Uppers
Shoes with synthetic fabric uppers (the area that extends from the laces down the side of the shoe to the sole) are lighter, washable and breathable -- so your feet don't get too hot. To lengthen the shoe's life, wear them only for running and dry them slowly when they are wet. Your running shoe should have a padded tongue to cushion against lace pressure and a padded ankle collar to cushion the ankle and help prevent Achilles tendinitis.
Midsoles
The midsole is located between the outsole and the upper. It is the most important part of any running shoe. It controls excessive foot motion and provides cushioning and shock absorption.
The primary materials used in midsoles are EVA (ethylene vinyl acetate) and PU (polyurethane). EVA is a foam that is light and has good cushioning, but breaks down fairly quickly. Compression-molded EVA is harder but more durable. PU, also a foam, is denser, heavier, and more durable than EVA.
Most shoes are also cushioned with gel, foam or various manufacturer-specific technologies that are encapsulated in the midsole. This cushioning lasts longer than previous methods and often adds stability as well as shock absorption.
Outsoles
The outsole is the treaded layer glued to the bottom of the midsole. It resists wear, provides traction, and absorbs shock. The outsole is usually blown rubber, hard carbon rubber, or a combination. Blown rubber is the lightest, but not as durable as carbon.
Stud or waffle outsoles are good for running on dirt or grass because they improve traction and stability. Ripple soles are better for running on asphalt or cement surfaces.
Heels
The heel counter is the inflexible material surrounding the heel. It must be rigid and durable to support and stabilize the heel. Because the internal heel counter material is thin and tends to lose its stiffness, an external counter is usually placed between the midsole and base of the heel counter.
A wedge, which adds height to the heel, enhances the shoe's ability to absorb shock and reduces strains. While this can ease problems like Achilles tendinitis, a shoe with a higher heel may feel less stable.
If you're having trouble finding a running shoe appropriate for your needs, check out The American Running's Running Shoe Database in "Programs & Resources."
Copyright, The American Running Association.