And this is because, the level of competition will matter when you are doing the mental activity, and in general when you are doing the physical activity then the level of competition never matters. And note that you are doing an activity which is always of one level in case of physical activity, and no sort of games where the competition does matter, and you need to beat the record all the time. You are in this situation challenged physically though always at one level which is fixed and known in advance, and how much exertion your body can withstand, and that is a competition in itself. However, no other competition or level is a factor in this scenario. Though in case of mental activity the requirement can vary at times, and hence requiring the varied level of competition.
