Knee
I quoted from、http://www.tahara-seikei.com/1000.htmthe pictures posted here.

A knee is bent and stretched out. There is the muscle and a ligament for this purpose.
Let’s study.
Bone
Femur Kneecap , so called “plate”
Meniscus

Like a right model, there are boards the outside and an inner side.

It is a role of cushion.
Muscle (right knee)

About 12 muscles adhere around a knee.
Front Right-hand side
Left-hand side Rear surface
Bending and stretching of a knee

Muscles are shrunk when a knee is bent and stretched out.
Which is the muscle which works when the right knee is stretched out?
Contracting muscle is shown in blue.

Meniscus is omitted.

 Which is the muscle when the right knee is bent?

When we see from the left (inner side of a leg)
　

When we see from the right (outside of a leg)

When we stretch out a knee, a fibula and tibia are stretched out because a thighbone (thighbone) and a quadriceps that adhere to the kneecap, that is, a plate shrink.


A knee bends by five muscles which are inside a leg, and two muscles which are outside being shrunk.

The ligament which fixes a bone and a bone is required.

The ligament of a right knee
Front in condition that a femur is raised.
Rear surface 　

The number of ligaments is five
fibular collateral, medial collateral, patella, anterior cruciate ligament, posterior cruciate ligament

Literally, fibular collateral ligament is reinforcement of the outside knee, and medial collateral ligament is the reinforcement of the inside.


Patella ligament is the reinforcement of the front part of a knee.

What are the anterior cruciate ligament and the posterior cruciate ligament doing?

The role of the anterior cruciate ligament

If a knee is stretched out early strongly, a lower leg (lower thigh) may project in front.

If it projects in front, we cannot walk. Then, it prevents projecting by placing a ligament like a string strong between bones. In the picture below, the anterior cruciate ligament is shown in yellow.

膝を曲げ、正面から：　front view when the knee bends、　膝を伸ばし後ろから：　back view when the knee is stright

We showed the relations between thighbone (bone on the knee), a crus bone (two bones of the knee), a meniscus and anterior cruciate ligament shown in yellow by an animation.

The meniscus shown in blue parts from the outside inward, and, the inside opens like an animation.
The anterior cruciate ligament adhered to tibial front adheres to femur which is in the slanting backside through the hole of meniscus.

The role of posterior cruciate ligament

If a knee is bent strongly quickly, there is the case that the leg under a knee swings past the maximum back.

When the ligament adhered to the back surface of the tibial adheres to the back medial surface of the thighbone, it prevents from coming off tibia from thighbone when a knee is bent.

We are looking at the back side of the knee.

The anterior cruciate ligament which is visible in front adheres to the backside of the knees.
The posterior cruciate ligament adheres to the bones of top and bottom in the back side of the knee.

This ligament prevent tibia from moving behind than normal.