Outsole: The length of textured rubber or exposed foam that lines the very bottom of the shoe from heel to toe. The outsole is what contacts the ground and is responsible for grip, traction, and protection. In trail shoes, this surface is commonly lined with “lugs” of varying size and depth. Some important things to keep in mind when looking at shoes’ outsoles include the rubber compound / type (softer = grippier, harder = more durable) and the size, depth, and layout of the lug pattern. Taller, more spaced-out lugs generally grip better in soft, deep, loose conditions like mud and snow, while shorter, more densely spaced lugs typically equate to better efficiency on firmer, hardpacked surfaces.
Midsole: The layer of material between the outsole and the upper. The midsole is the main part of the shoe that’s responsible for cushioning, energy return, protection, and motion control (pronation / supination). I.e., it’s one of the most important and defining parts of a shoe’s on-trail performance and feel. The midsole runs the length of the shoe and in most cases is made from plastic compounds (typically some sort of foam) of varying densities. Midsoles vary greatly in terms of how they actually feel on trail (e.g., some are super plush / squishy, while others feel very firm), which is why we always discuss the midsole in detail in our full shoe reviews.
Heel Crash Pad: A section of midsole that extends off the back of the shoe below the heel counter. Models stressing stability often make use of this feature to accommodate heel striking. It’s commonly found in maximal shoes (i.e., those with very thick midsoles), and less often in shoes with lower stack heights.
Heel / Pull Tab: Some trail models add a simple pull tab behind the heel to make getting the shoe on easier. This feature is especially helpful on tight-fitting models intended for races or workouts.
Overlay: Found on top of the upper material, overlays are typically synthetic strips of material serving a few purposes when used on trail running shoes: to add protection, increase the stability of the fit, and improve durability. They’re typically either stitched, glued, or heat-welded on. Generally, a shoe with lots of overlays will feel more rigid and supportive through the upper (which can be particularly useful when running on technical off-camber terrain), and potentially be more durable and less breathable.
Toe Cap: A lip of rubber or plastic that extends from the outsole beneath the shoe and attaches to the upper at the front of the toe box. This feature is much more pronounced in trail shoes and functions primarily to protect your toes from rocks and other trail debris.
Toe Spring: The degree to which a shoe’s outsole is curved upwards at the forefoot. Inflexible road shoes make use of this design aspect to encourage proper toe-off during the gait cycle. It is less commonly seen in trail models, but is related to rocker geometry (see below).
Lugs: Small points of raised rubber lining the outsole on most trail shoes. Measured in millimeters, lug lengths and patterns vary depending on the types of conditions the shoe is intended for. Longer lugs (up to 8 mm) are commonly found on models best suited for wet / muddy terrain, whereas models with lugs under 4 mm are often used as road-to-trail shoes.
Gaiter Attachment: Some trail running models offer attachment sites at the heel counter and the bottom of the tongue for lightweight gaiters. These accessories help prevent sediments and other debris from entering the shoe and are especially useful in sandy, dry conditions (as well as deep snow).
Stack Height: The distance between your foot and the ground (in millimeters), measured at both the front and back of the shoe. Stack height is a very valuable metric when thinking about the amount of cushioning a shoe offers. Measurements range from as little as 3 mm in minimal models to excesses of 30 mm in maximal models. Taking the difference between the front stack measurement and the rear stack measurement of a shoe will indicate its heel-to-toe drop (see below).
Heel-to-Toe Drop: Indicates the difference in height (in mm) between the heel and forefoot of a running shoe. These measurements vary from anywhere between 0 to 16. A shoe’s heel-to-toe drop is important when considering gait patterns, muscle recruitment, injury history, distance, and terrain. Typically a lower heel-to-toe drop encourages a more midfoot or forefoot-leading foot strike, while a very high heel-to-toe drop typically encourages a heel-first strike. This is also sometimes simply referred to as “drop.” In the context of trail running, shoes with higher drops tend to provide less ground feel than those with a lower differential, though they may be more comfortable if you heel strike.
Rocker: This refers to shoes that have a rounded midsole geometry designed to “rock” the foot forward through the gait cycle. Some models incorporate this design element from just the toe forward (toe rocker), while in other models, it runs the length of the shoe (heel-to-toe rocker). In theory, rockered sole geometry is included to boost running efficiency. Typically, you see the most radically rockered soles on shoes with thick midsoles / high stack heights. If you’re familiar with the term “rocker” in the context of surfboards, snowboards, or skis, it’s a similar concept in shoes, referring to the way the bottom of the shoe curves upward.
Rock Plate: A thin plastic (and in some cases, carbon fiber) piece located between a trail shoe’s midsole and outsole (or between the midsole and insole). Rock plates primarily serve to shield the foot from sharp objects; however, they often inadvertently add rigidity. In newer models, the use of segmented rock plates enables shoes to articulate without drastically sacrificing protection.
Performance Plate: Included to increase energy return and efficiency, performance plates are typically made from inflexible materials like carbon fiber or certain hard plastics. As of late, their use in popular high-end, speed-oriented road running shoes has spilled over into premium trail models, typically targeted at racers. By virtue of their rigidity, performance plates can often double as protective rock plates.