X ULTRA 4. Agile on the way up, stable & grippy on the way down. Drawing on our trail-running expertise, this shoe is lighter and more agile, so you stay nimble on uneven terrain. The new Advanced Chassis targets articulations on the outer foot to optimize stability without limiting your mobility. The All Terrain Contagrip outsole grabs the terrain, even when it’s wet or slippery, giving you more control especially on descents.