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.
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.”
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.)
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.