Fast track science

Sole science

At the top of the sport where performance is paramount, a quiet revolution is occurring in the design of running shoes.

The main functions of running shoes are ‘protection; improvement of performance; and to provide comfort’, according to Benno Nigg, founder and co-director of the Human Performance Laboratory at the University of Calgary, Canada, and author of The biomechanics of sports shoes. However, advances in nanotechnology are now making their mark on the running shoe in three areas: the trade-off between support and impact resistance; the design of the footplate; and improved water repellency.

Tony Ryan, ICI Professor of Materials Science at the University of Sheffield, UK, explains: ‘Running shoes are designed to absorb energy, to be comfortable and to protect your feet. To do this they have to be soft and squishy, but if you walked on a soft material [all day] it would eventually flatten out.’

To stop this, Ryan explains that manufacturers need to to add harder substances to the formulation to prevent the shape squishing too far and allow it to rebound after each step. As a result, the running shoe stays in shape. ‘The nanotechnology comes in because the soft and the hard bits we use are polymer molecules,’ he says.

A running shoe sole has three layers: the insole, the mid-sole, and the out-sole. The insole is a thin layer of man-made ethylene vinyl acetate (EVA). Out-soles are usually made of hard carbon rubber or softer blown rubber, though manufacturers then use an assortment of materials to produce different textures.

The core of the shoe is the mid-sole, which provides the bulk of the cushioning and is where the nanotechnology is used, although the precise design varies among manufacturers. Some manufacturers use a polyurethane sheath surrounding a gel or liquid silicone, for example, while others use polyurethane foam.

The exact formulation is a closely guarded secret. In both cases, however, the mix of polymers dictates the properties of the shoe. Thus polymer chemistry is crucial in the design of the mid-sole where combining polymers in the correct – usually proprietary – ratio gives the sole the desired balance between energy absorption and shape recovery.

Meanwhile, nanotechnology is also playing an increasingly important role in the design of the footplate. Adidas designed the Lone Star spike running shoe, for example, for the 400m Texan runner Jeremy Wariner. The Lone Star features the first full-length carbon nanotube reinforced plate and an innovative compression spike.

In the same way that NasCar racing cars have right and left tyre compounds and different suspension set-ups on either side of the car to help it take corners, Wariner’s running shoes have differently designed treads for the right and left foot to assist his cornering.

Adidas claims that the new shoe is ‘one of the most technologically advanced and lightest running shoes to ever hit the track’. After studying Wariner’s running pattern by high-speed video and pressure mapping, Adidas’ engineers and designers were able to see how Wariner uses each foot as he runs, which in turn enabled them to custom- design a shoe for his running style.

Mechanical and chemical bonds within the nanotubes inside the spike plate increase its structural integrity and durability, allowing it to be made in a single piece rather than the more common three. The plate is about one-third the thickness of Wariner’s previous spike plates and weighs 50% less.

‘We spent more than two years working with Jeremy Wariner … to design the Adidas Lone Star,’ says Mic Lussier, innovation team leader of Adidas America. ‘We know long-sprint races are won and lost in the curves so we looked at what he already does best and from there created the finest curve running shoes in the world. The Lone Star allows Jeremy to push even better in the turn [providing] him with … more stability and [mechanical] efficiency.’

The spike of the shoe is also innovative. It is a progressive-compression peg that acts like a piston on the track. It is formed from the same nanotubes as the sole plate but its length, taper, thickness and groove topology are mathematically designed to provide the optimum penetration/compression ratio to exploit the elastic properties of the track surface. Because the spike does not cut into the track Wariner does not have to expend effort pulling the spike free of the track surface. The shoe therefore quite literally puts a spring into Wariner’s step.

High rainfall levels this summer have also given an edge to another often overlooked feature of running shoes: water repellency. And here again, nanotechnology is lending a helping hand. British company P2i recently announced, for example, that it is applying its patented water-repelling nanotechnology to sports shoes, including running shoes.

Nick Rimmer, the company’s head of marketing, explains that the technology was originally developed, with funding from the MoD, in the 1990s, as a fluid-repellent covering on protective clothing used for biological and chemical warfare. It involves coating the material with a hydrophobic, nanoscopic (40–80nm) polymer layer based on perfluorinated carbon compounds, using a pulsed, ionised gas plasma.

Once applied, the ability of the treated fabric to repel water makes it ideal for sports applications since it keeps the weight of the product down. And in the exalted circles of the world’s top athletes even a few grams of water can make all the difference.

But how much of a difference has the revolution in sports shoes actually made to success on the track? Steve Haake, professor of sport science at Sheffield Hallam University, UK, estimates ‘1 or 2%... if we are talking about the top 25 athletes’. Nigg agrees: ‘I think that the performance aspect of shoes is rather substantial...The improvement is in the percentage [range].’

And the future looks poised to deliver yet more innovation to running shoes. Nike, for example, is developing shoes that actively feed back data while in use. The shoes have a fibre-optic force sensor connected to a communication port from which data on an athlete’s performance can be downloaded.

As the sole is compressed, the wavelength of the light passing through changes and this is picked up by the sensor, which can then be translated into a value for the force. Nike says that the shoe’s sensor system will have a host of applications beyond sports, including gaming and as control inputs for computers and other devices.

Tracking gold

When it comes to running in the Olympics, the other side of the excellence equation is the track surface. Throughout the history of the Olympic Games, the search has been on for the optimum surface to allow athletes to maximise their potential in bad weather as well as in good. Enhanced traction surfaces were used in the early days: dirt, grass, sand and crushed cinders; but by the 1950s surfaces were made of a combination of rubber compounds and asphalt – many of them still in use today.

The construction of running tracks has now become almost as competitive as the athletes who race on them. Track requirements are specified by the International Association of Athletics Federation (IAAF) and are exacting, says Martin Oakes, business development manager at BASF. Both BASF and its competitor Mondo are supplying tracks for the 2012 Olympics. BASF tracks are poured in layers and Mondo’s are assembled in prefabricated strips.

Mondo has been supplying running tracks since the early 1980s, and is now the manufacturer of choice for competition. The Mondo ‘carpet’ segments are cut to size then glued together seamlessly in the direction of the lane lines to eliminate any tripping hazard. The Mondo surface gives a consistent energy return, or bounce, as well as running shoe traction. Because of the tight specifications required to achieve a seamless ‘weld’ between sections, Mondo tracks are one of the most expensive systems to install, which explains why practice tracks, such as BASF’s, are usually constructed using a cheaper system.

Energy return is key to track performance, Haake emphasises, pointing out that ‘the stiffness and damping of [the best] surfaces are tuned to optimise energy return to the athletes so that they … fatigue less’. This means that track construction can be complex. For example, the top-of-the-range surface from BASF – its Conipur M system – consists of five layers of proprietary polymers that together provide a Class I track surface, the highest possible rating stipulated by the IAAF.

Nigg comments that he expects little further improvement in running track technology. ‘Current track and field surfaces are close to optimal within the existing rules,’ he says. ‘Compared with cinder tracks, the horizontal friction is much better.’

The refinement of sports shoes and surfaces are an inseparable component of the search for athletic excellence. So how would Usain Bolt have fared in the ancient Olympics?

Haake suggests that, while there are too many unknown variables to give a meaningful answer, ‘he [Bolt] would have beaten the average elite runner by about 20m in the 100m’. But 20m is 20% over the 100m sprint – a difference far larger than that attributable to either running shoes or surfaces.

It seems that the performance of our athletes is more than the sum of their racing appendages after all.

A sole obsession

Spikes were introduced in 1852 and by 1894 the Spalding Company was manufacturing low-cut running shoes made from pliable kangaroo leather with six-spike soles. Joseph William Foster founded the first sports shoe company in Bolton, UK in the 1890s. His grandson later took over in 1958 and renamed the firm Reebok. Reebok had an early association with the Olympic Games when it made a pair of bespoke thin leather running shoes for Lord Burghley in the 1924 Olympics. Soon running shoes were being made with stitched leather straps around the top of the shoe.

In 1920, Adolf Dassler and his brother began making dedicated running shoes for athletes with metal spikes for improved traction. By the time of the Berlin Olympics in 1936, his shoes had gained an international reputation and were being worn by world-class athletes such as Jesse Owens.

After WWII, Dassler started making training shoes from surplus tent canvas and rubber from fuel tanks. In 1948, he founded Adidas, but the company was soon to split into Addas, later again known as Adidas, and Puma, after a bitter dispute with his brother Rudolf. In 1949, Dassler added three coloured side strips to the shoe to give extra support. It was the origin of what we would recognise as the modern trainer.

In 1962, New Balance introduced the first ‘scientifically tested’ ultra-lightweight running shoe that weighed just 96g. In 1968, brush spikes – a layer of hundreds of spikes attached to the sole of the shoe – were introduced, replacing the traditional spiked running shoe. This so improved performance that they were later banned from competition.

Today’s synthetic shoes are made of lightweight mesh fabric uppers with synthetic soles and a middle layer of considerable mechanical and chemical complexity. They are designed for comfort but also performance.