When looking under a microscope comparing the muscle cell of an Olympic athlete and an average adult you notice there is a significant difference. What is the difference? Why does this difference exist?

More mitochondria in athlete's muscle cells.

Explanation:

There is a significant difference in the muscle cell of an Olympic athlete as compared to an average adult because Olympic athlete has more number of mitochondria present in their muscles cells as compared to muscle cell of an average adult. The main reason for this is that Athlete performs harder activities so they required more energy to do their activities so their muscles cells have large number of mitochondria to provide more energy for their activities.

the answer to you question is   jet streams