Tendinitis is the inflammation of a tendon. Tendons are thick cords of tissue that connect muscles to bone. 

Achilles tendinitis, or an inflammation of the Achilles tendon, is one of the most common causes of foot or ankle pain. Other types of foot/ankle tendinitis include posterior tibial tendinitis and peroneal tendinitis.

Causes
Tendinitis can result from an injury or over-use. Improper stretching prior to exertion or incorrect form during physical activity can also contribute to the development of tendinitis. Some people, including those with “flat feet,” tight tendons or arthritis, are particularly prone to tendinitis.

Symptoms
Pain is the most prominent symptom of tendinitis. The pain will be most noticeable when you try to move that part of your body. The involved tendon may swell.