The Achilles tendon is the strongest tendon in the leg, and serves an important purpose by allowing the ankle to move up and down in a stable manner. This tendon begins in the calf as a combination of two different muscles, and attaches to the back of the heel bone. It can easily be felt through the skin, and can usually be seen bulging out of the skin as well. The Achilles tendon can become injured through both trauma and chronic stress, and unfortunately both of these occur commonly. When a force is applied to the foot that pushes it upward, the Achilles tendon can become strained or even tear, especially if the tendon is actively contracting downward at the same time. These types of injuries occur commonly in running sports such as basketball, football, track, and soccer. If the tendon does not outright tear, it will become inflamed and painful. The inflammation can be constantly renewed if the tissue is not supported, if the activity that caused it is continued, or if bone spurs on the back of the heel bone continually irritate the tendon fibers. Additionally, there is an area in the tendon a couple inches above the heel bone where the tendon has poor blood supply, and is weaker structurally. This area is much more likely to be injured and become inflamed, and is also more likely to rupture.
Achilles tendonitis pain can be felt as a sharp stab or a dull ache on the back of the heel. Bending the ankle upward or forcefully bending it downward can cause pain, as can simply walking. Sometimes the area behind the heel can become swollen, however this is not frequent and is typically associated with a rupture, as is bruising. Shoes that have a hard heel counter (the material that wraps around the heel) can irritate the heel tissue, as can gently bumping the back of the heel into something.