Monitoring Shoe Wear
Running shoes are usually pretty sturdy, especially those made for heavier runners, but they don't last forever. The midsole of a typical running shoe is made of a foamy substance called EVA, ethylene vinyl acetate. With each stride, the EVA is flattened. It then rebounds to provide the cushioning you need.
As you run, the cushioning properties of the EVA diminish, and the foam is also affected by the perspiration from your feet. Eventually, the EVA just stays compacted. At that point, your running shoes have become a liability, and this happens long before the shoes show outward signs of wear.
A good pair of running shoes should last 300 to 500 miles. If your shoes start giving you problems before you get to 300 miles with them, seriously consider changing brands or styles. To keep track of the miles, make sure that your logbook indicates which shoes you were wearing during each run.
Running in worn-out shoes is asking for trouble. If you try to extend the use of running shoes that have lost their protective properties, you will end up injured. Shin splints and Achilles tendonitis are common problems associated with running in shoes past their prime.
How a running shoe feels is a good gauge of whether it's time to get a new pair, but there are other indicators. Look at the shoe from the back. If the heel is worn down or compressed on one side, it's probably time to change.
Such wear might also be a sign that it's time for you to visit a podiatrist, who can evaluate you for a pair of orthotics. These are inserts designed to correct flaws in your gait. If you show excessive wear on one part of your shoes, that may be indicative of a problem that can be solved with the inserts.