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IN THE Hollywood comedy Son of Flubber, made in 1963, absent-minded
professor Fred MacMurray invented a magical bouncy substance. Inserted into
the boots of the ailing football team, it gave them a lift and helped them
to win their big game. What was fantasy then is now the talk of the leisure
industry and the world of running.

Shoes are the most vital weapon in every runner’s armoury. Until recently,
all a runner wanted was a shoe that let you train for a race and run it
without injury. The two important words were ‘cushioning’, protection against
the impact of the foot with the road, and ‘stability’, the prevention of
exaggerated sideways movements of the foot that can lead to injury. Most
of the developments in the technology of running shoes have come in these
two areas.

To choose the right shoe, then, runners must take several things into
account: their weight, the terrain on which they run, how far and how fast
they run each week, and the way their feet and legs move when they run.

Feet tend to fall into two extremes, with many permutations in between.
At one extreme is the rigid, high-arched foot that does not spread out as
it strikes the ground. For such a foot, cushioning or shock absorption is
most important. At the other is the flat foot which spreads out as it hits
the ground. This type of foot needs a design that promotes stability or
controls sideways motion. These features prevent injuries caused by excessive
movement of the foot either inwards (over-pronation) or outwards (over-supination).

A number of devices can enhance a shoe’s stability. One such is the
heel counter, a piece of strong, flexible thermoplastic that cups around
the heel to hold it snugly in place. Heel counter are often extended along
the inside of the foot to prevent over-pronation; a pad of harder material
under the heel can also help here.

Cushioning is largely the province of the midsole. This layer of material,
thicker under the heel, runs the length of the foot between the upper sole
and the outsole. To cushion a shoe, manufacturers commonly use ethyl vinyl
acetate (EVA) and polyurethane. Both can come in different densities for
different needs: the more rigid the foot, the softer the material should
be. The first EVAs became compacted and lost their cushioning very quickly,
to the chagrin of consumers who expected more kilometres for their money.
Polyurethane, on the other hand, lasted well but was far less springy.

Manufacturers strove to get the best of both worlds. For example, New
Balance ‘encapsulated’ EVA inside a layer of polyurethane; Nike put air
in a polyurethane capsule. Over the years, EVA became lighter and could
be ‘compression moulded’, which means that all the changes in cushioning
take place before the runner buys the shoe, like buying pre-shrunk jeans.

Two years ago, Neill Alexander of the University of Leeds suggested
in these pages that ‘shoe designers might benefit from thinking more about
the natural bounce of the foot’, and this they have done (see ‘A spring
to your step,’ New ÐÓ°ÉÔ­´´, 30 April 1987). Alexander cited the experiment
of Thomas McMahon and his colleagues at Harvard University; they showed
that it was possible to ‘tune’ an indoor running track by making it slightly
elastic. This helped athletes to run 3 per cent faster than on conventional
tracks. Could this theory be applied not to the surface they ran on, but
to the shoe they ran in? The jury is still out. Some manufacturers say a
hearty ‘Yes’ and have concentrated their marketing activity in the direction
of energy return. Others say ‘No’ – either with emphatic statements of ‘It
can’t be done’ or by abstaining from the debate altogether.

The main protagonists of energy return are Reebok, whose Energy Return
System, trademarked ERS, uses hollow tubes of Du Pont’s Hytrel, encapsulated
in polyurethane, across the foot. Hytrel is a flexible thermoplastic that
quickly springs back into its original shape. Reebok claims that the ‘amount
of energy absorbed by the shoe is directly correlated to the energy returned’,
and that the ERS system is ‘tuned to perform at the right moment, giving
the release of stored energy back to the full benefit of the performer’.

Converse has developed its own midsole material called the Energy Wave
which is 10 per cent lighter than compression-moulded EVA (an advance in
itself). Converse claims that its new material gives 50 to 55 per cent energy
return compared with 40 to 45 per cent for Gel and EVA and 25 to 30 per
cent for Air and polyurethane. The company also claims a longer life for
the Energy Wave shoe. After 40 kilometres, its says, EVA has lost 20 per
cent of its cushioning compared with 0.5 per cent for Energy Wave. For many
runners this durability may prove a better selling point than the energy
a shoe promises to return.

Hi-Tec’s ABC (Air Ball Concept) is a concertina-like cylinder of pressurised
air under a skin of polyurethane, under the heel. Jim Walker, who developed
ABC, says that ‘many existing systems absorb and dissipate energy resulting
in a dampening effect. The concertina design allows compression on impact
before regaining its shape on lift-off, adding kinetic energy to the wearer’s
leg movement.’

Turntec’s Z03 uses Toblerone-shaped bars of ‘superball’ rubber under
the heel, although Gerry Turner of Turntec says that energy return is not
the be-all-and-end-all of its marketing drive. The cushioning and stability
offered by its ‘anatomical cradle’, in which the foot rests in a scooped-out
hollow, not on a flat board, are still the most important features of Turntec
shoes. And the Saucony Azura uses two materials from Du Pont, Hytrel and
Kevlar, in a Torsional Rigidity Bar which, Saucony claims, ‘enhances rebound’.

Nike has been the most outspoken critic of energy return, arguing that
softer, more compliant materials, not hard tubes, will offer both better
cushioning and, as a by-product, better energy return. In a 10-point corporate
statement, it accuses Reebok and other manufacturers of eliminating the
compliance factor from their tests of energy return, and says that energy
return has been developed at the expense of other, more important, features.
‘By Nike standards,’ says the statement, ‘the (Reebok) World Trainer is
heavy, inflexible and poorly cushioned.’ Nike, instead, has developed the
use of pressurised gas (for lightness and cushioning) in capsules of polyurethane
(for durability). The company claims that its air cushioning never compacts,
and cushions as well after 800 kilometres as it does after the first.

In a similar statement, Avia makes the point that ‘a truly functional
energy return system would have to capture the energy from heel strike beneath
the rearfoot and transfer it to the forefoot to effectively return any of
the energy to the athlete. To date, no such system exists in athletic footwear.’
Like Nike, it argues that energy return does not protect against injury,
and indeed, its most recent innovation, the ARC system – also using Du Pont’s
Hytrel – is specifically designed to dampen and dissipate shock on heel
strike, not to return it.

The fact is that most people who buy running shoes run to keep fit,
not to break records. Avia’s consultant, Peter Cavanagh of Penn State University,
and author of The Running Shoe Book, says: ‘Supposing for a moment that
true energy return to the body can be achieved, then it is certainly likely
to lead to faster times, and this will be important to runners at the elite
level . . . But for most runners, the goal is usually to expend energy –
thus most runners are concerned with maximising energy expenditure rather
than minimising it.’

Shoes on the rebound

Cavanagh also points out that no research group has yet published a
definitive ‘bench test’ to measure the energy that an athletic footwear
would return in, for example, a race. Don’t expect any benefit, says Cavanagh,
if your shoe first strikes the ground with the forefoot, yet the device
that is to return energy is in the rear of the shoe. ‘Even for a rearfoot
striker, it is not yet clear if or how stored energy can be recovered to
provide useful energy input to the runner.’ Running style, body weight and
footwear properties are likely to interact in a complex way that must be
understood before anyone can design a shoe that returns the maximum energy.
A device that is suitable for one runner may be ineffective when used by
another. Cavanagh cautions against simplistic conclusion: ‘A steel bell
on a concrete surface would give an excellent score for energy return but
no one would suggest that a shoe made from these materials would be acceptable.’

Some manufacturers have not entered the debate on energy return, though
most have listened in on it. They have instead concentrated on the protective
features of cushioning and stability. For the past two years, Asics (formerly
Tiger) has enjoyed some success with Gel, a lightweight midsole material
developed in Japan. Barry Bates, of the University of Oregon, and the American
consultant to Asics, is concerned that energy return is being developed
at the expense of stability. ‘Energy storage requires a material to have
a springlike quality so it can compress,’ he told the American magazine
Runners’ World, ‘and this compression has the potential to be very unstable
in the side-to-side direction.’

New Balance has concentrated on the midsole material, developing first
Encap-EVA in a skin of polyurethane, and now Evathane. It is based on polyurethane,
but the company claims it cushions as well as EVA.

Brooks claims that its HydroFlow system, which uses a two-chambered
bag of silicone fluid in the midsole, is the only shock absorption system
that takes the individual runner’s speed and weight into account (‘custom
cushioning’). When the heel strikes the ground, the fluid is forced from
one chamber to the other. The pressurised fluid cannot escape quickly from
the second chamber, so with a heavier or faster runner the pressure builds
up, making the bags stiffer and thus offering better cushioning. When pressure
from the heel is removed, the fluid flows back into the first chamber. HydroFlow
directs the shock ‘so that it stays within the shoe instead of passing up
into the body’ – quite the opposite of energy return. Brooks also claims
that its HydroFlow shoes last longer than shoes made from EVA or polyurethane,
and that HydroFlow provides more shock absorption than Air and Gel, which
are confined to a single chamber.

Adidas has been running a quite different race, addressing itself to
the rotation of the foot along its longitudinal axis. After cushioning and
support, it says, the next step is to guide the foot. The company says:
‘Currently, no shoe gives independent forefoot and heel movements, allowing
natural torsion movements to occur unhindered. When torsion movements are
restricted by too rigid a shoe, increased demand is placed on the locomotor
system from the lower leg and knee, up to the hip.’ Two elements in the
Adidas shoe put this right: a groove in the sole, and a torsion bar that
rotates along the longitudinal axis but has little lateral movement or flexion.
As a result, the foot can move naturally in the longitudinal plane.

Many runners are sceptical about the new developments, and often they
are displeased that old favourites have been replaced by super-duper new
models with different names. At the end of the day, what runners really
want is injury-free running, and lots of it, whether from one Pounds sterling
99 pair of shoes or three pairs at Pounds sterling 33. If the development
of strong, light materials for energy return helps to make shoes last longer,
runners, whatever their standard, will get more miles for their money. In
the current economic climate this no-frills, no-gimmick approach may well
be the one that sells most shoes.

Alison Turnball is a freelance sports writer and contributing editor
to Running magazine.