The sole of a hiking boot has three main components: the footbed, the insole and the outsole. Most hiking boots also have a midsole and/or shank.
The footbed is the part of the sole that the foot lies directly upon, and is generally removable. It moulds to the shape of the foot and is slightly padded. Its role is to provide support, particularly to the arch, as well as increased comfort. In some cases, the footbed is made of absorbent material, which captures perspiration and helps keep feet dry. In other cases, several more or less padded footbed layers are used to modify a boot’s inner volume. In more specialized boots, the footbed can be heat-moulded for a more customized fit. La Cordée’s footwear department provides such a service.
A typical footbed is made of ethylvinyl acetate (EVA) covered with a polyester or nylon knit or fabric. In some cases, the cover is made of leather rather than fabric. This part of the sole plays a key role in the adjustment of a boot’s fit. An arch that is tilted too far forward or backward or is too high or low, or a heel that is too wide or too narrow, can lead to discomfort and possibly injury. Sometimes, simply changing the footbed can greatly improve a boot’s fit. Some companies specialize in the manufacturing of footbeds that provide better support and/or greater shock absorption.
Insoles can be made of leather or synthetic materials, such as nylon. They provide rigidity and stability. The thicker and harder the insole, the stiffer the boot and the more difficult it is to bend and twist, resulting in greater stability. Today, injected nylon insoles are often used. This makes it possible to adjust the thickness of the insole, which varies over its entire length. Generally, the rear three quarters of the insole is thicker and more rigid, while the remaining portion tapers in the forefoot to ease walking. In some cases, the insole is made of a very thick and stiff leather. This classic construction is used less often. Even the thickest leather must be combined with a shank for added rigidity. In the case of very flexible boots, the insole is simply a thin sheet of polymer, such as texon or nylon.
The shank is an optional part of the sole. It is an extremely stiff rod, generally made of steel or fibreglass, placed between the insole and the midsole. Its role is to make the sole stiffer and more stable, and keep it from bending. The ¾-length shank, which is measured from the heel seat, protects the heel and the arch of the foot. The boot’s flex point, located behind the toe area, is free, facilitating walking. A full-length shank is used in footwear designed for long-distance hikes over rough terrain—mountaineering, for example. These types of boots are hardly flexible at all and provide excellent support for the underside of the foot. Traditionally, hiking boots had a leather insole and a ¾-length steel shank. Since steel is heavy and a good heat conductor, and since leather absorbs moisture and is slow drying, shanks are being used less and less. Stiff insoles made of injected nylon are being used instead, since they are light and do not conduct heat as well.
The midsole is placed between the insole and the outsole. In cases where there is no midsole, the insole is attached directly to the outsole. This is unusual, however, since it results in a boot that offers little shock absorption. The primary role of the midsole is to cushion the impact between the heel and the ground. A hiker carrying a heavy load can generate a force several times his or her weight. If the midsole fails to absorb the force of impact, the ankles, legs and knees must do so, resulting in an increased risk of injury after a full day of hiking. Modern midsoles are made of ethylvinyl acetate (EVA) or polyurethane rubber (PU), two elastomers that make walking more comfortable. EVA is a high-quality, lightweight and very absorbent material; it is also expensive and somewhat fragile. PU is heavier, but very sturdy (it won’t compress when used) and is less expensive. Many manufacturers use PU foam, especially in stiffer hiking or backpacking boots. Some boots are designed with an air cushion at the heel for added shock absorption. This air cushion is generally part of the midsole.
The outsole is the part of the sole that comes into contact with the ground. It is made of vulcanized rubber—similar to car tires, only denser. There are several possibilities in terms of composition. A sole that is soft and flexible to the touch will provide better grip. A sole that is hard and difficult to penetrate with a fingernail will not wear easily, but will provide less traction. The company Vibram, established in 1937, is recognized worldwide for its especially hard wearing soles.
The shape of the outsole is also very important. Some boots on the market have prominent multidirectional lugs. Others have small unidirectional lugs. Lug patterns and sizes vary so that wearers can perform a given activity optimally on a given terrain. An outsole must, however, have certain basic features. The heel, for example, must be thicker and more pronounced than the rest of the sole. Like the lugs, the heel helps brake on descent.
Generally, the heel of a hiking boot slopes downward, so that the contact area with the ground is greater than the area attached to the upper. A heel that does not slope downward should at least be flat. An upward slope would decrease the contact area. Lugs that function in more than one direction are another important feature of outsoles. Lugs that function laterally are recommended for hiking boots. They provide much more stability for crossing riverbeds or negotiating sloping trails.
Today, many outsoles have a preset curvature in the toe cap. The rocker eases walking, even if the sole is extremely rigid. This design provides greater comfort and limits strain on leg muscles.
One final point about outsoles: some are compatible with crampons, while others are not. Many people use hiking boots in winter. Some boots have grooves at the front to accommodate a rigid or semi-rigid crampon wire. The insole and the shank (if present) must, however, be somewhat rigid so that the crampon remains attached to the boot when the foot flexes.
