What's the Upper?
As its name suggests, the upper is the combination of materials that wrap around the top of the foot.
The upper consists of:
- Overlays – Synthetic materials that criss-cross the upper, and provide support and/or reinforcement.
- Heel Counter – Rigid or semi-rigid device in the heel that holds the foot in place.
- Collar – Foam padding that surrounds the sides and rear of the shoe, and helps hold the foot in place.
- Toe-box – Front portion of the upper that creates the space above and around the toes. Running shoe toe boxes should be roomy to allow the foot to expand.
- Last – The last is how the upper is attached to the midsole. The last is molded to fit the volume and width of each particular size. There are three different types of lasts: slip, strobel and combination.
Types of Lasts
- Slip – The upper is wrapped around the bottom of the last and the materials are stitched together down the middle.
- Strobel – The upper is attached to a thin material that is patterned on the shape or silhouette of the midsole. Traditional strobel lasts use a thin cloth material, while newer variations use a thin sheet of EVA for a more cushioned feel.
- Combination – Traditional combination last attaches the upper to fiberboard in the rear of the shoe and the front is slip-or strobel -lasted. Newer variations use an EVA (see below) sheet in the rear of the shoe while the front is slip-or strobel-lasted.
What's the Midsole?
The midsole is typically made of plastic materials that feel and behave like foam or rubber. Midsole composition dictates the durability or longevity of the shoe, as well as the quality of the ride. Cushioning and pronation control technologies are located in the midsole.
- Compression-molded EVA – The most common insole material. EVA stands for ethylene vinyl acetate, a combination of two types of plastics.
- Polyurethane – Another type of material that behaves like plastic or rubber, and is used in midsoles. Polyurethane is not as commonly used as EVA because it tends to be heavier and firmer.
- Durometer – Midsole resistance to indentation is indicated in durometers. The higher the durometer, the stiffer and more resiliant the midsole.
- Cushioning devices – Cushioning devices are placed in the heel and forefoot to absorb shock and resist compression. Each shoe manufacturer produces their own unique types of cushioning devices, but the function of these devices is similar across brands.
- Dual-or multi-density midsole – A midsole that has two different densities of material is dual-density and a midsole with more than two densities is multi-density. Shoes that are designed to prevent over-pronation usually have dual-or multi-density midsoles.
- Medial Post – A device located on the medial, or inside, part of the shoe. A medial post is placed in a shoe to reduce over-pronation. Medial posts are usually made out of plastic.
- Thermal Plastic Unit – Used to replace or support the midsole material. Most often, thermal plastic is used in the bottom of the midsole at the mid-foot. Thermal plastic units can also be used in the midsole to reduce over-pronation.
What's the Outsole?
Typically made of rubber, the outsole is the bottom of the shoe. The outsole provides traction and contributes to how soft or firm the shoe "rides" as well as the torsion rigidity and flexibility.
- Carbon Rubber – Durable rubber compound that makes up the majority of running shoe outsoles.
- Blown Rubber – Air-injected rubber that is lighter, softer and more flexible than traditional rubber. Blown rubber is most often found in the forefoot. It provides a great feel but is less durable than carbon rubber.
- Shape – Outsoles range in shape from curved to straight. Curved shoes tend to be less stable, and are best for neutral runners to supinators, while more rigid straight shoes are best for overpronators.