When you make wet footprints on the bathroom floor, you will see that there are blank patches in the middle between the toe marks and the heel marks. This is because the middle parts of the feet are raised off the ground by two sets of muscles, called the tibial and the peronal muscles which makes the arches of the feet.
If these muscles weaken, the feet spread out and the arches sink to the ground. The footprints are flat, with no gaps in the middle. This is called having flat feet, or fallen arches.
Sometimes people get flat feet because their jobs keep them standing around too much. Policewomen, waitresses and nurses often suffer from flat feet.
The cure is exercise which tighten up the muscles again. Supports to be put inside shoes are worse than useless. They make the muscles slacken, weaker instead of stronger.