The Asics Fluid Axis is a design feature in the Asics running shoes that is design to allow the shoes to flex along the lines of the subtalar joint axis. Curated Resources: http://www.podiatry-arena.com/podiatry-forum/showthread.php?t=85327 (Podiatry Arena) http://podiapaedia.org/wiki/sports-medicine/individual-sports/running/running-shoes/asics/asics-running-shoe-technology/asics-fluid-axis/ (PodiaPaedia) http://www.podiatrystudent.net/fluidaxis-from-asics-2/22666/ (Podiatry Students)