By Kate Kelland, Health and Science Correspondent
LONDON (Reuters) - Despite the cold and many other potential hazards, naked from the ankle down is the way Anna Toombs likes it, and she gets plenty of catcalls in the street as a result.
The 35-year-old co-founder of the personal training company Barefoot Running UK says she's lost count of the times people yell "where are your shoes?" as she and partner David Robinson negotiate London's parks and pavements to indulge their passion and train their clients.
"People give you a lot of weird looks," says Robinson.
They are also getting a lot of inquiries.
A surge of interest in "natural," or barefoot, training has seen runners around the world kick off their arch-supporting, motion-controlling, heel-cushioning shoes and try to feel the ground beneath their feet.
Top scientists -- from sports physicians to podiatrists to evolutionary biologists -- are jumping in too.
At a recent sports science conference in London, hundreds of participants, many of them shod but a few daringly barefooted, flocked to a two-hour long discussion about the merits or otherwise of running without shoes.
"It's a really polarized debate -- there are what you might call the barefoot evangelicals on one side and the aggressive anti-barefoots on the other," says Ross Tucker, an expert in exercise physiology at South Africa's University of Cape Town and a middle- and long-distance running coach.
BORN TO RUN?
The current barefoot trend has its roots in the book "Born to Run," by Christopher McDougall. In it, he tells of time spent with Mexico's Tarahumara tribe who can run huge distances barefoot, often very fast, apparently without suffering the injuries that plague many keen runners in the developed world.
The debate centers on whether running in shoes with cushioned heels and supportive structures changes the way people move so dramatically that it's more likely to cause injuries.
Proponents of barefoot running say the natural way is more likely to prompt a runner to land on the padded and springy part of the foot, toward the front, rather than strike the ground with the heel as many shod runners do.
In a study published in the scientific journal Nature last year, Daniel Lieberman, an evolutionary biology professor at Harvard University, sought to find out how our ancestors, who ran and hunted for millions of years in bare feet or simple moccasins, coped with the impact of the foot hitting the ground.
Lieberman and colleagues from Britain and Kenya studied runners who had always run barefoot, those who had always worn shoes and runners who had abandoned shoes.
They found that barefoot endurance runners often land on the fore-foot before bringing down the heel, while shod runners mostly rear-foot strike, prompted by the raised and cushioned heels of modern running shoes.
In a series of analyses, they found that even on hard surfaces, barefoot runners who fore-foot strike generate smaller "collision forces" -- less impact -- than rear-foot strikers in shoes. Barefoot runners also had a springier step and used their calf and foot muscles more efficiently.
Lieberman, who spoke at the conference after an early-morning barefoot run along the banks of London's Thames, is keen to stress that the scientific evidence on whether barefoot running is better in terms of injuries is still very unclear.
"A lot of people are arguing on the basis of passion, anecdote, emotion or financial gain -- but what's quite true is there are no good data saying whether it's better for you or worse for you," he said.
Having said that, he has already voted with his feet.
As has fellow biology professor Daniel Howell, who teaches human anatomy and physiology at Liberty University in the United States.
Howell, dubbed the "Barefoot Professor" by his students after he began living his life 95 percent shoe-free, admits he's an extremist.
He's spent almost all of the past six years in bare feet, he's run thousands of miles in all weathers and across many terrains without footwear, and he refers to shoes rather suspiciously as "devices."
"Barefoot is the natural condition. It's the most natural way to be," he told the conference. "Walking and running are extremely complicated from a biomechanical perspective ... and if you add a device to your foot, it alters it."
"When you put on a device, it changes the way you stand, the way you walk and the way you run. Those changes are unnatural, and generally negative."
SHOES, OR DEVICES?
While it's true that almost all modern athletes use running shoes in international sporting competitions, a few barefooters have been trailblazers for the cause.
Back in 1960 Ethiopia's Abebe Bikila, one of the world's greatest Olympic marathon runners, won the first of his consecutive gold medals without shoes, covering the 26.2 miles in 2 hours, 15 minutes and 17 seconds. And in 1984, South African barefoot runner Zola Budd set a track world record when she ran 5,000 meters in 15 minutes and 1.83 seconds.
Simon Bartold, a sports podiatrist and international research consultant for the sports brand Asics, says most athletes, amateur or otherwise, should stick to wearing shoes.
"I'd come down pretty heavily in favor of footwear," he said. "It does offer some real protection and some real performance advantages over barefoot."
Still, Asics and other big running shoe brands like Nike, New Balance and Saucony see no reason to be excluded from this new and potentially lucrative form of the sport just because it's about running in bare feet.
A nifty rebranding of the trend to "natural" or "minimalist" running has opened up a potential new market in "barefoot running shoes" that promise to be the closest thing to wearing nothing at all.
For Howell, even minimalist shoes are a step too far. "For most people, under most circumstances, most of the time, barefoot is the healthiest and most natural way to be," he said.
Toombs, whose clients often come to her with injuries or illnesses that are restricting their movement, is concerned that scientific rows about the biomechanics of foot strikes, and efforts by sports brands to cash in, are robbing barefoot running of its best bits.
Formerly an enthusiastic shod runner, she says training without shoes is partly about getting back to nature, but it's also about learning a better way to run, using the body's bounce and balance to improve form and reduce impact.
"With barefoot running ... each time my foot strikes the ground, it lands slightly differently," she told Reuters. "In other words it's adjusting to what's underneath it."
"I'm constantly scanning the terrain, dodging rougher areas and taking a much more meandering line, which works different sets of muscles. It's almost like dancing. But the moment I put shoes on, most of that sensitivity is gone."
(Editing by Sonya Hepinstall)