Named For The Woman-Specific Double X Chromosome Dna Sequence, The Run Xx Nitro Stands Behind Puma'S Run For Her Motto With A Progressive, Female-Specific Design. Featuring An Updated Fit That Hugs A Woman'S Foot At The Heel, Instep, And Arch, The Running Shoe'S New, Firmer Durometer Nitro Foam Midsole Is The Perfect Balance Of Durable And Lightweight. Runguide Promotes Proper Alignment And Increased Stability For Longer Distances.