[Christian]

Hi Scott,

thanks again for sharing your expert knowledge! The Book Club idea is awesome and I greatly enjoyed watching your first video on Youtube! I am based in Munich and unable to attend in person due to the time difference. But maybe I can contribute to the Book Club session about chapter 2 with a question.

My mental model used to be: Fast twitch fibres produce pyruvate and slow twitch fibres slurp it up. It got challenged by the information that there is a continuum of fibre types. I guess the intermediate fibres can do both, produce and consume pyruvate? Actually, where does pyruvate production happen? Is it another organelle or does it happen in the cytoplasm of the cell?

Maybe I should change my mental model in the following way:
In a muscle cell some organelles produce pyruvate and the mitochondria slurp it up. It would mean drawing the vaccum cleaner metaphor on page 50 with the pyruvate producing organelle at the bottom and the mitochondria at the top.

The fact that the distribution of mitochondria accross muscle fibres is skewed would just be a detail (nevertheless important for the order of muscle fibre recruitment). The goal of metabolic endurance training would be to increase the quantity of mitocondria and their ability to recycle ATP aerobically in ALL muscle fibres. And the threshold would be defined by the net balance of pyruvate production vs consumption in ALL of the fibres that are currently recruited.

Am I on the right track here, or do I get something wrong?

Have a nice session today! I am already looking forward to watching it. 🙂

Best regards
Christian

[Scott Johnston]

Christian:

How prescient of you!  I addressed just this question in last night’s book group lecture. I think your mental model is good.  As mathematician George Box said: All models are wrong (meaning that they are not perfect representations of reality), but some are useful.

Glycolysis and its resultant pyruvate production occur in the cell’s cytosol outside the mitochondria.  A process of converting it to acetyl-coA allows it to pass through the mitochondrial membrane and get used in the Kreb’s cycle for aerobic metabolism.

Glycolysis can and will take place in the full spectrum of muscle fiber types.  The faster the FT fibers the more they will rely upon glycolysis and the less mitochondrial content they have.  Thus they are less endurance endowed.

Yes, one of the main goals of endurance training is to increase the mitochondrial content of faster and faster twitch fibers. Improving their endurance characteristics can provide propelling force for longer before fatiguing.  As you point out.  This moves the AnT upward in terms of power output.  You can run faster for longer (i.e., more endurance).

The vacuum analogy is a perfect example of George Box’s little ditty. The vacuum is comprised of: mitochondria in those same muscle cells where pyruvate is being produced, more remote mitochondria, the liver, and the heart.

While the fibers do apparently exist along a continuum thing of the slower twitch fibers acting as the vacuum for the next level up the FT scale.  This is what I talked about in the book group’s second physiology chapter. I think it will help you.  But you essentially have it, at least at the macro level we need to understand it.

Scott

[Christian]

Hello Scott,

thank you for your reply and your latest lecture! It helped a lot indeed, in particular your thought experiment where we increase speed step by step and imagine what takes place in our body. I imagined an armada of vacuum cleaners, one in each muscle cell, where the cells with bigger vacuum cleaners help out the cells with smaller vacuum cleaners.

I don’t know if you know the old series “Once Upon a time… Life”. It would be cool to have an episode about endurance – of course with some vacuum cleaners involved. 🙂

[Author]

Posts