Peter Attia

There's cardiac output, so that's heart rate and stroke volume.

Then there's the oxygen carrying capacity of the blood, namely hemoglobin.

And then there's the muscle's ability to extract this.

But as I said a moment ago, it's the cardiac output that is the main driver here, and it is the one we are most sensitive to in reduction.

So again, what drives cardiac output?

primarily is stroke volume, how much blood comes out of the heart with each pump, and heart rate.

And of course, when you're at a VO2 max effort, you're getting to maximum heart rate.

So somewhere between 70% and 85% of the variability in VO2 max is accounted for just by this one variable.

In the show notes, we'll include a whole bunch more detail on this if anybody kind of wants to nerd out on this stuff.

I love this stuff, but I don't want to spend any more time on it right here.

So as I kind of alluded to, it's very tempting to, and I want to apologize if I've ever created the impression or oversimplified this, and it's possible that I have.

Zone two is what you do exclusively to build your base and high intensity workouts is the only thing you do to build your peak.

As I said, these systems work together.

And if all you did was zone two, you would absolutely get a wider base.

You would also raise your peak.

Similarly, if you did higher intensity training, you would increase your peak, but you would also widen your base a little bit.

The key, as we'll get into in the nuance, is what is the optimized way to utilize time around different volume and intensity requirements?

So how much total work can you do?

How much cardiorespiratory fitness training can you do?

That's probably the single biggest determinant.