From Jonathan Cluett, M.D.,
Your Guide to Orthopedics.
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

A tendon is a tough yet flexible band of fibrous tissue. The tendon is the structure in your body that connects your muscles to the bones. The skeletal muscles in your body are responsible for moving your bones, thus enabling you to walk, jump, lift, and move in many ways. When a muscle contracts it pulls on a bone to cause movements. The structure that transmits the force of the muscle contraction to the bone is called a tendon.

Tendons come in many shapes and sizes. Some are very small, like the ones that cause movements of your fingers, and some are much larger, such as your Achilles tendon in your heel. When functioning normally, these tendons glide easily and smoothly as the muscle contracts.

What is tendonitis?
Sometimes the tendons become inflamed for a variety of reasons, and the action of pulling the muscle becomes irritating. If the normal smooth gliding motion of your tendon is impaired, the tendon will become inflamed and movement will become painful. This is called tendonitis, and literally means inflammation of the tendon.

What causes tendonitis?
The most common cause of tendonitis is overuse. Commonly, individuals begin an exercise program, or increase their level of exercise, and begin to experience symptoms of tendonitis. The tendon is unaccustomed to the new level of demand, and this overuse will cause an inflammation and tendonitis.

Another common cause of symptoms of tendonitis is due to age-related changes of the tendon. As people age, the tendons loose their elasticity and ability to glide as smoothly as they used to. With increasing age, individuals are more prone to developing symptoms of tendonitis. The cause of these age-related changes is not entirely understood, but may be due to changes in the blood vessels that supply nutrition to the tendons.

Sometimes, there is an anatomical cause for tendonitis. If the tendon does not have a smooth path to glide along, it will be more likely to become irritated and inflamed. In these unusual situations, surgical treatment may be necessary to realign the tendon.

Read on for more information about different types of tendonitis...