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.
"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.
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.
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."