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 is sandwiched between the outsole and the insole), but it is largely responsible for the cushioning your feet, knees, hips and entire body receive each time your feet hit the ground. It can also help promote foot stability and prevent conditions such as over pronation (when your foot rolls too far inward) or under pronation (when your foot does not roll inward enough), which are associated with flat feet and high arches.

Midsoles are generally made from 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 one of these foam bubbles is filled with gas. The result is a material that is lightweight and flexible. Each time you land on an EVA midsole, your shoe breathes a little. The gas is pushed out and then let back in once your 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 because of its density. That said, PU tends to hold up better than EVA. Over time, the bubbles that make EVA midsoles so light and bouncy to begin with permanently lose some of their air. PU gives less bounce in the beginning, 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 maintain an EVA core.

In addition to the midsole, you will want to carefully examine the shoes’ other features as well. With regards to the outsole, it is normally most important to examine the quality of traction it provides. Features to look for with insoles include arch support and contoured foot beds.

A word to the wise: often Shoe companies will use special terms or “company lingo” when it comes time to advertise these straightforward terms. For example, Asic’s Speva(TM) is in fact a fancy way of saying EVA (i.e., spEVA). Most high quality running shoes favor EVA over PU, or some combination of the two. You probably will not find too many PU-only midsoles, unless you are shopping in the vintage section.