The knee is the largest joint in the body and works like a hinge.

The longer version

As the largest joint in the body, the knee has to carry body weight while still allowing movement, which is a demanding combination. Its main motion is flexion and extension, and the shapes of the bone ends plus strong ligaments on either side keep that motion close to a single plane.

Two crescent-shaped pads of cartilage sit between the bones, spreading the load and improving the fit. Fluid inside the joint capsule lubricates the surfaces and nourishes the cartilage, which has almost no blood supply of its own. Small rotations are possible when the joint is bent, which is why twisting forces are awkward for it.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.