The Achilles tendon is a group of tough fibrous tissue that connects the calf muscles to the heel bone. It functions to lift the heel while walking or running. Achilles tendonitis is defined as the inflammation of the Achilles tendon. Constant overuse or repetitive activities can cause Achilles tendonitis. Participating in sports such as tennis also can cause Achilles tendonitis. Sudden increase in physical activities and wearing improper shoes can also cause tendonitis.