Peter Attia

So another way to think about this is through the lens of the fibers that are involved.

And again, these are terms we've used on the podcast before, but the goal of this podcast is to kind of tie this all together.

So at lower intensities, you have these type one or slow twitch muscle fibers.

And again, I think the term slow twitch, it does to some extent reflect the speed with which they twitch.

But I think a more important way to think about them is they're slow to fatigue and they're more endurance based fibers.

So again, at lower intensities, they're the ones that are doing all the work.

very rich in mitochondria, deep red.

They excel at oxidizing fat and they're very, very efficient.

As the intensity increases, we have to start recruiting more of the type two fibers.

These are fast twitch fibers, which again are more contractile in their force, but they are also fast to fatigue.

They have less mitochondria and they're going to recruit and rely more heavily on glycolysis.

that's happening outside the mitochondria.

So initially, lactate, which again, kind of gets a bit of a bad rap, but again, we've done an entire podcast on this and we'll link to the podcasts on this topic.

But the most important of these is definitely the one with George Brooks.

Initially, the lactate gets recycled locally.

So it's shuttled to neighboring type 1 fibers.

It gets generated in a type 2 fiber, gets shuttled to a type 1 fiber, gets converted back into pyruvate,

And then the pyruvate goes into the mitochondria to produce more ATP.

That's called the lactate shuttle.

But again, these things are constrained by demand.