Tendinitis (also called tendonitis) is a condition that occurs when a tendon — the tough cord that connects muscle to bone — becomes inflamed or irritated. It’s commonly caused by repetitive motion or overuse and often affects areas such as the shoulders, elbows, wrists, knees, and heels. Tendonitis (tendinitis) is a condition where the connective tissues between your muscles and bones (tendons) inflame. Often caused by repetitive activities, tendonitis can be painful. Learn how repetitive stress injuries cause tendonitis, the common symptoms, and treatment options to ease pain and restore function. Tendinitis is inflammation of the tendons, which are the tapered ends of muscles that attach to bones. Tendons are in action every time muscles contract and bones move. As such, tendonitis is often the result of overuse, such as repetitive movements at work. Tendinitis is most common in the shoulder, elbow, knee, wrist, and heel, although it can happen anywhere that tendons are found in the body. For uncertain reasons, tendinitis is also common in people with diabetes. Tendinitis is new or sudden swelling and irritation, called inflammation, of a tendon. Tendons are thick fibrous cords that attach muscle to bone. Tendinitis causes pain and tenderness just outside a joint. Tendinitis can happen in any tendon. It's common around shoulders, elbows, wrists, hips, knees and heels. Tendonitis - an easy to understand guide covering causes, diagnosis, symptoms, treatment and prevention plus additional in depth medical information. Video: What is tendonitis? This animation explains what tendonitis is and what causes it. Tendonitis is an inflammation of a tendon—the tough cord of tissue that connects muscles to bones. Tendonitis can affect any tendon, but is most commonly seen in the wrist and fingers. Often, a physical exam alone can diagnose tendinitis. Your healthcare professional may press on the affected joint and move the joint into different positions. Sometimes your healthcare professional may use imaging tests to help with diagnosis. Imaging tests may include: X-rays.