Named for the woman-specific double X chromosome DNA sequence, the Run XX Nitro WNS running shoes stand 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 Nitro midsole is the perfect balance of durability and lightweight. The run guide promotes proper alignment and increased stability for longer distances.