Tendonitis (or more formally tendinitis) is an inflammation of a tendon, the strong stringy tissues that attach muscles to bones that allow muscles to move the bones when the muscle contracts. Through overuse, these tendons become inflammed, painful and unable to pull through their full range of movement. Any tendon can become inflammed, but tendons that are responsible for large ranges of movement are the most likely to do so, such as those in the heel, knee, shoulder and elbow. It is very common in athletes, although it can affect anyone.