Like shock absorbers in your car, the arches in your feet handle the jolt that comes with each stride. Hoist a heavy pack and the impact increases. In extreme cases, arches can collapse, spread, and permanently elongate by a full size or more.
Here's a simple way to find out if your feet elongate. Have someone carefully trace both of your bare feet on a piece of paper as you sit and again as you stand. You may want to try this wearing a heavy pack as well.
If the two tracings are identical, you're among the fortunate hikers with good shocks. If one or both are marginally larger, don't worry. "Mild elongation (less than 1/4 inch) is okay, as long as you buy a properly sized boot," says Phil Oren, owner of FitSystem, an Arizona-based company devoted to teaching better boot fitting. Fail to accommodate your pack-lengthened feet, though, and you risk toes bumping the front of your boots, which can lead to a painful black toe, toes bent permanently out of shape (hammertoe), or a crop of other problems. Oren recommends that you have your feet measured-while standing-when buying boots.
If the second tracing of your feet is more than 1/3 inch longer, it could mean your arch is collapsing. "Look into getting a footbed-either off-the-shelf or made by a podiatrist," advises Oren.