Everybody is different. Just as people look different in the street, internal organs, muscles and bone are also different. During exercise testing for research we have seen an average 25 year old male with normal maximal oxygen consumption of 45 ml/kg/min. So what? The following week another untrained young male came in and had a max uptake of 70 ml/kg/min - Olympic endurance athlete levels. In the same way, the composition of muscle is different with varying amounts of slow and fast twitch fibres (the genetic markers of natural endurance or sprint performance). Just as we are made differently we also train and de-train differently. Some people may benefit from the training effect faster and gain more with less training. Other people have to train hard constantly for little or no gain. Consequently, some people can not exercise for weeks and still retain their fitness levels; others lose them after only a few days rest.