Many psychologists believe that extrinsic motivation

Many psychologists believe that extrinsic motivation is driven by an external rewarding factor such as monetary wealth, greatness, grades, and acknowledgment. This kind of motivation originates from outside of the person as opposed to intrinsic motivation which arises from within the person.

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)