Explosive fast twitched fighters gassing does not mean they do not have heart: Rumble, Tyson, Joshua, even Aldo all are fast twitched explosive fighters, but that muscle requires oxygen, and when you don't have enough oxygen, physics takes over and not all the heart in the world will make you go from sluggish to fresh. Is normal. It is like building a video game character and when you build up power other areas will suffer. There is a reason running backs and wide receivers have different bodies and skill sets, just as quarter horses and thoroughbreds - yet, we do not call quarter horses heart less because they have more muscle, can travel higher top speeds but can not carry them longer.