TOKYO (Reuters Life!) - A snazzy sneaker with asymmetric soles is sweeping Japan, casting a spell on millions of youngsters with the catchy boast of helping them run faster.
The "Syunsoku" brand of sneakers has sold over 10 million pairs since being launched in 2003, with sales more than doubling every year.
Total sales almost equal the number of Sony PlayStation Portable units sold in Japan, suggesting that one in every two elementary school kids owns a pair, says manufacturer Achilles Corp.
"After I begun wearing them, I was picked to be a relay runner," says four-year-old Kohei, showing off his pair of red-lined silver Syunsokus, which mean "Nimble Feet" in English.
Kohei's friend, Kei Yamashita, owns two pairs -- one in green and the other in blue.
Behind the Syunsoku boom is cutting-edge technology specially developed to help children run at top speed without losing balance -- especially around athletic tracks.
The shoes have asymmetrical soles with more rubber spikes on the right. The design gives runners better stability and grip against centripetal force when running anticlockwise around a track, according to Achilles.
Other features include elastic laces to help with knotting.
Tokyo-based Achilles says it sold 4.5 million pairs of the shoes in the business year ended in March, and sales this year seem even stronger.
"It's mostly word-of-mouth that's fueling the sales," says Yutaka Tsubata, a senior manager of Achilles' product planning development.
"Our product concept was to help fast runners run faster, and to give dreams to those who cannot. It is every kids' dream to be a hero on sports day."
The company plans to start making an adult line next year.
Toddler Kohei's mother, Yukie Ota, says she was not convinced the sneakers actually made her son a faster runner.
"But they make him believe that he's faster than ever... and I guess that's good enough," she said.
Reporting by Mariko Katsumura; Editing by David Fox