Getting the Boot

The 2004 UEFA Euro soccer tournament in Portugal showcased the talent, flair, and excitement of one of the world’s most popular sports. Along with the glitz and glamour of soccer’s finest stars, discerning spectators saw the latest in cutting edge technology. The soccer boot (the traditional name for a soccer shoe), which serves as the primary medium linking a player to the turf and the ball, could arguably be called the single most important piece of equipment in the sport. For this reason, companies annually invest millions of dollars and spend thousands of hours completing research to ensure that players receive as much of a competitive edge as possible.

Although the base design of the soccer boot has changed very little over the past 70 years [1], the modern shoe contains numerous innovations, some more visible than others, that allow a player to push beyond the normal boundaries of performance. At its core, soccer boot technology aims to improve a player’s game in three major categories: movement, ball interaction, and comfort/protection. A better understanding of the significant contributions engineering has made to the sport of soccer will be gained by examining the new innovations in these three areas.

As the player’s primary point of interaction with the playing surface, the soccer boot must provide excellent traction. Better traction allows for greater acceleration and directional control. The innovation that allows for this traction is the cleats or studs that run along the bottom of the shoe, an improvement resulting from the demands of poor weather and field conditions [1]. Borrowing from field hockey shoes of the same era, shoemakers attached plugs of leather to the base of the shoe in the 1890’s [1]. Safety rules regulating the type of materials that could be used in the manufacture of the studs required the use of leather plugs instead of more dangerous metal tacks.

Over time, as players improved and game styles changed, cleat versatility, design and placement became important areas of development. In terms of versatility, removable studs, pioneered in the 1920’s [1], allowed players unprecedented control over traction capabilities. Players could tailor the stud length according to field conditions, choosing longer studs for better grip in wet conditions and shorter studs for firmer, dry surfaces. The ability to swap out studs was such a significant innovation that it remains a feature on many high-end soccer boots manufactured today.

Cleat design focuses mainly on the shape and material used in construction. While older shoes used hardened, leather studs, modern shoes commonly feature durable, tough plastics or metals such as steel or magnesium, which have the benefit of low weight. Plastic cleats have the advantage of being directly molded from the same piece of material as the sole of the shoe, which reduces manufacturing costs. Commonly, studs incorporate a circular design with a wider diameter near the sole and a slightly smaller diameter closer to the point of contact with the ground. Alternatively, some companies have adopted more radical stud designs such as angular, teeth-like cleats, called blades, designed to provide more aggressive grip in predetermined positions.

Cleat placement also contributes significantly to the functionality of a soccer boot. A standard arrangement of four studs in the forefoot and two studs in the heel allows for even weight distribution and has been the style of placement most common since the advent of studs. Using advanced pressure sensors and computers, however, modern day researchers for numerous shoe companies have discovered other arrangements suited to better performance. Circular patterns optimize omni directional acceleration [1] while laterally arranged studs provide an advantage in directional changes to the left or right. This means that in addition to improving traction, cleat arrangement can also affect a player’s agility.

The soccer boot also serves as the primary point of contact with the ball. In order to facilitate the execution of spectacular dribbling maneuvers, a player requires sensitivity and a feel for the ball, a characteristic often referred to as “touch”. The better the touch, the better a player can control a ball. The material used to manufacture the upper portion of the boot often contributes the most to the overall touch since the upper portion provides the lone barrier between the foot and the ball. Soccer boots of the past often used heavier calf or bovine leathers [1], a thick rigid barrier that inhibited touch. In the nineteenth century, however, shoemakers began using kangaroo leather, a supple and slender yet strong and durable leather often considered the finest natural material for soccer “uppers”. In fact, the thin and glove-like property of kangaroo leather enhances the feel of the ball against the foot, significantly improving touch. To this day, many high-end soccer shoes still incorporate kangaroo leather in the construction of the upper since it remains the best natural material available.

In recent years, synthetic materials, such as Nike’s proprietary NikeSkin, have also been developed in an effort to emulate the properties of leather with improved weather resistance and durability. Additionally, synthetic material often helps reduce the overall weight of the boot. A few shoes even combine kangaroo leather with synthetic material by constructing the front of the shoe with the kangaroo leather to provide good touch and the rest of the shoe with synthetic materials to improve durability and reduce weight.

Soccer boots must provide a good surface with which to strike a ball. For passing and shooting, players require a clean surface to generate power and provide consistent accuracy. Older soccer boots crudely fashioned out of thick leather provided no benefit in this area, but modern day research has led to many different types of innovations that all strive to accomplish the goal of giving players the best available platform from which to kick the ball.

One method incorporated by soccer companies involves shifting the laces closer to the outside edge of the boot—an innovation often referred to as asymmetrical lacing. By using an asymmetrical lacing system, the front and inside front of the boot offer more smooth space for the ball to strike. Without the bump of lacing to interfere with ball contact, the player achieves cleaner, more consistent contact with the ball.

Released earlier this year, the latest evolution of Adidas’ Predator line of soccer shoes epitomizes the focus towards finding innovations that assist a player to help improve performance (see Fig. 1). The new shoe incorporates two important features that specifically aid in helping to achieve more powerful and accurate strikes. First, a rubber-like compound has been directly injected onto the surface of the forefoot [2]. In addition to helping improve ball contact, the frictional qualities of the compound also help players apply spin to the ball as they kick. The increased spin on the ball allows a player to curve the ball’s trajectory to avoid other players or make the shot more difficult for the goalkeeper to defend. Fig. 2 provides more detail about this phenomenon.

The second, more revolutionary feature introduced in the latest Predator actually involves focusing more mass near the front of the shoe. Using a three-dimensional computer simulation of a human leg (anatomically correct with joints, muscles and bones), engineers refined the boot design on the computer while Adidas simultaneously conducted prototype trials with human athletes [3]. Moving the center of mass closer to the point of impact helps optimize power transfer during contact with the ball [2]. Testing with the boot showed increases in average ball speed, which translates into less time for the goalie to react to the shot [3] and consequently a better chance for a player to score a goal. The inspiration for this feature came from similar design theories used to construct tennis rackets, golf clubs and baseball bats [3].

Function is not the only factor determining shoe design. With all the running and kicking done by players over the course of a soccer game, comfort and safety compose a major portion of players’ needs. By improving the comfort and safety of a shoe, players can focus their attention on the game with confidence in the construction of the boot.

The traditional soccer shoe made in the early nineteenth century focused on providing protection for the foot because the first versions of the game involved much more physical contact than seen today [1]. Shoes of that time extended above the ankle to provide ankle protection and were often manufactured using heavier, thicker types of leather to provide further protection for the rest of the foot [1]. As the game evolved towards reduced physicality and increased emphasis on speed and skill, the shoes began to lose the ankle protection. With regard to comfort and safety, design initiatives became focused on improving the smaller sized boot we are familiar with today. Two important features, the sole and the surrounding structure, contribute a great deal to the overall comfort and safety of the modern soccer boot.

As the interface between the foot and the ground, the sole’s function is to protect the foot and maintain players’ comfort by absorbing the shock of repeated impact with the playing surface. To serve this purpose, shoe manufacturers insert cushioning into the sole of the shoe. This cushioning resembles the typical shock-absorbing, man-made materials used in running and athletic shoes, but is designed on a smaller scale to be more weight efficient. Engineers analyze data from pressure sensors to determine the ideal amount of cushioning that provides optimum protection and energy dissipation without compromising the performance characteristics of the shoe. Although manufacturers typically fix the amount of cushioning, Adidas now offers a shoe with three exchangeable soles featuring different characteristics, so the shoe’s “chassis” can be chosen to best suit the needs of individual players [2].

Just like a good ballet shoe supports the dancer, the structure of the soccer boot supports the player. The close, glove-like fit of the soccer boot helps it provide protection at the critical locations. The heel counter at the rear of the boot helps to cradle the heel and lock the foot in place [4]. Running shoes generally have heel counters embedded within the shoe’s heel padding, whereas modern soccer boots often have them on the outside of the heel. The external heel counter provides much more rigid support with increased fit and impact protection for the heel. The lacing system, another important element of the shoe structure, helps to secure the boot to the foot. In addition to improving ball contact, the asymmetric lacing system employed by some companies also helps improve comfort by moving pressure away from the top middle of the foot—an area that can be particularly sensitive.

The Nike Air Zoom Total 90 III soccer boot has been designed as “the world’s most comfortable boot”—according to the company [5] —and although this statement may or may not be true, the shoe definitely showcases a variety of technologies intended to assist in increasing comfort. To begin, the outside of the shoe features asymmetrical lacing and an external heel counter, elements common in modern soccer boots. Additionally, the Total 90 has a sole that incorporates two types of cushioning derived from running shoe technology. The mid section of the sole features a compressed foam material specifically designed for shock-absorption and distribution of pressure, and the heel of the sole features a Zoom Air cushion that provides lightweight, supplementary cushioning. The shoe also features support bars that run from the front to the back of the shoe. This structural strengthening helps provide greater rigidity as well as stability during flexing [5].

The impetus for this innovative technology came from a desire to create sophisticated products to help athletes. For assistance, engineers at Nike often call upon high-speed cameras that capture information from soccer kicks over 30 times faster than typical consumer cameras, thereby allowing engineers to meticulously examine every mechanical detail of even the simplest of soccer movements [5]. This data, combined with results from pressure measurements taken outside and inside the shoe, provides engineers with a complete picture of the human-shoe interaction.

While deceptively simple, soccer boots actually showcase a huge amount of effort in research and development. Engineers that design for large shoe companies utilize the most sophisticated analytical equipment and software available to produce technology that assists athletes in competition. Soccer shoes have progressed a great deal from the nineteenth century both in look and function. The modern soccer boot provides players with advantages in movement, ball control, and shooting, while also allowing maximum comfort and safety. The influence of engineers is clearly manifested through each new advancement, and engineers will continue to be the driving force of modern day soccer for many World Cups to come