The numbering system indicates the cushioning of the shoe and follows a scale ranging from 0 (barefoot) to 10 (normal running shoe), i.e. Free 3.0 being the least and Free 7.0(discontinued) being the most cushioned model so far. Runners are advised to gradually break in the shoe rather than immediately running long distances as to prevent muscle cramps and other discomforts, such as the notorious shin splints.
Nike started the Free series with the Free 5.0 in 2004 and in 2006 released a new version of the shoe, the Nike Free 5.0 V2, that addressed some of the durability issues in the earlier model. Among other changes, the heel and sidewalls of the shoe were redesigned to prevent tearing.
Since then, the Nike Free 5.0 V3 (2007), Nike Free 5.0 V4 (2009), and Nike Free 5.0 V5 (2013) models have been released along with various other models from the Nike Free 3.0 V1, V2, V3, V4, and V5 to the Nike Free 7.0 V1 and V2.
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