For years, running shoes have been designed to stabilize, cushion and otherwise pamper the foot. Now, Nike Inc. is trying to sell runners on a new shoe that mimics running barefoot on grass — a concept that runs counter to the past three decades of shoe technology and is already prompting widespread debate among runners.

The $85 shoe, dubbed Nike Free 5.0, has been in limited distribution since last year, selling mostly in specialty running shops. The Free 5.0 and four other Free shoe styles now are moving into department stores and mainstream shoe and sporting-goods chains, backed by a big Nike marketing campaign. The philosophy behind the Free is that running barefoot strengthens tiny muscles in the foot that typically atrophy inside more protective running shoes. The increased effort acts much the way a weight workout does for the biceps, say Nike designers, ultimately making feet stronger, faster and less prone to injury. Nike is marketing the shoe as a "training tool."

Getting runners to adopt such a counterintuitive shoe is challenging, however. Nike includes with the shoe a training guide that includes drills and programs for the Free. It suggests wearing the shoes "around the house" for the first two weeks. Runners are then advised to start 3-to-5-mile runs in the third or fourth week. Nike recently advised on its Web site, "You can't just put these on and run."