My son keeps on growing overnight a shoe size - these have been barely worn before we had to get the next size up. Get ready to power through uphills and descents with the X ULTRA 4. It is as agile as a trail-running shoe but with all the stability and grip you need to tackle technical terrain. Equipped with a new chassis, the shoe is even better at protecting sensitive articulations without compromising freedom of movement. Height Low Foot protection High Foot support Stable