I still have a hard time explaining this one to people. Fitness is a muscular issue not a heart issue. YES – your heart is a muscle – but it is also an involuntary organ. Your pecs are not. And the fibers are completely different.
Training a client today I had an epiphany (of sorts) – "What if I had a Jarvik heart? Could I become fit with IT in my chest?" 
Why sure I could.
If I was out of shape (not unhealthy) and had one of these gizmo’s inside of me, as the weeks went by of my fitness program I’d become fitter and fitter.
But if cardio fitness is due to the heart becoming stronger and fitter, how is this possible as an artificial heart cannot become stronger.
Indeed.
Enter the mitochondria.
These little gadgets proliferate within the muscles as you become fitter. They are complex, energy producing organelles which are found in all eukaryotic cells in both plants and animals.
These little dudes are the site of "cellular respiration." Certain cells which requires large amounts of energy contain large numbers of mitochondria. So, the fitter and stronger you get, the more of these little gum drops you get.
And the more of these you get
(specifically the little myofibrils on the very end),
the more little gadgets you get.
The hearts got nothing to do with it.
When you drive your car long and hard, your engine doesn’t get stronger. It just wears out sooner.
Take it from Buzz Aldrin – (or was it Neil Armstrong?):
"I believe my heart has a finite number of heartbeats available to it. I’m not about to waste any of them doing exercise."
Except for strength training that is – 20 minutes, twice a week. Not many heartbeats wasted this way!
