Running shoes wear out before they stop working. .

This article is the first of two parts.

'To say running shoes wear out between 300 to 500 miles is like saying all car tires wear out between 10,000 and 30,000 miles without taking into consideration car make and model, driving conditions, driver and other factors. Changing your tires too early will waste money unnecessarily, but waiting too long could result in disaster. The same can be said when evaluating the life of a shoe. If, for example, you place a pronator in a humid running climate in a highly-cushioned shoe built for a supinator, the shoe may only last 50 miles.

With most pronators, every time the foot strikes the ground the arch crushes the medial aspect (inside) of the midsole, as the foot excessively rolls inward during the stance phase. Shoes built for pronators, then, should emphasize motion control of the rearfoot. However, shoes made for supinators, who excessively roll their foot outward during midstance, should emphasize shock-absorption over motion control in order to cushion the lateral (outside) edge of the foot. A humid climate will also contribute to the shoe's rapid breakdown because running in a wet shoe will overstretch the shoe's upper aspect (the usually canvas portion) while overcompressing the shoe's lower aspect (the three layers of sole) (Figure 1).

Running in a shoe that no longer provides traction, support and cushioning can lead to a number of musculoskeletal complaints and injuries, among them heel pain, shin splints and stress fractures. The key to pain-free, healthy running is knowing what shoes to purchase according to your foot type and running style, as well as the reasons running shoes wear out. You should learn to recognize failing shoes so that you may retire them before the shock absorption is gone, the traction is worn out and the support mechanism is damaged. In the store, compare your used shoe to a new one of the same model, and gain a feel for the decreased performance.

Reasons for Premature Wear

The Right Shoe for the Wrong Job. When purchasing shoes, looking for quality materials and good fit is not enough. You must find sport-specific shoes as well. The majority of stores selling running or walking shoes do not have a knowledgeable salesperson to evaluate the needs of your feet. While a running shoe should provide overall shock absorption and optimal heel control, walking shoes should be lightweight and provide extra shock absorption under the ball of the foot to reduce tenderness in this area, as well as prevent heel pain. Walking shoes should be more rigid than running shoes so the foot can roll off them while walking, as opposed to bending through them as you run.

Foot vs. Shoe. Understanding your body type and foot characteristics is the second part of finding the shoe that will last the longest. A runner with high arches will tend to under pronate during midstance. This runner will need a shoe with a softer midfoot and one that is more cushioned. If that runner uses a firmer shoe that is made only to stabilize the midfoot, the shoe will wear out along the lateral edge very quickly. Conversely, old running shoes belonging to pronators tend to display overcompression of the medial arch (as opposed to the lateral edge) of the midsole, and extensive depreciation of the lateral regions of the heel counter and medial forefoot. Pronators, who are also frequently low-arched, need shoes designed for motion control--specifically, control of the rearfoot with hard midsoles.

Your body weight is a big factor in determining which shoe is best for you; in general, the more you weigh, the more cushion your feet need to withstand the pavement pounding. A disastrous combination for both shoe and runner would be a large runner with low arches training long distances in lightweight trainers.

Now that we've established an introduction to shoe parts and runner types, the second half of this article will examine in detail ways to tell when your shoes are worn out.

Bruce R. Wilk, PT, OCS, is a certified physical therapist and the director of Orthopedic Rehabilitation Specialists in Miami, FL. Maritza M. Valdez, BS, works with Bruce as a physical therapy aide.