Midsoles – EVA Versus PU

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 outsole and insole), but it’s largely responsible for the cushioning your feet, knees, hips, and entire body receive every time your feet hit the ground. It can also help promote foot stability and prevent conditions like overpronation (when the foot rolls in too far) or underpronation (when the foot doesn’t roll in 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 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 light and flexible material. Every time you land on an EVA midsole, your shoe breathes a little. The gas is expelled and then let back in once the foot leaves the ground.

PU is also a type of foam. It stands for polyurethane. The material is heavier than EVA and is generally less preferred by runners due to its density. That being said, PU tends to hold up better than EVA. Over time, the bubbles that make EVA midsoles so light and springy at first lose some of their air permanently. 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 gets the most stress) and then keep an EVA core.

In addition to the midsole, you’ll also want to carefully examine other features of the shoes. When it comes to the sole, the most important thing is usually to examine the quality of the traction it provides. Features to look for with insoles include arch support and contoured insoles.

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 (ie spEVA). Most high-end running shoes prefer EVA over PU, or some combination of the two. You probably won’t find too many PU-only midsoles, unless you’re shopping in the vintage section.