Jose and sarah are running in the cross country meet. their skeletal and muscular systems work to move them through the course. as they run, they breathe faster and harder and their heart rate increases. this illustrates that body systems

There muscles will keep moving and keep tiring till when they rest it increases there level

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)