The Knee

The knee joint connects the upper and the lower leg. The femur receives the weight of the torso through the pelvis directly, and that weight transmits through the knee joint to the lower leg, and through the ankle joint to the foot.

There are strong ligaments, cartilages, and muscles that support the movements at the knee joint. And of course, the function of this joint integrates the movement from the joints above (the hip) and below (the ankle).

To begin with, if you simply look at the bone structure, it’s a little bit easier to manage the movements of the leg.

The shape of the lower end of the femur is round, and the top end of the tibia is flat. But the semilunar shape of the menisci resolves this "bad fit" of the two bones, allowing the femur to glide on the tibia head efficiently. This structure—and being able to visualize the femur gliding on top of the tibia—helps to connect both the upper and lower leg bones during movement.

It’s not a split movement at the knee joint; the femur's weight or the direction of the movement is always transmitted and connected to the tibia and the fibula (through the top plate of the tibia), whether the feet are on the ground or not. Even though the rotation of the femur is added to the movement and things get a little more complicated, the connection between the femur and the tibia applies the same way. When this connection is lost, you will most likely grip the joints above (the hip) and below (the ankle).

Simply put, just looking at the skeleton at the knee joint, the femur is always sitting on top of the tibia. Even if the intention of the movement comes from the bottom end (the foot), the connection of the upper and lower leg remains the same.

When the knee is flexed, the movement looks like the knee is moving forward (both the femur and the tibia are pointing forward). But if you move with the awareness that the femur is moving on top of the tibia and the tibia is transmitting the weight through the ankle and into the foot, the connection of the flexing motion doesn't break at the knee joint, and it will ground you to the foot.

And just a bit more to share: how the ligaments and cartilages are designed to support the knee joint is amazing! (I won't mention the muscles for now...)

This is explained very well in the book Body³ by Thomas Myers. I love the way he explains it—it helps me to understand a complicated anatomy.

Here are some quick notes from his book about the names and functions of the ligaments and cartilages at the knee:

• The Medial Collateral Ligament: Supports the inside of the knee by running vertically from the femur to the tibia, preventing the femur and the joint from collapsing medially.

• The Lateral Collateral Ligament: Supports the outside of the knee, attaching from the femur to the fibula, preventing the femur and the joint from collapsing laterally. Both ligaments are further supported by muscles that run outside of them, preventing abduction and adduction of the knee and permitting only a very small transverse motion.

• The Cruciate Ligaments: Inside the knee joint are the two cruciate ligaments. It's complicated, but to summarize, they prevent the knee from hyperextending very far beyond a straight line, and they act in concert to keep the surface of the femur from rolling or sliding off the knee in all its various positions.

• The Menisci: Inside the joint, on either side of the cruciates, lie the semilunar cartilages, or menisci. They act like a cushioned plate to fit the rounded ends of the femur to the flat top of the tibia.

  
  

And so on...

Image is from POCKET Anatomy & Physiology by Shirley A. Jones