A faculty member at a college where I worked (I think he was semi-retired/part time) was perplexed because he couldn't regularly get the surge of physical power that sometimes occurred when he jogged to the point of near collapse. He couldn't understand why something so good couldn't be more readily available. I told him that he was using up his life-force reserves i.e. when a flood of adrenal hormones had to kick in due the extreme demands using up his available energy etc., and I reminded him about the scientific investigations that had been done on animals faced with unrelenting heavy stress and their pattern sustained resistance followed by terminal collapse when they had no more to give. I think he realized the need to moderate his exercise regimen, but his biological gas tank did run out permanently within a year or so. Thus it's a judgement call as to what amount of exertion develops higher capacity without hastening the depletion of irreplaceable reserve.