The Mach 19 features our updated spike last which should offer a true to size fit, meaning you should be not have to adjust your sizing from models like the Ghost/Adrenaline GTS when sizing into the Mach 19. The new collar lining, tongue, and midfoot saddle use a suede-like material to help keep the fit secure through your run. Updated design to improve spike durability. We've updated the heel material to help soften the heel and also give the heel better grip.