This month we'll begin a 3-part series on tennis footwear. Areas that will be covered are:

• the components of a tennis shoe and the purpose of these components;
• foot motion in tennis play, forces on the foot and leg;
• shoe and surface interaction;
• foot conditions and injuries;
• technology, tools and footwear testing;
• footwear materials.

Tennis footwear is an important component of your game equipment. It can support or damage your performance and comfort. Most importantly athletic shoes protect the foot preventing abrasions and injuries. Footwear stability in court sports like tennis and basketball is incredibly important since it is estimated that as many as 45% of all lower extremity injuries occur in the foot and ankle.

In this first column we'll look at the components of an athletic shoe and the purpose of those components. If you have an old shoe you are ready to toss in the trash, or even better, one where the upper surface has split and separated from the soles, it might be helpful to take it apart and examine your own shoe and find these components. (Of course, if you are wearing athletic shoes as you read this column you can just look down and inspect them!)

ATHLETIC SHOE COMPONENTS

An athletic shoe has four basic components or layers: the upper, insert, midsole and outsole. Each component has a purpose. Dependent on the sport these components can vary a great deal in appearance and functionality.

Upper

The upper surface is the portion which covers the top of your foot. Although our choice of shoe may be affected by the attractiveness of the upper, its purpose is to serve as an attachment to the lower portions of the shoe (the mid- and outsole). Sometimes it is difficult to separate the functional parts from the aesthetic on a shoe's upper.

The upper wraps around, encloses your foot and holds the soles to the bottom of your feet. It also provides a foot enclosure method (generally shoestring laces, Velcro or today in running the "slip on") and has a heel counter. The heel counter conforms to your foot's heel to provide some rigidity and stability in the heel area preventing your heel from slipping off the soles onto the upper. Clearly your foot slipping off the bottom of the sole and sliding onto the upper could cause an injury.

Insert

The bottom of your foot sits on the insert. The insert positions the arch support, can be used to draw moisture and heat away from the foot, provides a little cushioning and cover for the midsole, and sometimes with an additive helps prevent foot odor. (The arch is the curve in your foot which provides structural support for your foot.) Custom made inserts (orthotics) can be designed to compensate for genetic or other support problems of the foot. Arch support systems can vary with the sport, the activity level of the player, foot size, arch type, age and gender.

The insert can be key in preventing blisters (caused by irritating the skin through rubbing or burning). [Just an interesting side note - researchers at the University of Wisconsin at Madison developed a mathematical equation to describe the number of rubs before the formation of blisters occurs. The equation considers the skin's frictional properties, temperature, effects of sweating, and the pressure and coefficient of friction between the skin and the contact surface. It was developed as a guide for footwear design and prevention of foot injuries.]

Midsole

The midsole contains the shoe's cushioning system. In one of the subsequent parts of this series on footwear, we'll present and explain current research in cushioning systems. For that reason we will leave those in-depth discussions for the future column. Let's just define the primary purposes of a cushioning system.

The cushioning system is meant to distribute the impact force to prevent the full transfer of that force to the foot and leg. Cushioning systems have also been developed to provide energy return. The concept - the cushioning device provides a rebound, spring or lift to the foot, in lieu of your muscles contracting (reducing the player's energy output) to achieve the same effect.

Outsole

The outsole provides traction and reduces wear on the midsole (which increases the durability of the shoe). Today's outsoles address sport specific movements (running versus pivoting) and playing surface types. Different areas of the outsole are designed for the distinct frictional needs of specific movements. A visit to your favorite athletic footwear store will supply you with a wide variety of different patterns on the bottom of the outsoles.

SHOE DESIGN PARAMETERS

In addition to comfort and fit key shoe design parameters are flexibility, stability, durability and traction. Let's examine what these terms mean and how the components contribute to these attributes.

Flexibility

Flexibility refers to the shoe's pliancy - its ability to yield and bend. Depending on the sport, flexibility is built in or minimized in a shoe. For example, a football place-kicker requires a stiff area in the forefoot (the area between the ball of the foot and the toes), while a running shoe deforms with the human foot's natural flex lines. There are tradeoffs. A highly flexible shoe can jeopardize stability and durability. As you can imagine, the cushioning system used in the midsole may compromise flexibility. However, athletic shoe scientists and designers endeavor to meld flexibility into the shoe's midsole and outsole along the foot's natural flex lines.

Durability

Durability, of course means, the shoe's ability to endure and perform over time - to last and continue to maintain its stability, traction, flexibility and shape (fit). Durability is dependent on the materials, structure and construction of the shoe and is managed in each shoe component differently.

For example, the bottom of the outsole wears based on the amount of friction it encounters. Researchers have isolated friction contact points to reduce wear in those areas. Durability of the upper is dependent on the materials used commensurate with the sport and foot motions. A case in point, baseball pitchers may wear the toes of their shoes faster than players in other positions. In the midsole, the materials and construction of the cushioning system govern its durability. (Again we'll be discussing cushioning systems separately in this series.)

Stability

The upper and midsole are key to providing stability to the foot. Stability is the capacity to resist forces which would cause motion or a change of motion. "High top" or "three quarter" height uppers may provide additional support to the ankle to prevent sprained ankles. To add stability, heel counters have been integrated from the midsole to the upper. One concern for designers is that cushioning systems may be so soft that they compromise stability - an important design consideration.

Traction

Traction created by the friction between the outsole and the surface allows the shoe to grip the surface. As surfaces, conditions and player motion change, traction may need to vary. An athletic shoe needs to grip well when running but not when pivoting. For hard court sports you do not want the shoe to grab the surface and stop so suddenly that you turn an ankle.

Okay! At this point we all have some common information and are ready to deal with some specific issues in tennis. Next month we'll go through specific footwork patterns in tennis and look at the forces placed on the lower extremities.

However, here's the challenge. Aside from running, which primarily has one direction of motion (full steam straight ahead), most other sports have a large variation in foot motions. Consequently (and possibly unfortunately) most studies and research in footwear deal with running and walking. There are generic and sport specific injury patterns and it is no surprise to you that running injuries differ from court sports like tennis and basketball.

Just a quick final note. I received an email from Crawford Lindsey on Thanksgiving Day. The book, The Physics and Technology of Tennis by Howard Brody, Rod Cross and Crawford Lindsey has just been published. You might remember we spoke to Crawford about this work in the Tennis SET August 2002 column. The book is available through the Tennis Server's online bookstore.

Thank you for sending in your tennis footwork questions. I will try to incorporate and answer all those questions into the series.

Happy Holidays from my family to all of you! Jani

References

Chang, W. and Seireg, A., Frictional Properties Of The Skin And Blister Formation, Third Symposium on Footwear Biomechanics, Tokyo, Japan, 1997.

Cooke, A and Dixon, S., Sports Science and Engineering in Education: Sport Shoe Design, http://www.cookassociates.com, 2001-2002.

Larsen, J., Slam Dunk Science, A Sport Research Lab In the Classroom, http://www.scire.com/sds/sdsmenu.html, 1995-2002.

Segresser, B., Injuries And Sport Shoe Design: Wish And Reality, 2001 Footwear Symposium, Zurich, Switzerland, July 5-7, 2001.

Xia, B. and Robinson, J., 3D Kinematic Evaluation Of Footwear Stability In Lateral Movements, Third Symposium on Footwear Biomechanics, Tokyo, Japan, 1997.