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running shoes 101
Every thing you ever wanted to know about Cushioning Technology
A most important feature in running shoes, the cushioning technology is located in the midsole of the shoe. The midsole provides cushion for the runner between the shoe upper and the outsole. The midsole provides many important features:
- Protects the foot from feeling hard or sharp objects
- Provides stability against over–pronation with dual densities and contoured foot bed.
- Rebounds for easier take-off
- Provides cushioning for the impact forces of 2.5 to 3 times body weight at heel strike.
Midsoles come in two basic materials: EVA and PU
Each material has its own positive and negative attributes. EVA is light, flexible and has good cushioning features but over time and repeated impacts it tends to compress and lose some of its some resilience. Runners notice this and say their shoes feel “flat”. The technical term would be the foam has taken a “compression set”. This is much like the indent you get in your couch cushions after years of use.
Every thing you ever wanted to know about EVA but were afraid to ask:
EVA is a copolymer or cross-linked foam made of ethylene and vinyl acetate. It is made up of hundreds of thousands of foam cells that contain air or gas. When you land on EVA foam you compress this EVA and the gas gets pushed out and then sucked back in again. Over time these cells don’t rebound as well and don’t take as much air back in. To compensate for this issue, a great majority of good running shoes use a compression molded EVA midsole. By compressing EVA in a pressurized mold the midsole forms a thick skin which helps the EVA stay resilient longer. It also helps provides structural side rigidity to the midsole. (they also add PU components as below)
What is PU, besides the bad smell or week old running socks and how does it work?
PU is short for Polyurethane which is a heavier and often a more dense foam than EVA. Polyurethane acts much like EVA, except it has stronger cell walls that take much longer to take a compression set. PU is an excellent foam, but to some runners it seems to have less “bounce” or rebound when running. Running shoes made entirely of PU last a long time, but they tend to be too heavy for most runners.
The best of both worldes PU & EVA:
A large majority of good running shoes are now made with a combination of the two foams, PU and compression molded EVA drawing the benefits from both materials into one midsole. By putting PU into the high stress and high impact area running shoes can be designed for the best of both worlds.
Understanding claims about "cushioning technologies"
So many shoe companies use the same basic midsole materials that each has thought it important to differentiate their product. It is important to understand that these added components or branded formulas are designed to convince you that their cushioning technology is superior. If you ask runners who wear different brands, you will find that just about all well made running shoes have midsoles that will perform well for hundreds of miles.
The midsole technology by itself should not be the only criteria when determining which shoes work for you. The amount of miles you can run in a shoe is determined more by your own wear pattern, foot-strike, weight and personal preference. For many runners it’s just as often that you change a pair of shoes because the sole is worn, or the shoes are too stinky, than it is that the midsole has failed.
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