I always considered martial powers as supernatural. Not magic per se, lets say more like psionics: the power from your soul/willpower channeled trough your body as pure energy to enhance your martial performance to another level. People who played Pillars of Eternity would probably understand what I'm trying to describe. The things that differentiate adventurers from normal guard, mages, priest is not fate or talent, its Willpower with a big W. That Willpower can be used in different ways: the intense focus of wizard to learn the rules of the universe, the rage-induced resilience to bump axes with your bare chest as a barbarian, the draining power to awaken the magic in your genes and yes...even the ability to strike 4 hits while the other opponent can hope for 2 and push beyond normal physical limitations of human body to disarm 4 enemies in 6 second should you wish to do so.