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NAOMI W. is used to heavy loads. The firewood, food and water supplies that
she carries often weigh nearly as much as she does. She is one of the Luo, a
people who live on the western flatlands of Kenya. She is no great athlete. In
fact, she is no fitter or healthier than you are. But when it comes to carrying,
Naomi leaves elite American soldiers for dust.

Over the past fifteen years, researchers have been struggling to understand
how the Luo women do it. There have been several false starts. The idea that the
Luo might have special load-carrying genes or unusually well-developed neck
muscles, for instance, was confounded when the researchers began to look further
afield. They discovered that the ability to carry extraordinarily heavy weights
with ease isn鈥檛 confined to one small group鈥攊t shows up all across Africa,
India and Asia, and even among North American Indians. In culture after culture,
load carriers were found to suffer less breathlessness, less fatigue and less
back pain than their counterparts in the West. So how do they do it? This year
the answer has finally been revealed, and it鈥檚 neither muscles nor magic. It鈥檚
all about rhythm.

The mystery dates back to 1977. Norman Heglund, then a graduate student at
Harvard University, was in Kenya with collaborators Geoffrey Maloiy and Dick
Taylor to examine the energy use of elephants, giraffes and buffaloes. He had
brought a treadmill for the animals to walk on and equipment to measure their
rate of oxygen consumption. But instead of studying the animals, Heglund found
his attention caught by the activities of his Luo hosts. 鈥淵ou would see a Luo
woman carrying water on top of her head with children hanging off her front and
a man walking beside carrying nothing but a stick,鈥� he says.

He began to wonder how the women could carry so much on their heads without
appearing to get tired. Eventually he approached the wives of some of the
researchers at Kenya鈥檚 East African Veterinary Research Organization. They
included not only Luo women, but also women of the Kikuyu from the Central
Highlands, who could carry equally impressive loads. Instead of balancing them
on their heads, the Kikuyu carry loads behind them slung from a strap across
their forehead鈥攚hich, over time, can lead to a permanently grooved
skull.

Heglund wanted to know how much extra energy the women used when transporting
a load. To find out, he needed to measure how much oxygen they consumed, since
this is directly related to how much energy is being expended鈥� the more
heavily you breathe, the more energy you鈥檙e using. So he asked for volunteers to
breathe into his apparatus while carrying loads on a treadmill. 鈥淭he women were
quite shy and we had to cover the windows to the lab where we made the
measurements,鈥� says Heglund.

The first volunteer stepped onto the treadmill and began to walk steadily at
about 3 kilometres an hour while Heglund smoothly deposited a load of about 10
kilograms onto her head. He asked her to try to continue at the same speed and
glanced across at the oxygen meter. 鈥淭here wasn鈥檛 even a blip,鈥� he says, 鈥渟he
just carried on as if the load wasn鈥檛 there.鈥� In fact, Heglund found he could
load up to a fifth of the women鈥檚 body weight on their heads before they began
to breathe more deeply. And even when more was added, it didn鈥檛 seem to be
costing them nearly as much energy as it should.

Intrigued, Heglund began to investigate further. He came across a 10-year-old
paper on the load-bearing performances of American army recruits carrying
rucksacks. A team from the US Army Research Institute of Environmental Medicine
in Natick, Massachusetts, had produced detailed guides to the amount of energy
required to carry different loads under differing conditions. When Heglund
compared these figures with his results for the African women, he found the
women outperformed the soldiers at all but the very slowest walking speeds. When
loaded with 70 per cent of their body weight, the army recruits鈥� oxygen
consumption more than doubled. But the women were only breathing one and a half
times as heavily as usual鈥攁 far more sustainable level of exertion.

How could the women beat the highly trained, immensely fit soldiers? Perhaps,
Heglund thought, they were walking very smoothly, to minimise their load鈥檚
vertical motion and save themselves from having to hoist it up at every step.
Maybe they had amazingly strong neck muscles, developed over years of training,
which the army recruits lacked. But time and again, as loaded women walked by in
the street, Heglund realised he was way off target. 鈥淚 was just throwing lots of
ideas around because I didn鈥檛 know,鈥� he admits. As far as he could see, the
women of the Luo and Kikuyu looked and walked just like anyone else.

Realising he was in over his head, Heglund approached Giovanni Cavagna, an
expert in the study of walking from the University of Milan in Italy. Cavagna
and his colleagues Rudolfo Margaria and Francesco Saibene had developed a model
of walking to solve a completely different problem: how people walk on the Moon
(see 鈥淲alking on the Moon鈥�). Heglund hoped that this same model would
help solve the mystery of how the African women walk.

Cavagna鈥檚 model compares a person walking to the swinging of a pendulum. He
came across this idea by thinking about the way weight is transferred from foot
to foot as a person walks. Walking feet can be pictured as neighbouring corners
on a square wheel.

Every time a person takes a step, it鈥檚 as if the square wheel flips round
onto its next side. As the wheel rotates, the point of contact moves from being
a flat edge to a corner of the square, to the next flat edge, to the next
corner, and so on. As it does so, its centre of mass moves from being over the
flat edge, to behind the corner in contact with the ground, to directly over the
corner, to being in front of the corner, before the wheel finally falls onto the
next flat edge. The trajectory traced out by the centre of mass, Cavagna
realised, is exactly that of an upside-down pendulum (see Diagram).

That鈥檚 intriguing, because the energy of a swinging pendulum is exchanged in
a very particular way. As the pendulum swings, it converts energy seamlessly
from potential energy to kinetic. Potential energy is related to your height in
a gravitational field鈥攖he higher up you are, the more potential energy you
have. That鈥檚 why it鈥檚 easier to walk downstairs than upstairs. When you walk
downstairs you鈥檙e using the potential energy stored when you hauled yourself
upstairs. Kinetic energy, on the other hand, is all about how quickly you鈥檙e
moving. The faster you鈥檙e going, the more kinetic energy you have. For a
pendulum, the extra potential energy at the high parts of the swing is converted
into speed at the low part of the swing. Back and forth the pendulum moves, its
energy shifting from one form to the other.

Cavagna believes people walk like the swinging of an upside-down pendulum.
From the highest point of the step, with their weight on one foot, people swing
down to the other foot, converting gravitational potential energy to kinetic
energy of forward motion. This motion drives them forwards onto the other leg
and into the upward swing as the cycle starts again.

To see if his ideas were right, Cavagna and his colleagues traced the motion
of adults鈥� centre of mass and measured their speed as they walked. For most of
the step, he found walkers did achieve a continuous exchange of potential and
kinetic energy, just like a pendulum. But all in all, they were only 65 per cent
as efficient as a perfect pendulum, because they took time out from the energy
exchange at the beginning of each step鈥攚hen both feet are on the
ground鈥攁t the end and at the middle. 鈥淎t the beginning, energy has to be
added by the muscles, like the spring in a pendulum clock鈥� says Cavagna. 鈥淏ut in
the middle of the step, it鈥檚 just wasted.鈥�

When Cavagna heard about the amazing abilities of the African women, he
suggested that they might be avoiding part of this wastage. Maybe African women
were better pendulums than your average European or American woman. To put
Cavagna鈥檚 idea to the test, Heglund needed to see how the pendulum idea applied
to people carrying heavy weights. So he returned to Kenya to make detailed
measurements on Luo and Kikuyu women walking with and without loads. As a
control group, his collaborators Massimo Penta and Patrick Willems later took
the same measurements from volunteers at the Catholic University of Louvain in
Belgium.

Though the results were obtained nearly 5 years ago, it was only this summer
that a computer program written by Willems and Mario Legramandi at the
University of Milan made it possible to analyse the pendular energy transfer
within each step cycle. It turned out that unloaded African women walk just like
Europeans, adding energy at the start (and end) of each step and wasting it in
the middle. But when the Luo and Kikuyu women are carrying loads, imperceptible
changes begin to emerge.

The difference occurs in the middle of the step, when the walker is balanced
on one leg with her centre of mass above one foot and the second foot passing
forwards. Here the spare energy is almost entirely in potential form, and the
walker begins to lose height and gain speed. Instead of exchanging all the
energy, most European walkers waste it: for around 15 milliseconds, they lose
height faster than they gain speed, continuing to decelerate as if they were
still gaining height until they lurch down at the end of the step. But in loaded
Luo and Kikuyu women, the energy exchange ceased for less than 10 milliseconds.
In fact, in the best load-carriers it didn鈥檛 cease at all.

This explains why the women can carry some of the weight on their heads for
free. Without a load, they are imperfect pendulums, managing only 65 per cent
efficiency. But as their load increases, they pause less and less in the middle
of the step, and become better pendulums. Carrying 20 per cent of their body
weight, for instance, they averaged 70 per cent of the full pendulum efficiency,
and their energy consumption was exactly the same as when they were unloaded.
When they were loaded with more than 20 per cent of their body weight, however,
the women failed to improve their efficiency any further and the load began to
take its toll. But they were still better pendulums than their European
counterparts.

Now Cavagna knows the phase of the step where the gaits of loaded African and
European volunteers diverge, he can start to understand how the load helps their
pendular performance. 鈥淭he exact mechanism is still under study,鈥� he says, 鈥渂ut
we know the African women are swinging down with better pendular motion than the
Europeans. Possibly the load helps them to bring the changes in kinetic and
potential energy more into phase.鈥�

So can we all learn to walk the walk? Cavagna鈥檚 not sure. He points out that
the difference in gait is too small for the human eye to see, and there鈥檚 also
tremendous variation between different women. 鈥淚 think it鈥檚 maybe something you
can learn by carrying in childhood.鈥�

Meantime, there鈥檚 the rest of the world to think of. While Heglund and
Cavagna were studying the Kikuyu and Luo women, they began to receive reports of
amazing load-carrying feats among other peoples. This summer, Heglund and his
colleagues B茅n茅dicte Schepens and Willems began tests with the
Sherpas of Nepal, where both men and women carry huge loads on their heads.
Heglund believes there鈥檚 no biological reason why Luo and Kikuyu men could not
carry loads like the women. 鈥淚n my opinion it is just a cultural phenomenon,鈥� he
says. The Sherpas don鈥檛 carry the first 20 per cent of the load on their heads
for free, but they seem to use less energy than the Luo and Kikuyu at loads of
between 40 and 60 per cent of their body weight.

Heglund doesn鈥檛 yet know how Sherpas achieve such economy, but unlike last
time round, he knows where to start. 鈥淲hatever it is they鈥檙e doing, I don鈥檛
think it鈥檒l be extra muscles or a different anatomy, 鈥� he says. 鈥淚t鈥檒l be
something subtle in how they鈥檙e walking.鈥� And he鈥檚 betting heavily that it鈥檚 all
to do with the way they swing.

IN 1969, Neil Armstrong took the famous small step from Apollo onto the
surface of the Moon. But if you look at the film closely, says Giovanni Cavagna
from the University of Milan, you鈥檒l see it was really a leap. Cavagna and his
colleagues Rodolpho Margaria and Francesco Saibene had predicted that astronauts
would be unable to walk normally on the Moon. As they watched the first Moon
landing, they received the first confirmation of their ideas.

Cavagna and his team made their prediction by comparing walking to the
swinging of a pendulum. When a pendulum swings it converts gravitational
potential energy to kinetic energy. The gravitational potential energy is
calculated as mgh, where m is the mass, g is the
acceleration due to gravity and h is the height of the centre of mass.
The kinetic energy is 陆(mv²), where v is the velocity of the centre
of mass. In walking, just as in a pendulum, these types of energy are
continuously exchanged. Because mass figures in both quantities, a change in
mass should have no effect on a person鈥檚 walking efficiency. 鈥淚f you look at a
child and a man, you find both walk beautifully,鈥� says Cavagna.

However, the strength of the gravitational field, g, only figures in
the potential energy equation. So, Cavagna argues, a change in the field does
affect your ability to convert potential energy into kinetic energy.

Cavagna, Willems and Heglund have tested these ideas on one of the European
Space Agency鈥檚 鈥渧omit comets鈥�, planes which fly in a variety of trajectories to
enable trainee astronauts to experience conditions of increased and reduced
gravity. They found that the stronger the gravity, the greater the speed at
which a person has to walk in order to achieve the optimum pendulum transfer. On
Earth, the ideal walking speed is 5 kilometres per hour. On a planet with
gravity one and a half times that on Earth, the ideal would increase to 7
kilometres an hour.

What about lower gravity? On Mars, where gravity is 40 per cent that of the
Earth, Cavagna says, astronauts will have to walk very slowly. On the Moon,
gravity is so low that it would be impossible to walk slowly enough to effect a
sensible pendulum transfer. Which is why moonwalkers are reduced to
bouncing.