Evolutionary biologist Daniel E. Lieberman caused an international stir nearly a decade ago when he published a paper showing that running in cushioned sneakers encourages people to hit the ground harder than running barefoot.
Lieberman, a professor of biological sciences at Harvard University,
also started running barefoot himself as an experiment and kept doing it
because he enjoyed it. Every spring, after running the Boston Marathon,
he would trade his traditional sneakers for a pair of minimal shoes or
no footwear at all. The more he ran barefoot, the more callused and
protected his feet became. “But I could still feel the ground just as
well as when my calluses were really thin,” Lieberman says. From an
evolutionary standpoint, it made sense that callused feet would still
feel: they are the body’s only contact with the ground, and ancient
people could not afford to lose that sensation, he thought.
Now Lieberman and his colleagues at Harvard and in Germany and Kenya have conducted another study, published Wednesday in Nature,
that confirms his suspicions. It finds that although calluses thicken
as people walk barefoot more often, there is no trade-off in sensation
from that extra protection. Essentially, the hard surface of the callus
transmits mechanical force through the foot to the nerves deep inside
the skin equally well as an unprotected sole. Advertisement
Calluses are made out of the protein keratin, the same material as
fingernails, glued together with another special protein. “There’s no
viscosity to calluses, so forces from the ground go right into deeper
layers of skin, and you don’t lose any information,” Lieberman says.
Lieberman and his colleagues measured the sensitivity of the sole to
mechanical stimuli, showing that people with thick calluses were as
sensitive to vibrations as those with thin or no calluses. The
researchers compared calluses and foot sensitivity among 81 people from
western Kenya, some of whom regularly went without footwear and some of
whom did not. They also collected similar data from 22 people in Boston.
With cushioned shoes, the stiffness of the sole slows the rate at
which the body hits the ground, making the impact more comfortable, but
the force is the same, Lieberman says. “The energy that gets shot up
your leg is about three times bigger in a cushioned shoe than if you’re
barefoot,” he says, adding that “we have no idea what that means” for
joint health. It is theoretically possible, he says, that this extra
impact is behind the doubling of rates of arthritis of the knee since
World War II—about the time that technological advances in footwear
design allowed for more cushioned soles. But there is no solid evidence
to support such a connection.
In some ways, walking barefoot is better for the body than wearing
deeply cushioned soles, Lieberman says. But he insists he is not
antifootwear: “I’m not saying people shouldn’t wear shoes.” Rather he
thinks that scientists do not yet understand the impact of footwear on
the body over the course of millions of steps. Lieberman says it would
be challenging to study the effects of wearing shoes for millions upon
millions of steps over the course of 70-plus years in humans, but he is
currently exploring the impact of such cushioning on animal locomotion.
Balance might also be a casualty of soft soles. People’s feet become
less sensitive as they age. If they have also lost touch with the
ground, they might become more vulnerable to falls, Lieberman explains.
“If your feet can’t sense what’s going on on the ground, maybe you’re
more susceptible and more vulnerable [to falls], and shoes may be a part
of that,” he says. “If we can give people’s brains, their reflexes,
more information, that might help them.” Advertisement
Gymnasts and martial artists go barefoot to increase their connection
with the ground, and Formula One race car drivers wear hard-soled shoes
that actually boost their sensitivity, according to one study.
With today’s cushioned shoes, “we add comfort, but we reduce
functionality,” says Thorsten Sterzing, a footwear scientist who designs
high-performance shoes. He was not involved in the new research but
hopes to build on it in his own work. Too often, people opt for footwear
that fits society’s idea of beauty, yet that does not promote healthy
walking, he says. Studies like Lieberman’s can lead to better-designed
shoes that complement the body’s natural abilities rather than undermine
them.
Kristiaan D’Août, a senior lecturer in musculoskeletal biology at the
University of Liverpool in England, says the foot is one of the least
understood structures in the body because of individual variation, the
complexity of foot bones and ligaments, and because so much of what
happens inside the foot is impossible to see. D’Août was not involved in
the recent paper, but he conducts related work and wrote a commentary
about the study that appears in the same issue of Nature. In
one of his research studies, D’Août had participants wear minimal shoes
for six months. Although they were uncomfortable at first, “quite a few
people prefer them now,” he says. “One of the things that I would really
hope would come out of this research and footwear research in general
is that people will start to realize that shoes can be quite invasive.”
(D’Août admits he usually wears regular shoes himself because of the
wet, gloomy weather in Liverpool.)
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People have probably been wearing shoes for about 40,000 years,
Lieberman says, although some suspect that Neandertals remained
shoeless. In some parts of the world, including India and Kenya, where
Lieberman conducts research, many people still go their whole lives
without wearing shoes. “I find it unimaginable to be barefoot in middle
of Europe in the Ice Age, but then again, all the other animals in
Europe during the Ice Age were barefoot—so maybe our cousins, the
Neandertals, were able to handle it just fine,” Lieberman says.
Still, he says, he has no plans to test going barefoot himself during a New England winter. Advertisement
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