A shoe’s midsole (the material between the footbed and the outsole) is one of its most-important, yet least-understood components. We discuss it in almost every footwear review we write, but what does all that jargon mean? Here’s a quick explainer on the tech that keeps your feet from feeling sore at the end of every hike.
EVA vs. PU — Comfort vs. Durability
Most hiking footwear midsoles are made out of these two materials. EVA, or ethylene-vinyl acetate, is a relatively low-density foam that provides cushion with a low weight penalty. Because of EVA’s low density, though, it tends to compress faster than other materials. So while those nimble, ultracomfy trail running shoes might float you down the trail for a year, after a while you’ll start to feel the ground underneath your feet more.
PU, or polyurethane, is denser and heavier than EVA. While it generally provides less cushion, it offers a bit more support and, most importantly, is way more durable. If you want your hiking footwear to feel comfortable under a heavy pack and to last you for many years, PU is the way to go.
It’s also helpful to know that even among these two materials, different formulas and densities exist for each, and designers will sometimes blend the two together, taking the best qualities from EVA and PU.
Heel-to-Toe Drop — Natural stride vs. Heel strikes
Initially the provenance of hardcore runners, heel drop discussions have made their way into hiking footwear. Heel drop is, simply, the difference in stack height (more on that in a second) between the heel and toe of a shoe, measured in millimeters. If the height of the heel is 25 millimeters and the forefoot is 17 millimeters, the heel-to-toe drop is 8 millimeters, which is about average.
How large (or small) the drop is affects how you stride while wearing the shoe. Moderate and high heel-to-toe drops (from about 5 millimeters to 12 millimeters) makes your heel strike the ground first, which should work for wearers that already hike and run that way, or for trails that have a lot of descent. Zero-drop and minimal-drop footwear (0 millimeters to 4 millimeters) promotes a more “natural” stride that for some hikers is easier on their feet and legs, and results in less fatigue at the end of the day. Minimalist footwear can be tough for some folks to adjust to, though, so isn’t for everybody.
Stack Height — Cushion vs. Minimalist feel
This measurement illustrates how thick the midsole is at various points along its length. A large stack height indicates a shoe with lots of material between your foot and the ground, and usually more cushion. A shoe with low stack height will provide more ground feel and nimbleness, but offers less protection. (No stack height is a barefoot model.)
Note: Stack height and heel-to-toe drop don’t necessarily correlate. There are plenty of hiking shoes with beefy midsoles that have minimal- or zero-drop designs (HOKA and Altra are at the forefront of this trend).
By trying out different combinations of the traits above, you’ll be able to find a midsole that works for your hiking style.