A tendon is a tough, rope-like cord of tissue that connects muscle to bone. When you bend your arm, wiggle your fingers, or jump, your muscles pull on tendons, which pull on your bones to create movement. Without tendons, your muscles would contract uselessly, unable to move your skeleton at all.

You can feel your own tendons. Make a fist and look at the back of your hand: those ridge-like cords running from your knuckles toward your wrist are tendons. The largest tendon in your body, the Achilles tendon, connects your calf muscle to your heel bone. It's named after the Greek hero Achilles, whose only vulnerable spot was his heel.

Tendons are incredibly strong, built from tightly bundled fibers of a protein called collagen. They need to be strong because they handle enormous forces: your Achilles tendon supports forces several times your body weight every time you run or jump. But tendons can be injured through overuse or sudden stress. A torn tendon is a serious injury that can take months to heal because tendons have a limited blood supply.

Athletes, musicians, and anyone who repeats the same motion frequently need to take care of their tendons to avoid injuries like tendinitis, where a tendon becomes painfully inflamed.