Nike Free introduced Nike’s barefoot-inspired running concept in 2005.
Nike developed the shoe after observing athletes training barefoot on grass at Stanford University. The design aimed to let the foot move more naturally than in a conventional structured running shoe, using deep flex grooves and a flexible sole.
The number in models such as Nike Free 5.0 originally referred to a scale describing how close the shoe was intended to feel to barefoot movement. Nike later changed its naming system, so the number should not be treated as a precise measurement of cushioning or flexibility.
Nike Free is not the same product as Nike Cortez or Nike Pegasus. Cortez is an older classic running design, while Pegasus is a long-running cushioned training series. Free was specifically associated with flexibility and natural-feeling movement.