Jim Paisner's 3-year-old grandson calls it running with "naked feet."
When the 62-year-old Massachusetts man heads outside for his five-mile run, even in the dead of a New England winter, he might be all bundled up against the cold -- but usually not all the way down.
"[Unless] I worry that I'll freeze my tootsies off ... I just run barefoot," he said.
Paisner, who runs a wholesale medical supplies company in Concord, Mass., said he's been a runner for most of his adult life, but has been dogged by injuries for the past few years. Even when he was younger, he said there was a "perfectly linear relationship between how much I ran and how much my knees hurt."
Frustrated, he searched for something that could keep the pain at bay. When he happened upon a Web site on barefoot running, he decided to try it out.
He said he started last April, just two ouch-filled driveway-lengths at a time. It took him a month to run one mile. Three months to reach five.
But now, he said, he's running like he never has.
Modern Running Shoe Invented in 1970s
"My knees feel great. My ankles feel great. I finish a run and my muscles can be tired, my legs [can be] tired, but my knees don't feel weird at all," he said. "For me, this, so far, has been amazing. ... There's just a pleasure that's come back to running."
Recent research indicates that there might be a scientific reason why.
Just this week, the journal Nature published a study by a Harvard evolutionary biologist showing that the way humans evolved to run is usually not the way they run now.
"We started running millions of years ago. The modern shoe was only invented in the 1970s," said Daniel Lieberman, a professor of human evolutionary biology at Harvard and the lead author of the study. "The motivation for the study was we wanted to know how humans ran before we got the big [running] shoes."
Lieberman and his partners studied several different groups of runners from the United States and Kenya, and found that barefoot runners tended to land toward the front or middle of the foot, but runners in modern cushioned shoes tended to land on the heel of the foot.
Study: Barefoot Running May Protect feet, Lower Limbs From Impact-Related Injuries
Where the foot first strikes the ground makes a difference, he said.
When you land on your heel, Lieberman said, a large proportion of your body weight comes to a dead stop and the impact is like someone hitting the bottom of your foot with a hammer 1.5 to three times your body weight.
But when you land toward the front of your foot, much less mass is involved in the collision, he added, reducing the impact and shock.
That kind of gait was probably more common when humans ran barefoot or in minimal shoes, the study said, and "may protect the feet and lower limbs from some of the impact-related injuries now experienced by a high percentage of runners."
While Lieberman said that part of the research was funded by a company that makes minimalist shoes (which encourage a barefoot-like running experience), it didn't have a role in the process or outcome.
Not About Shoe or Shoeless But About Strike Style
He also emphasized that his research does not include data on injury or show that barefoot running is better for you, only that "it's a reasonable hypothesis that needs to be tested."
Shoed runners also can run toward the front or middle of the foot, and he said that there are trade-offs that accompany all styles of running.
While front-strike running might be collision-free and require less energy to run the same distances, it requires more calf and foot strength and more stretching of the Achilles tendon.
Lieberman personally enjoys running in minimalist shoes that make for a barefoot-like experience but said he isn't trying to overthrow the shoed majority.
"This is not about barefoot versus non-barefoot. It's about strike style," he said, adding that whichever style people choose, they need to be careful.
"About a third of runners get injured," he said. "What we need to do is try to figure out how different styles of running cause people to be injured."
Still, for those runners who choose to go shoe-free (or come as close to the experience as possible), Lieberman's study indicated that barefoot running is a trend with growing support.
"Its got a lot of gravitas. ... It is a radical paradigm shift," said "Barefoot" Ted McDonald, a long-distance barefoot runner and coach whose experience is featured in the best-selling book by Christopher McDougall, "Born to Run."
Barefoot Runner: Movement Is Reaching Tipping Point
In addition to coaching and speaking about barefoot running, McDonald maintains a Google group called "Minimalist Runners" that includes more than 1,000 members.
About five years ago, he estimated tens of thousands of people would count themselves among barefoot or minimalist runners. Now, he said that number is in the hundreds of thousands.
"I think [barefoot running] has been eccentric in our generation from the get-go. ... You're like a smoker, you're willing to take a huge risk," he said. But now, "we're reaching the tipping point. ... It's a trend that won't be going away anytime soon."
Companies such as Vibram, Terra Plana and Feelmax have started to cater to the minimalist crowd with thin-soled shoes that are conducive to a front- or mid-foot strike gait. The Nike Free also offers a similar experience.
'Chief Runner' Issues Warning About Barefoot Running
In December, a study by the American Academy of Physical Medicine and Rehabilitation suggested that running shoes could increase pressure on joints compared to barefoot running.
"Once you take off your shoe, you no longer pound the ground," said McDonald. "My way of approaching it is getting people to run more like dancers move across the stage -- with form and grace and strength."
Though he runs with a minimalist shoe on occasion, whenever he can he goes completely sans sole. Like Paisner, he said that barefoot runners' feet become tougher and stronger and adapt to handle direct contact with the open road. Both say they've never experienced any major cuts or bruises from running barefoot.
But the barefoot movement is not without its detractors.
In an e-mail to about 3 million subscribers, Michael Gotfredson, founder and "chief runner" of Road Runner Sports, a retailer of running shoes based in San Diego, Calif. that says it's the "world's largest running store," issued a "warning about barefoot running."
"Don't follow blindly the latest trend. The barefoot running thing is an injury waiting to happen," he wrote. "Ever walked on the beach or a sidewalk and seen shards by the dozens? Don't step on those if you plan on running in your future."
Sports Doctor: More Data on Barefoot Running Is Needed
Gotfredson told ABCNews.com that he sent the warning because of "the nonsense in the papers I've been reading about barefoot running."
"It's a fad," he said. "You'd have to have such pristine conditions to make it safe."
Still, he said he doesn't take issue with the biomechanics of barefoot running.
"If you don't need support, if all you need is protection, that's fine," he said, but he added that the reality his company sees time and again is that people need support.
He said that whatever works for individuals is the appropriate way to run, and though people can change their style of running it's a "major project."
Dr. Pierre D'Hemecourt, the medical director for the Boston Athletic Association, which organizes the Boston Marathon, said he was "cautiously interested" in barefoot running.
"I think it's very interesting. I'm very pro looking at it. I think it makes some very interesting biomechanical statements," he said of the recent study in Nature. But "a lot of people running in the regular shoes are doing quite well with that."
He said that runners could ease their way into the barefoot or minimalist approach to running with shoes that have smaller heels and arches. But he emphasized that more studies are really needed to look at injury rates associated with different styles of running.
"I think I would chalk it up as a very interesting study," he said. "I think that if people were interested in testing the waters I would take very small doses at a time."