different than other areas of cartilage, this cartilage is not great at absorbing force, and instead more helpful for reducing friction.

Muscles and Bands – “The Movers”

Now we have all reviewed the makeup of the knee, let’s talk about how it moves.

Things that Move the Knee

These are the major movers that live around the knee joint. This means that when they contract they have the direct ability to make the knee move. This isn’t a comprehensive list, but covers the major players:

  • Quadriceps (AKA quads)
    The quads are made up of four different muscles, they are really good at getting our knees straight, and doing it with a lot of power
  • Hamstrings
    The hamstrings are made up of three different muscles. They bend our knees towards our butts. They are also a big dynamic stabilizer of the knee. Meaning they help support the knee when things are moving quickly.
  • Illiotibial band (AKA IT band)
    The IT band starts at the hip and attaches outside the lower part of the knee.  Although as the name implies, the IT Band is not a muscle, it’s is a band of fascia, but it’s appropriate to put it here because it’s an extension of a muscle and it’s often treated like one. We’re inclined to smash, roll, scrape, and stretch it to “loosen” it up, but you will not change the very strong fascia of the IT Band.
  • Adductors (AKA groin muscles)
    There are also three groin muscles that pull out leg towards the midline.
  • Gastrocnemius (AKA calf)
    You may have never thought of the calf behind a part of the knee, but the way our calf muscles attach to the leg it will also help bend the knee.

Stability

Taking a step back and looking at the big picture we can see how important the knee is, but linking the pieces together we can’t forget about what happens above and below the joint.

The stability that comes from the ankle and the hip directly impact how the knee works.

It should make sense though. Now that we have reviewed the bones and muscles that make up the knee, you may have noticed some overlap between the bones of the knee and bones of the hip or ankle. That is because they are the same.

For example, the top part of the knee joint (the femur) is the same bone in our hips. So it can be easy to see how what happens at the hip directly affects the knees.

We won’t dig into the specifics of how these are intertwined in this article, but understanding how closely these joints are related can be helpful to improving the stability of the knee by looking above and below the joint.

From here we’ll start laying out specific pain points that affect the knee, but more importantly, how to fix them.