When deciding which running shoe to buy, perhaps the most important feature of the shoe is its midsole. You can’t see the midsole (it’s sandwiched between the sole and the insole), but it’s largely responsible for the cushioning your feet, knees, hips, and your entire body receive every time your feet hit the ground. It can also help promote foot stability and prevent conditions such as excessive pronation (when the foot turns too much inward) or insufficient pronation (when the foot does not turn inward enough), which are associated with flat feet and high arches. .
Midsoles are generally made of two basic materials: EVA and PU.
EVA stands for Ethylene Vinyl Acetate. It is not made of plastic or rubber, but foam. Each midsole is made up of thousands of foam bubbles that act like cells. Each of these foam bubbles is filled with gas. The result is a lightweight and flexible material. Every time you land on an EVA midsole, your shoe breathes a little. The gas is expelled and then allowed to enter once the foot leaves the ground.
PU is also a type of foam. It means polyurethane. The material is heavier than EVA and is generally less preferred by runners due to its density. That said, PU tends to hold up better than EVA. Over time, the bubbles that make EVA midsoles so light and springy to begin with permanently lose some of their air. PU gives less bounce at first, but its bounce lasts longer.
Some midsoles are made with a combination of the two midsoles. The classic design is to put PU on the outside (where the shoe receives the most stress) and then keep an EVA core.
In addition to the midsole, you will also want to carefully examine the other features of the shoes. With regard to the sole, it is usually more important to examine the quality of traction it provides. Features to look for with insoles include contoured arch support and footrests.
A word to the wise: shoe companies often use special terms or “company jargon” when it comes time to advertise these simple terms. For example, Asic’s Speva (TM) is actually a fancy way of saying EVA (i.e. spEVA). Most high-end running shoes prefer EVA over PU, or some combination of the two. You probably won’t find too many PU midsoles, unless you’re shopping in the vintage section.