I know I am late so if for the future people the answer is...
The second principle of training is "Intensity." Intensity is basically the level of difficulty an exercise is. The balance one needs to find during a workout is important to burn energy and prevent any injury. One way an individual can find a good balance is by determining their heart rate. It is important to stay inside that zone.
I got it right on edge ( A 100%)
Have a good day :)