Answer:
Variety: Bob used to run for a cardiovascular workout. After sustaining shin splints...
Balance: Tina used to think that doing only cardiovascular exercise was the ideal way to keep fit. Now she realizes...
Moderation: In high school, Charlie used to run until his knees would almost give out. Now that he is older, he values...
Step-by-step explanation:
In Bob's case, we can see that he practices exercises using the principle of variety. This is possible to notice, as he does different exercises, preventing a single part of his body to be overloaded, or his body to get used to the exercise and stop showing results.
In Tina's case, we can see the principle of balance, as she balances cardio exercises with strength and resistance exercises.
In Charlie's case, we can see the principle of moderation, as he realized that it's not okay to run until his knees buckle and began to use a more regular and no-nonsense running schedule.