What does the body use for energy?

[BrokenBricks]

The concern about exceding the "taregt" heart rate is limited to cardio, i belive. When doing cardio, once your body resorts to using fat for energy
(after blood sugar, muscle glycogen and others) there is a maximum rate that your body can access that energy. Your heart rate is a good indicator of the body's current energy demand, and if your heart rate indicates a level of enegry demand that outpaces the rate of fat access then you have a situation where muscle is broken down and metabolised.

This is different than when lifting weights. Your muslces should be adequatly loaded with glycogen form your preworkout meals and shake to supply energy and a very fast pace. It is a much simpler task to burn muscle glycogen than fat and so your body can produce a given amount of calories quicker. This prevents muscle canabalism.

But if you make a habit of working would while in a state of shity nutrition (like the state you SHOULD do cardio in) you would be smart to keep your average heart rate below to cardio target. You could if you wanted to, make a cardio workout out of lifting weights.

[Farmer]

I know, it was my pitch at humor.

Peace