Tendinitis, also known as tendonitis, is an inflammation or irritation of a tendon, the thick fibrous cords that attach muscle to bone. This condition is typically caused by repetitive, minor impact on the affected area, or from a sudden, more serious injury. Tendinitis can affect any tendon, but it’s most commonly found in the shoulders, elbows, wrists, knees, and heels. Pathology Behind Tendinitis Tendons are tough, flexible bands of fibrous tissue that connect muscles to b