Nikita is an aspiring model. in order to fit into the standards of the fashion industry, she often starves herself. even though she has a slim build, she never thinks she is thin enough. nikita is mostly likely to be suffering from:

She’s suffering from Anorexia (eating disorder).

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)