You sure about that? I mean my absolute maximum effort would probably kill me. Maybe what we should say is, "all we can do is try pretty hard but somewhat less than our best, most of the time".
It is always hard to know what my best really is, and a personal best is not a static thing; what a person is capable can expand. So I am always trying to push myself to the edge and then keep trying to do more . . . and I'm not always sure when I'm pushing myself too hard, and when I'm not pushing myself hard enough for maximal self-improvement.