The main purpose of footwear is to protect and support our feet. Trainers were originally worn for sport, but are now worn universally in everyday life for comfort and for fashion. From a science perspective, research and development in the field of chemistry has contributed greatly to the evolution of the trainer.

Trainer technology and manufacture

Sportsmen and women have always been interested in comfortable and appropriate footwear since the ancient Roman games, when thin leather sandals were worn.

The British term 'trainer' is an abbreviation of 'training shoe'. In the United States, the term ‘sneakers’ is used for leisure and athletic shoes. Specifically, sneakers refer to a style of footwear made of versatile materials, typically featuring a sole made of rubber with an upper made of leather or canvas. They were given the nickname 'sneakers' because they were so quiet, a person wearing them could sneak up on someone!

Rubber-soled shoes called plimsolls started to be worn in the late 18th century, but they were very basic and not entirely comfortable. The first references to shoes designed specifically for running didn't appear until 1852, when a race in which runners wore shoes with spiked soles was held. In 1900 the first sneaker, or all-purpose athletic shoe, was designed. Made mainly of canvas, this sneaker featured a rubber rim only made possible by Charles Goodyear's 1839 discovery of vulcanised rubber.

Known about for centuries, rubber was finally made commercially more useful when Goodyear heated and combined it with sulphur, which prevented it from hardening and losing its elasticity. Around 1892, the U.S. Rubber Company began producing a more comfortable rubber sneaker with canvas upper which began to be mass produced by 1917. Rubber helped to cushion the impact of running on hard surfaces. However, it did not last when used as a running shoe material. It was not durable, and leather, although not ideal, returned as the preferred material. In addition to being expensive, leather running shoes caused chafing, and runners had to buy chamois liners to protect their feet.

Charles Goodyear and one of the other big products in which vulcanised rubber is used - tyres.

In 1925, Adolph ‘Adi’ Dassler, a German shoemaker, decided to focus on athletic shoes, founding a business with his brother, Rudolph. The Dasslers' running shoes provided both arch support and speed lacing. Jesse Owens is reported to have worn Dassler shoes during the 1936 Olympic Games in Berlin where he won four gold medals. In 1948, the brothers started their own companies, and created two of today’s most well-known trainer brands.

During the 1960s, a company named New Balance examined how running impacts the foot. As a result of their research, they developed an orthopaedic running shoe with a rippled sole and wedge heel to absorb shock. Many runners also began to request shoes that provided support with a lightweight construction, using new materials like nylon, invented during World War II.

Sales of sneakers and trainers really took off in the mid 1980s, when Michael Jordan signed a contract to wear a Nike shoe called Air Jordans — the most famous sneaker ever made. As companies like Nike, Reebok and Adidas competed, they changed the way sneakers looked, adding colour and doing away with laces. Sneakers began to be produced for all sports. Nike’s Air Force used little pockets of gas to create better cushioning, while Reebok introduced The Pump, air pumped into shoes to make them fit more snugly. Technological surprises continued. Spira Footwear, for example, has built a spring in the soles to reduce foot stress. Innovations like these always come with a price. Trainers often cost more than £60 a pair.

The science behind trainer technology

Trainers are made from a combination of materials. The sole has three layers: insole, midsole, and outsole. The insole is a thin layer of man-made ethylene vinyl acetate (EVA). Ethylene vinyl acetate is the co-polymer of ethylene and vinyl acetate. It is a polymer that has a degree of softness and flexibility, yet can be processed like other thermoplastics. The material has good definition and gloss, barrier properties, low-temperature toughness, resistance to stress and cracking, hot liquid and water proof properties, with resistance to UV radiation.	Ethylene vinyl acetate
	IUPAC name but-3-enoic acid; ethene

The components of the midsole, which provides the bulk of the cushioning, varies among manufacturers. Generally it consists of polyurethane surrounding another material such as gel or liquid silicone, or polyurethane foam given a special brand name by the manufacturer. In some cases the polyurethane may surround capsules of compressed air.

A polyurethane is a polymer composed of a chain of organic units joined by carbamate (urethane) bonds. Polyurethane polymers are formed by combining two bi- or higher functional monomers. One contains two or more isocyanate functional groups and the other contains two or more hydroxyl groups. More complicated monomers are also used. The alcohol and the isocyanate groups combine to form a urethane bond:

ROH + R'NCO → ROC(O)N(H)R' (R and R' are alkyl or aryl groups)

This combining process, sometimes called condensation, typically requires the presence of a catalyst.

One of the most desirable properties of polyurethanes is their ability to be turned into foam. Blowing agents such as water, certain halocarbons such as HFC-245fa (1,1,1,3,3-pentafluoropropane) and HFC-134a (1,1,1,2-tetrafluoroethane), and hydrocarbons such as n-pentane, can be incorporated into the poly side or added as an auxiliary stream.

Outsoles are usually made of carbon rubber, which is hard, or blown rubber, a softer type, although manufacturers use an assortment of materials to produce different textures on the outsole.

The rest of the shoe covering is usually a synthetic material such as artificial suede or a nylon weave with plastic pads or boards supporting the shape. There may be a leather overlay or nylon overlay with leather attachments. Cloth is usually limited to the laces fitted through plastic eyelets, and nails have given way to an adhesive known as cement lasting that bonds the various components together.

Stephen Nugus
Science Subject Advisor

Read more about the science behind trainer construction:

1. Shoe Making - How Shoes Are Made, Textile Exchange
http://www.teonline.com/knowledge-centre/shoe-making-how-shoes-made.html

2. Running Shoe - How Products Are Made: Vol.1
http://www.madehow.com/index.html

3. Polyurethane - from Wikipedia, the free encyclopaedia
http://en.wikipedia.org/wiki/Polyurethane

4. Ethylene-vinyl acetate - from Wikipedia, the free encyclopaedia
http://en.wikipedia.org/wiki/Ethylene-vinyl_acetate