Achilles tendinitis - The Achilles tendon runs down the back of the leg and connects to the calf muscle. It can become inflamed from overuse and inflexibility. Younger runners tend to strain the Achilles just above the heel, but as runners age, tendinitis usually occurs higher, where the Achilles connects to the calf muscle. An inflammed Achilles feels tender and stiff.
Causes - Running tends to tighten the calf muscle.

When the muscle becomes too tight, it doesn't allow for the normal biomechanics of running, and the Achilles tendon becomes strained and inflamed. Running steep hills or increasing your weekly mileage too quickly can lead to inflammation of the tendon. If you continue to run despite the pain, the inflammation can turn into partial tears of the tendon. Eventually, part of the tendon will die, and the weakened remaining tendon can easily rupture.

Treatment - Take an anti-inflammatory such as ibuprofen or naproxen sodium two or three times a day. Massage the Achilles with ice and take a few days off. In some cases, a quarter-inch to half-inch heel lift will alleviate the stress on the tendon. If you still have pain after a couple of weeks, you should see a sports-oriented physical therapist or podiatrist.

Prevention - Since tight calf muscles and a tight Achilles usually lead to Achilles tendinitis, stretching the calf and tendon are imperative. Contrary to conventional wisdom, it's best to stretch your tendon after you run, not before. That way, your tendon is fully warmed up and receptive to a slow, gradual stretch. Never stretch to the point of pain. Also, consider switching to a firmer, motion-control shoe to limit rearfoot motion and overpronation, and make certain that there isn't any pressure or rubbing from your shoes on the Achilles tendon. Eliminate or cut back on hill training.