Running shoes are the single most important piece of equipment in both track and distance running. A well-constructed shoe, that balances protection of the athlete from undue physical stress with lightweight construction and responsiveness, will assist runners in the achievement of their ultimate goal: to run as fast as possible.
An effective running shoe must combine the features of shock absorbency, motion control when the foot strikes the ground, flexibility and responsiveness, and a measure of durability. Running shoe science began a remarkable progression that included the work of Adi Dassler (1900–1978) of Germany, the founder of Adidas, and the later creations of Bill Bowerman (1911–1999), the American track coach who developed the Nike "waffle" outsole in the early 1970s.
Each component of the modern running shoe has a specific function. The outsole is the outer tread of the shoe; it is usually made from a carbon rubber compound and provides traction for the runner. The midsole is the part of the shoe construction that provides both cushioning and stability to the runner. The midsole will appear to be made of a foam material, usually ethylene vinyl acetate (EVA), an extremely lightweight material, or polyurethane. It is common for running shoes to have a post implanted in the midsole to provide further stability. Running shoes often have different densities of materials in the mid-sole construction, with the medial part of the midsole (inner) composed of a harder EVA, and the lateral (outer) side made of a softer material. This design is intended to counter the effects of "pronation," the inward movement of the foot on the contact with the running surface; 80% of runners tend to pronate. The midsole may also include a liquid or semi-gel, air, or specialized plastic compound to further absorb shock. Most distance runners will generate forces that are approximately three times their body weight on impact with each foot strike.
The upper is the part of the running shoe that encases the foot. It is padded and it is usually a synthetic material and typically washable. The heel counter is a hard, cup-shaped device set against the heel of the runner to promote stability and to limit the movements of the heel on impact (both laterally and vertically).
Many modern running shoes are built to accommodate a foot orthotic, used to correct the structural imbalances that are a primary cause of running injuries.
With each stride, the runner delivers a force through the shoe into the ground, as with classic Newtonian physics, every such action produces an equal and opposite reaction, with forces of impact directed into the foot. The more efficiently such forces may be distributed through shoe construction, the more responsive the shoe to the next stride and the less likely the musculoskeletal structure will be to unduly absorb these forces. The construction of the quintessential perfect running shoe is a marriage of the contrasting features of cushioning and responsiveness.