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Design in Running, Court, and Fitness Shoes
As recently as thirty-five years ago,
athletic shoes consisted of just a few shoes that were used for a wide
variety of athletic events. There were a few tennis and basketball
shoes. There were no shoes marketed specifically as walking shoes.
Aerobics or fitness shoes were nonexistent. Running shoes only amounted
to a few in number. However, in today's athletic
shoe stores, the number of brands and styles of shoes for all types of
sports is staggering. There are shoes made specifically for wrestling,
rock climbing and windsurfing in addition to the more common sports
such as running, basketball, tennis, racquetball, aerobic dance and
walking. In the running shoe market alone, there are nine major shoe
manufacturers with each manufacturer having about five to ten running
shoe models within their line. Even though the increased selection of
shoes increases the possibility of finding just the right shoe for each
set of feet, the large selection of models creates a large degree of
confusion among the consumer. It is actually this
diversity and complexity within athletic shoes that is their most
interesting aspect. Shoes that have different shapes, are made of
different materials, and which are put together by different
construction methods all will function on the foot differently. The
purpose of this article is to explain the major structural differences
between the three broadest categories of athletic shoes (i.e. running
shoes, court shoes and fitness shoes) so that their functional
differences may be better appreciated. Running Shoes 
Running,
like walking, is considered a straight ahead sport since it involves no
sudden stops, turns or other maneuvers. Most runners land on their
heels and then propel off of their toes. This heel to toe cycle is
repeated hundreds and thousands of times every running session. The
major biomechanical differences between running and walking are that in
running there is always one point during running when both feet are off
the ground and also during running the impact forces which the foot
absorbs are at least twice as great as that found in walking. Most
runners strike on the outside of the heel, rapidly pronate, stay
pronated for a brief instant and then resupinate as the heel leaves the
ground during the push-off phase of running. [Pronation of the foot is
a rolling inward of the ankle in which the arch flattens. Supination of
the foot is a rolling outward of the ankle in which the arch increases
in height.] Due to the large degree of variation within the population,
there are a large number of runners who pronate excessively during
running causing a multitude of running injuries such as posterior
tibial tendinitis, plantar fasciitis and pes anserinus bursitis, to
name a few. Because of the increased impact
forces and increased excessive pronation seen in running, running shoes
must be designed both to help reduce excessive shock to the body and
also help reduce pronation in the foot (Fig. 1). Unfortunately, the
same shoe design characteristics that are best at helping to control
pronation also tend to lessen the ability of the shoe to cushion the
foot. And conversely, any shoe designed to maximize the cushioning of
the foot during running will tend to have decreased ability in helping
to control pronation. To better understand how
the characteristics of running shoe design affect foot function it is
important to detail the structural components of the running shoe.
Every shoe is made of two basic parts, the sole and the upper. The sole
protects the foot from the ground and provides a layer of cushion for
the foot. The upper covers the top and sides of the foot to provide a
comfortable fit between the foot and the shoe and to improve stability
of the foot on the shoe sole. In the running
shoe, the sole is made up of two distinct layers, the outersole and the
midsole. The outersole is the part of the sole that contacts the
ground. It is made of a thin layer of relatively hard, abrasion
resistant material which functions to resist wear, provide traction and
allow flexibility in the forefoot for propulsion. Many
running shoes use a rubber compound with a high carbon content in the
heel and forefoot area, which is similar in composition to an
automobile tire, so that the outersole will resist the abrasion that
comes from the heel striking the ground. Running shoe outersoles also
are constructed with studs or ridges in the midfoot and forefoot area
to aid traction on soft or slippery surfaces, such as wet grass or
slick pavement. In addition, most running shoe outersoles also
incorporate some form of transverse grooves placed in the area of the
forefoot so that the shoe will be more flexible in the forefoot once
the heel leaves the ground during the push-off phase. The
midsole, however, is the part of the running shoe that either makes it
work well or makes it work poorly. The midsole is sandwiched between
the upper and the outersole. The upper is glued or bonded to the top
surface of the midsole. The midsole is the most important part of the
running shoe because its design and construction largely determine
whether the running shoe will be a shoe which is good at providing
cushioning, good at controlling pronation, good for heavy runners or
good for nothing. Running shoe midsoles are
designed so that there is thick cushioning under both the heel and
forefoot to help provide cushioning to the heel and forefoot. The total
height of the midsole and outersole under the heel is generally about 1
inch and the total height of the midsole and outersole under the
forefoot is about 5/8"". The 3/8"" difference of sole thickness between
the heel and forefoot in many running shoes tends to be preferred by
most runners and also reduces the strain on the Achilles tendon,
therefore, reducing the likelihood of Achilles tendinitis. The
midsole may be constructed of various materials to provide cushioning
and pronation control. The two most common materials used in the
construction of running shoe midsoles is ethyl vinyl acetate (EVA) or
polyurethane (PU). EVA is a copolymer of ethylene and vinyl acetate
that has microscopic air bubbles within it that makes it lightweight
and very cushiony. PU also has a microscopic air bubble structure like
EVA but is generally firmer and more resistant to compression than EVA. Running
shoe manufacturers use combinations of different densities of EVA
and/or PU within the midsole of the shoe, along with gel packets, air
bags, plastic plates and other exotic materials to provide what they
believe is the proper amount of cushioning and pronation control for
the shoe. Many running shoe midsoles have a firmer midsole material or
a hard plate under the medial heel and a softer midsole material under
the lateral heel so that the medial heel resists compression more than
the lateral heel when the heel strikes the ground in running [Medial is
toward the big toe, lateral is toward the little toe]. This ""dynamic
varus wedge"" effect does effectively help control foot pronation to
some extent. The softest midsole material is generally placed under the
forefoot since most runners find that good forefoot cushioning is a
very desirable feature when running on hard surfaces. The upper
of the running shoe is usually made of a combination of lightweight
nylon and thin synthetic or natural leather to reduce the total weight
of the shoe. Since running involves at least a thousand footstrikes per
mile, a lightweight running shoe is critical to insure that the runner
can move at a faster pace with less fatigue. One drawback to the
lightweight materials used in running shoe uppers is that they all tend
to suffer in side to side stability since the thin material in the
upper is ineffective at resisting medial and lateral shifting of the
foot on top of the sole of the shoe. The upper of a running shoe
also incorporates a stiff heel counter that is commonly stiffer than in
other athletic shoes to help control excessive pronation or supination
during running. Most running shoes also incorporate a raised padded
""Achilles tendon protector"" within the design of their upper to
supposedly help protect the Achilles tendon. Most runners find that the
""Achilles tendon protector"" serves only as a convenient handle by
which to pull their running shoes on with and serves little importance
in protecting the Achilles tendon from injury. Within the
interior of today's running shoes are removable insoles known as
sockliners. Sockliners serve to cushion the foot and provide some arch
support. Many sockliners in more expensive running shoes serve to
support the arch of the foot more effectively than those seen in
cheaper shoes. Nearly all sockliners can be removed easily from the
shoes so that custom foot orthoses may be added to the shoe to replace
the sockliner if needed. One more important fact about running
shoe design is that running shoes make excellent walking shoes. Since
running and walking are both straight-ahead activities, their basic
shoe designs are quite similar. In fact, I recommend running shoes for
my patients who walk for exercise in favor of many walking shoes since
running shoes are lighter, more comfortable and biomechanically more
efficient at helping control excessive foot pronation than the majority
of walking shoes. Court Shoes Court sports include
tennis, racquetball, basketball, squash, badminton and volleyball.
Because court sports require sudden starts, stops and side to side
motions, the best shoe construction for court sports is much different
than that required for running (Fig. 2). The sudden side-to-side
movements seen in court sports tend to make the foot slide forcefully
either in a medial or lateral direction on the shoe sole. For example,
if a tennis player is moving quickly toward the right and then uses the
right foot to come to a complete stop, the foot will tend to slide
laterally on top of the shoe sole. The only thing preventing the foot
from sliding directly laterally off of the shoe sole is the upper of
that shoe. It is because of this necessity for side-to-side stability
that court shoes must be constructed much differently than running
shoes. 
Like
running shoes, court shoes come in all shapes and sizes depending not
only on the sport which the shoe is designed for but also on the
manufacturer. Unlike running shoes in which the upper of the shoe
always ends just below the ankle bones (i.e. a low-cut shoe), the upper
of court shoes may extend partially over the ankle bones to about the
ankle joint level (i.e. a mid-cut shoe) or may extend above the ankle
bones completely covering them (i.e. a high-cut or high-top shoe). Many
basketball shoes tend to be made of a higher cut than other court shoes
due to the relatively great frequency of ankle sprains seen with
basketball. All other shoe design parameters being equal, the higher
the cut of the upper of the shoe, the better that shoe will be at
preventing ankle instability during the activity and the heavier that
shoe will be. Since the goal in a well designed court shoe is to
make the upper hold the foot on top of the sole, the uppers of court
shoes are thicker and made of heavier weight materials than running
shoes or fitness shoes. The uppers of court shoes are constructed of
thicker leathers or synthetic leathers than either running or fitness
shoes. Lightweight and thin materials such as nylon are used less
frequently in court shoe uppers. In addition, many tennis shoes may
have an extra layer of synthetic or natural leather toe box
reinforcement to prevent the upper from wearing through in the toe box
area from the scuffing which occurs during tennis serves. Many
court shoes also are constructed with an extended outersole or midsole
which rises up on the sides to the bottom edge of the upper to give
added strength to the sole/upper interface. As a result of the use of
thicker upper materials and the side reinforcement of the sole up onto
the upper, court shoes are nearly always heavier than the same size of
running shoe. The outersole of court shoes are usually made of a
non-marking rubber compound for traction on outdoor or indoor courts.
Court shoes have a much lower profile of tread patterns on their
outersoles than running shoes since court sports are nearly always
played on a dry, flat and smooth surface. In addition, court shoes
often have a circular designs constructed into the outersole under the
forefoot area of the sole to act as a ""pivot point"" for the shoe
during rotational motions of the foot on the playing surface. Like
running shoes, court shoe midsoles are predominantly made of either EVA
or PU. However, the midsoles of court shoes are firmer and thinner than
running shoes to reduce the instability of the court shoe during
side-to-side movements. Shoes with firmer soles have better
side-to-side stability since the force of body weight through the foot
will not deform a firm sole as much compared to a cushiony sole. The
more that a shoe sole deforms under the forces which the foot exert on
it during aggressive maneuvers, the more likely the shoe sole will tilt
to one side or the other which may lead to either pronation or
supination instability at the ankle joint complex. Thicker soles
increase the height of the foot and ankle from the ground that, in
turn, increases the distance of the ankle joint complex from the
ground. The higher that the ankle joint complex is from the ground, the
longer is the lever arm for the reaction force from the ground to cause
a either a pronation or supination force on the foot and ankle.
Therefore, the thinner soles of court shoes decrease the likelihood of
ankle sprains since the ground has a much shorter lever arm to produce
pronation or supination forces on the ankle joint complex. Fitness Shoes About
fifteen to twenty years ago there was a dramatic increase in the
popularity of aerobic dance. At that time, the shoes worn for aerobic
dance were either running or court shoes. Unfortunately, since running
and court shoes were not specifically designed for the demands of
aerobic dance, many injuries occurred. Those aerobic dancers wearing
running shoes had good cushion to the forefoot, but suffered from ankle
sprains due to the lack of lateral stability in running shoes. Those
dancers wearing court shoes had good side-to-side stability, but
suffered from painful symptoms in the forefoot due to the lack of
cushioning in the forefoot in court shoes. 
Shoe
manufacturers responded with the aerobics shoe that blended
technologies from both the running shoe and court shoe. The result was
a shoe with a midsole thickness and degree of cushioning midway between
that of court shoes and running shoes. In addition, the aerobics shoe
had an upper that was midway between the court and running shoe in
material weight and thickness. Today, shoes made for aerobic
dance are very similar in design to those shoes made for the various
activities available in a health or fitness club. Therefore, shoes made
for aerobic dance and cross-training are now known as "fitness shoes".
Understanding the construction of fitness shoes is important since they
not only are a very popular style of shoe, but their relatively recent
birth into the shoe marketplace demonstrates the ability of shoe
manufacturers to design a totally new and unique style of shoe to meet
the biomechanical demands of a new sport (Fig. 3). The fitness
shoe has been designed using technological features from both running
shoes and court shoes to create a shoe that is actually a better
all-purpose shoe than either the court shoe or the running shoe. It is
lighter in weight and more well cushioned than the court shoe and much
more able to resist side to side movements of the foot than a running
shoe. The upper of fitness shoes can range from a low-cut to a
high-cut with the most popular height being a mid-cut. The mid-cut
upper is a very popular style in fitness shoes since it does provide
extra lateral stability without adding a great deal of extra weight to
the shoe. The fitness shoe upper is made from a combination of thinner
natural or synthetic leather and nylon that decreases the weight of the
shoe compared to a court shoe. However, since the fitness shoe upper is
more substantial than the upper found in running shoes, the lateral
stability of the fitness shoe is greater than in the running shoe. Like
court shoes, many fitness shoes use an extended outersole or midsole on
the medial and lateral sides of the upper to provide extra bonding
strength to the sole/upper junction. The extended midsole is now very
popular in fitness shoes and does provide an extra degree of lateral
stability to the shoe. The outersole of fitness shoes are very
similar to court shoes being made from non-marking rubber compounds in
a low profile. However, the midsole in a fitness shoe is thicker than
that seen in the court shoe to provide extra cushioning to the forefoot
and rearfoot during aerobic dance, running and other impact activities.
Even though the midsole in a fitness shoe is not as thick as that in
running shoes, the fitness shoe can safely have a thicker midsole in
its design since the side to side activities seen in fitness shoes are
not as aggressive as that seen in court sports. Conclusion Certainly
in the case of all the shoes described, it is clear that the structure
of the shoe determines how the shoe will affect the function of the
foot within that shoe. Whether it is the composition of the outersole,
midsole or upper, or it is how the sole is attached to the upper, or it
is any other shoe design parameter, the construction of athletic shoes
must match the biomechanical requirements of the specific athletic
activity in order for the shoe to be useful and desirable for the
athlete.
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