Why EVA in running shoes is important
On our wall, just about every shoe is made with some kind of etheylne vinyl acetate (EVA). During the history of the technical running shoe, EVA has become the preferred material for midsoles and sockliners.
EVA is the prominent material in most technical running shoes. It’s a foam that can compress without flattening under large loads and then rebound back to its original height. It’s waterproof, flexible, durable and lightweight. Finally, it’s relatively inexpensive and factories can use a number of techniques to form parts from it.
A good way to think about EVA is to think about the conditions in which a running shoe needs to perform.
EVA compresses and rebounds
The act of standing on a shoe is demanding because it places upwards of a hundred pounds, depending on the person’s body weight and balance, on the shoe’s midsole, which is the part that you stand on. The standard midsole thickness is thin–about 15mm or just less than ¾ of an inch. It has to compress under the weight of standing, but not too much or you won’t feel any cushion. When you remove the shoe, the midsole should rebound.
But technical running shoes are made for running and that activity creates much larger forces than standing. When running, three times your bodyweight lands on each midsole. For the average man, that’s about 600 pounds. So the midsole’s thin material has to be durable enough to compress without flattening under 600 or more pounds and then instantaneously rebound as soon as the foot takes off but before it lands again. EVA can do this–repeatedly for months.
Foremost, the shoe has to be comfortable. Through trial and error manufacturers have learned that the midsole has to feel flexible, cushioned and lightweight. EVA materials have those properties.
A lot of running happens in the rain or wet environments. Up to a pint of water can accumulate in a shoe during exercise. So the midsole has to be waterproof. Just 3mm of EVA is enough to keep out water.