Ahead of Sunday's New York City Marathon, The New York Times reports that a growing contingent of runners is choosing to tackle the pavement without shoes. Running barefoot is not new — the Ancient Greeks' access to Nikes being somewhat limited — but the counter-intuitive practice has been creeping up in popularity, says The Times, as more runners become convinced that meticulously designed running shoes do more harm than good. (Watch an expert praise barefoot running.) Here's a quick guide to this seemingly painful trend:

Why run barefoot?
Advocates believe that running shoes "keep feet so restricted that they may actually be making your feet lazy, weak and more prone to injury," says Tara Parker-Pope in The Wall Street Journal. The argument is that humans evolved to run without shoes and that expensive sneakers contribute to ailments — including stress fractures and shin splints — that often plague long-distance runners.

When did this craze start?
A few elite runners were already ditching their shoes decades ago: Ethiopian Abebe Bikila won a 1960 marathon in Rome in bare feet. But the trend has really picked up in the last five years, partly due to Christopher McDougall's 2009 bestseller, Born to Run, about the Tarahumara Indians, a "Mexican tribe of ultrarunners who race from 50 to 200 miles straight without shoes," yet consistently avoid common running injuries. McDougall's book led to the creation of The Barefoot Runners Society, currently 1,345 members strong.

Has the trend inspired new products?
Yes. While true devotees shun footwear, companies are offering "lightweight, thin-soled shoes designed to mimic the feel of running barefoot" for those who want to hedge their bets. Time magazine describes the popular Vibram Five Fingers — which weigh just 5.7 ounces and have been seen "on the feet of everyone from Matthew McConaughey to former NFL star Eddie George" — as "thin rubber foot gloves."

How have running-shoe companies reacted?
The $25-billion industry is "racing backward to catch up." "In response to barefoot fever and the success of the FiveFingers," says The Huffington Post, Nike, New Balance, Saucony, and other prominent brands have all released their own versions. Still, consumers still spend $5 billion on traditional, padded sneakers; "most runners believe that cushioning prevents injuries, and that perception isn't likely to change quickly."

What has the medical community said?
Plenty. Medical professionals quoted in The Times claim "there is no scientific research that shows barefoot running reduces injuries." Other doctors believe that while shoeless running isn't inherently dangerous, "bare feet aren't meant to support running on modern day hard-top surfaces." For all of running shoes' flaws, they "excel at diminishing the force of impact on hard ground."  

Sources: The New York Times, The Wall Street Journal, Popular Mechanics, NPR, Runner's World, Huffington Post