What are the determining factors that bring changes in population?

You can literally find this on google of each science career and search up what type of training you need and how many years in school

The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)