Built for moving fast on uneven terrain, these shoes are designed for runners and "fast and light" scramblers who need shock absorption and superior traction. Trail running shoes can be worn for short workouts (an hour-long jaunt) and longer sessions (a 24-hour ultra-marathon). They require less break-in time than day hiking and backpacking boots, and can often be used right out of the box.
Things to consider when selecting trail runners:
If you run on sharp, uneven surfaces, look for a well-cushioned midsole that will absorb shock. Rocky terrain also requires a stiffer shoe that offers some protection from impact. Some trail shoes feature an impact-absorbing strike plate embedded into the sole for protection from sharp rocks and stumps. This plate also provides torsional rigidity to prevent foot fatigue and reduce the chance of turning your ankle.
Smoother trails such as packed dirt or soft forest cover, will suit a more flexible midsole with less under-foot protection, but will still require plenty of cushioning.
Loose terrain requires an aggressive tread pattern to provide traction and help braking on steep descents. Softer, stickier rubber will enhances grip on smooth or slippery rock sections, but tends to wear quickly. Most outsoles are made of compound rubber that provides both durability and grippiness.
An external rubber toe bumper or an internal plastic toe counter at the instep area around the big toe (and in some cases even the entire toebox) will protect against accidental stubbing and tripping – a definite advantage on steep, scree-covered descents.
Lightweight nylon and mesh uppers suit hot dry conditions. These materials are light and wonderfully breathable but are prone to abrasion. Mesh can also be practical for localized wet conditions (such as stream crossings), as it allows water to drain and lets the shoes dry rapidly.
For more prolonged wet conditions Gore-Tex® offers excellent water-resistance, but it can feel hot and sweaty. If your trails are often muddy look for deep lugs that enhance grip in muddy terrain and are designed to be self-cleaning, so you're not weighed down by gobs of mud.
The toebox of a trail running shoe generally fits wider than a standard running shoe. This accommodates foot swelling on longer runs and adds comfort on steep downhills. The heel and midfoot should fit snugly enough to prevent instability on trails.
For most runners, the outside of the heel is the first part of the foot to hit the ground. The foot then rolls inward and flattens, absorbing the shock of the body's weight landing. This process is called pronation.
Runners with low arches or flat feet may over-pronate: the foot rolls and flattens too much, which can lead to injuries. Motion control shoes, also called medially posted shoes, are designed to limit excessive pronation.
Motion control/posted shoes are not right for everyone. People with normal feet don't generally over-pronate. People with very high arches may actually under-pronate. For both these groups, shoes that restrict the foot's ability to pronate and absorb shock can increase the risk of injury. They're usually better off with a "neutral" shoe that allows nearly unencumbered pronation.
Over-pronators tend to show more wear on the inside soles of their shoes, under-pronators on the outsides. But nothing replaces professional medical advice. If you have "problem feet", consult with a podiatrist. If you use orthotic inserts, check with your practitioner about which type of running shoes to use.