If you feel the back of your ankle, you will find the tough, ropelike tendon that joins your calf muscle to your heel bone. That's the job of tendons throughout your body: connecting bone and muscle so you can move.
Consider that the Latin word tendere means "to stretch." That's an apt beginning for the word tendon, a tough but stretchy fibrous tissue (sinew). A tendon is made of dense bundles of fibrous collagen that form ropelike connectors that allow muscles and bones to work together. Athletes often suffer injuries to tendons, most often to the rotator cuff in the shoulder, the Achilles tendon in the leg, the patellar tendon in the knee, and the biceps muscle in the arm.