The Achilles tendon is the largest tendon in the human body. It connects your heel bone (calcaneus) to your calf muscles and is used when walking, running, and jumping. Achilles tendonitis occurs when the tendon has been overstressed or overworked. This is usually caused by sports, especially if an athlete suddenly changes the intensity or amount of exercise. Tight calf muscles and lack of flexibility to the calf and ankle can also cause stress to the tendon and over stretch it. As we get older, our tendons become less flexible, which is why middle-age athletes tend to be more susceptible to Achilles tendonitis. Poor athletic equipment or a sudden change in footwear can also cause Achilles tendonitis. Overuse is a major factor seen in runners who add too much mileage to their runs too quickly or for those who frequent hill training. And lastly, poor running form can also aggravate the achilles - typically due to overstriding.