Login | April 25, 2025
Significance of Z2 cardio training
PETE GLADDEN
Pete’s World
Published: April 21, 2025
When you think about burning calories, attaining a good level of cardiorespiratory fitness and just getting the most bang for your cardio buck, I’m willing to bet that most people immediately think of high intensity exercise.
But what if I told you that’s a small piece of the overall cardio puzzle?
Truth be told, if you’re looking to burn calories, get aerobically fit and put in some amazing high intensity cardio, you’re totally missing the boat if you’re not concentrating on zone 2 workouts.
And this dictate isn’t just confined to those athletic types, it also pertains to folks who are just trying to shed some excess pounds.
Indeed, the rule is this: The majority of your aerobic workouts should be done at the lower end of those heart rate (HR) zone charts - at a zone 2 intensity level.
So let me begin the discussion with an abbreviated definition of just what this HR zone 2 actually entails.
Zone 2 can be defined as a HR that lies between 60% and 70% of one’s maximal HR (the HR one reaches with an all out maximal effort to failure).
Thus, Z2 is at an intensity that still allows one to talk - albeit not like a Chatty Kathy.
Thus, at first glance it would appear that zone 2 is so low a HR that it wouldn’t really elicit any kind of aerobic training effect, let alone any kind of physiological effect on the body’s muscles.
It just seem too easy, right? Wrong.
The beauty of zone 2 training is that it does indeed have profound physiological effects and these effects occur way down at the cellular level.
You see, working out in zone 2 helps to increase what’s called the mitochondrial density in our skeletal muscles.
Mitochondria?
Mitochondria are these micro energy-making machines within a cell.
And most all human cells, animal cells and plant cells contain oodles of these little energy-making organelles.
So the benefit increasing one’s mitochondrial density in the muscles is because the more energy the mitochondria can produce the more one’s able to fuel his/her physical activities.
Now I’m going to dive into the weeds a bit in order to get to the much bigger picture.
So during exercise our body needs more fuel.
Seems logical, right that more activity means more energy is needed for the muscles?
Now the fuel comes from two primary sources in the body.
The first is via carbohydrates from glucose in the bloodstream and glycogen (a stored form of glucose) in the muscles and liver.
The second source is from fat that’s stored within the muscles and fat tissues.
So the interesting thing here is that the type of fuel that’s used - carbohydrate or fat - is predicated on the intensely and length of the exercise session.
Intense work leans more towards the fast-acting carbohydrate fuel source while lower intensity work leans more towards the slow acting fat fuel source.
Now when you take into consideration that our bodies contain around about a couple of thousand calories worth of the carbohydrate fuel source as opposed to tens of thousands of calories worth of the fat fuel source, you can see that our bodies are rich in fatty fuel.
And here’s where that mitochondria comes in.
When you do zone 2 training, your type 1 muscle fibers (those endurance/slow twitch fibers) are persuaded to make more mitochondria that are better at burning fat for fuel.
And ditto for higher intensity exercise where those type 2 fast twitch muscle fibers are persuaded to make more mitochondria that are better at burning carbohydrate for fuel.
Thus, becoming more proficient working at a zone 2 intensity level means a higher proficiency at burning fats as a fuel source.
And if you look at how professional cyclists and runners train, you’ll see that 60-75% of their training is in zone 2.
This is because zone 2 training causes favorable adaptations in fat metabolism, allowing them to bike and run faster while still burning fat for fuel rather than using up their preciously small carbohydrate fuel source.
Now realize that each of these mitochondrial adaptations - better at fat burning/better at carbohydrate burning - matter because taken together they provide metabolically flexibility.
So yes, high intensity levels are necessary for one’s overall cardiorespiratory fitness, but they’re ultimately built upon that super critical zone 2 foundation where fat utilization predominates…which is why zone 2 is so darned important.