What are tendons and tendon sheaths?

Our bones, muscles and joints work together in a coordinated way to move our body and give it stability. Tendons and ligaments play an important role here, too: Tendons connect muscles to bones, allowing us to move, and ligaments help to hold things in place.

What are tendons made of, and what do they do?

Tendons are made out of connective tissue that has a lot of strong collagen fibers in it. This means that they are very resistant to tearing, but not very stretchy. Compared to muscles, they have fewer blood vessels in them too. Because of this, they are prone to injury when overstrained and take a lot longer to heal than other types of tissue in the body.

The bones are connected to the muscles, which cause the bones to move when the muscles tighten and relax. In some parts of the body, the muscle is directly attached to the bone. But this isn’t always possible because there often isn’t enough room on the bone, or the bone and muscle are too far apart. Tendons then act as space-saving “connectors” that transfer the movement of the muscle to the bone. One end of the tendon is attached to a muscle. The other end is firmly attached to the membrane covering the bone (the periosteum) or to the bone itself.


Illustration: Tendons connect muscles to bones – Example here: the bicepsTendons connect muscles to bones – Example here: the biceps

What do tendon sheaths do?

Some tendons run through narrow tunnels made of bones and ligaments, and some are stretched over bumpy parts of bones, for instance in the wrist and foot. In these places, the tendons are often protected by layers of connective tissue known as tendon sheaths. Tendon sheaths are filled with a lubricating fluid, allowing the tendons to move smoothly and freely through them.