This category began with the Adrenalin. Adrenalin was the first alpine freeride boot with the features of a ski mountaineering boot. Endorphin, Adrenalin, and Axon remain the stiffest alpine boots with these functions. The Axon has Dynafit binding fittings so you can have a setup that is 2 pounds lighter than with the most popular frame bindings. Endorphin remains a bit stiffer than Adrenalin and has sexier, Magnesium buckles.