What is tendonitis?  Tendonitis (also spelled tendinitis) is inflammation of a tendon.  What is a tendon? A tendon is a flexible connective tissue structure that connects a muscle to a bone. What is INFLAMMATION?  Inflammation is the localized response of the body to injury caused by chemical, physical or biological agents. Most tendonitis that is seen by health care providers falls under the physical category and is related to overusing the tendon, most commonly in athletic endeavors.  The localized response of inflammation within and around a tendon is associated with several noticeable physical symptoms.  They are: redness, swelling, heat, pain, loss of function or motion of the tendon, and sometimes fever.  Tendonitis can affect any tendon of the body, but there are several tendons of the lower extremity that have a high likelihood of developing tendonitis.