Even with all the new technology and research in running shoes, why do the experts still recommend that we replace our shoes every five hundred miles? It seems to me that as the price of shoes increase, their life span should increase as well - Everlasting Shoes
There are two reasons why the experts recommend that you replace your running shoes every five hundred miles: (1) The Earth is mostly very hard and (2) your body is mostly very soft. Were it not for these two properties, a runners need to replace their running shoes would be of minimal importance. Imagine a world where the streets were paved with diamonds, our running shoes were constructed of high gauge iron, and our bodies were made of reinforced steel. Ignoring, for a moment, the numerous ill effects on the universe within such a world, we would find that your desires for an ever lasting shoe would be more plausible.
When you consider the physical interaction between your malleable body and the Earths solid hardness, it is not difficult to imagine why your rubber sole shoes wear down approaching the five hundred mile mark of repeated pounding. With each crushing stomp of pronation your soft, rubber-shoe sole makes a sub-atomic physical bond with the road (literally becoming "one" with the planet). This interaction is called friction and it is the adversary of runners world-wide.
It is friction that we curse when suffering from Chrondromalacia Patella, it is friction we despise when the first symptom of a stress fracture is found in a callus formation on the outer edge of our tibias. Friction is the enemy, and the source of all our woes.
As friction is our adversary, so too is gravity: that downward sucking force that cements us to our planets surface despite all efforts to remain airborne through the efforts of running. Future Lunar or Martian marathoners will laugh at us Earth-bound runners as they traverse their lightened loads across the "magnificent desolation" of their worlds. We are cursed to live upon the third planet from our star, and remain subjected to the 32 meters/second acceleration that bonds us to our own spinning globe.
With each mile that you run, your running shoes wear down in a way closely related to your bio-mechanics. Those who over-pronate tend to have the inner soles of their shoes "scraped away", while under-pronators have the outer soles depleted. Flat footed "neutral" runners find that their soles wear down over the middle arch and "ball" of the foot. In every case, it is the constant pounding friction that demands such replacement. The "five hundred mile" limit is more of a guideline than it is a strict rule. With some poorly constructed shoes, the limit will be closer to two hundred miles.
Failure to replace older, worn-out shoes will lead to a running injury. Remember that the Earth is unlikely to yield to the pounding force you place upon it with each run and your body's mechanical motion is likewise unlikely to change. Thus if you are running with a shoe that is unable to compensate for your form, weight and persistence, it will be your body that succumbs to the uncorrected stress. The results could be painful and nasty.
Many runners chose to purchase an extra pair of shoes, which they "alternate" in use, to prolong the need to purchase new shoes with increased frequency. I typically purchase two to three pairs each year, spaced out every four months.
Technology can only do so much to prevent you from breaking some part of your body. In the end, it is the physical limitations induced upon us by nature that require frequent replacement of our running shoes. And if it is the natural forces of the planet that you seek to overcome, remember we have eight others in our solar system with plenty of wild spaces for you to consider trotting upon. But if your solution takes you to these other worlds remember that you will still be faced with the evil powers of friction. And think of this: replacing your running shoes is significantly less expensive than replacing your space suit!
Run long and taper.