You can easily inflame your Achilles tendon if the back of your shoe is not cut out in the manner shown (see figure at right). Although most shoes provide this cut out, always check to make sure yours do.
Foot orthotics are shoe inserts designed to help control irregular foot motion. When used for race walking, they add stability to lighter, overly flexible shoes. Orthotics fall into three broad categories: rigid, primarily for changing foot function; soft, serving chiefly for protection; and semi-rigid, combining functional control and protection. They also vary in length. More flexible orthotics offer greater comfort but deteriorate quicker. More rigid orthotics often last many years, but beware: they can cause blisters.
The image to the upper-right is one of my own orthotics: rigid and very durable. I cannot race walk without them. Having bad knees since high school, I comfortably walk many miles in less supportive shoes by using orthotics. But if I try to walk even a few miles without them, my knees tighten up almost immediately. Lately I train in Asics Tiger Paws that would fail the sole flexibility test out of the box. With orthotics, however, they have arch support and bend near the toe box.
Plan ahead if you want orthotics. They tend to take over a month to order and a reasonable amount of time to break in. Typically made by a podiatrist, they can cost several hundred dollars and often are not covered by medical insurance. Though expensive, orthotics can be a very worthwhile investment.