When shopping for shoes, whether online or at a specialty running store, styling, color and price might be the first things to catch your eye. Next is probably the tread, and then comes the try-on test to determine comfort and fit. What you may not notice though, is that there is actually a lot more under your foot than you realize.

The basics are an insole (the piece against your foot), a midsole (where the cushioning lives) and the outsole (where “the rubber meets the road”). For trail running shoes, some may also have a rock plate added to the mix. Made of a firm material, often some type of plastic or carbon fiber, rock plates are generally embedded in the midsole or placed between the midsole and outsole to provide underfoot protection from sharp rocks and stone bruises.

“They basically provide protection from ‘ouch’,” says Golden Harper, co-founder of Altra Running. “Protection gives you confidence and confidence gives you speed.”

Rock plates can be solid or made of a durable film and run the full length of a shoe, smaller and placed specifically under the heel and forefoot, or even have perforated or shaped designs to make them more flexible.

This flexible, anatomically shaped rock plate will be a key part of several Altra trail running shoes in 2018. The design offers a reduction in weight from a full-sized plate and also allows for a greater range of dexterity and flexibility without inhibiting protection.

“I incorporate rock plates for protection in thinner-soled shoes,” says Tony Post, founder of Topo Athletic. “When a rock plate is split, runners will have more mobility and a more natural ride.”

In some cases truly minimal shoes eliminate rock plates in order to save weight. And generously cushioned and lugged trail shoes may not incorporate them because the shoes already have sufficient protection in the design.

Whether or not you’ll benefit from having the extra protection in your shoes depends largely upon where you run. But the weight increase from the plates is minimal, and, as Harper points out, they make shoes more versatile. Meaning your favorite trail shoe can take you wherever you want to go, rocks or not.

Sometimes you only know a shoe has a rock plate if description says so. Others, like this Salomon shoe give you visual cues as to what’s in the midsole.