[Scott Johnston]

Thanks for your critique of the Zone 2 article.

However, you must understand I am not attempting to, nor am I qualified to, present an entire course on the metabolism of exercise in one article written for lay people.  You are correct that there is much, much more to say about the biochemistry of metabolism as it relates to endurance performance. But where does one draw the line in detail? How far down the rabbit hole does one go before they lose the audience?  Is it necessary to be able to explain every step of the electron transport chain to have a general idea of the function of  mitochondria?  How important are these details to the athlete or coach?

The audience I am addressing is largely not well versed in exercise physiology (nor am I).  I am attempting to make accessible some of the more important points related to endurance training without going too deeply into the physiology.  As I have mentioned in many places, we create models for complex systems to help us grasp them.  A model I am using,  the vacuum cleaner, is of course not physiologically 100% correct but it helps people understand that lactate can be moved to other tissues where it can be used as fuel for metabolism.  In my mind that’s the important point that when I first wrote about this many, many years ago, was not even mentioned in training literature.  I had to dig deep into the science literature to figure it out.  Of course the heart has a great affinity for lactate as a fuel.  And of course the Cori (gluconeogensis) cycle in the liver is another place where lactate can be converted to a more usable form.  But are those trainable?  Not that I know of. Whereas the lactate shuttle and the aerobic capacity of  ST muscles are extremely trainable and directly related to performance.  Does the understanding of the heart muscle and the Cori cycle in the liver help improve performance?  Or does it mainly interest the physiologist?

I could  have explained that the enzyme Lactate Dehydrogenase (LDH) has two isoforms. The isoform present in Fast Twitch muscle fibers converts pyruvic acid into lactic acid while isoform of LDH in Slow Twitch muscles converts lactic acid to pyruvic acid.  But the vacuum analogy gets the same message across in a simpler fashion.  I also didn’t discuss the transport proteins MCT1 and MCT4 responsible for the lactate shuttle. The vacuum cleaner analogy encompasses all this in one simple image that most people seem to be able to grasp easily.

As for the CP omission: I make no excuse for that. I also don’t discuss protein as a fuel for a similar reason.  For events lasting many hours the contribution to the total energy supply from CP is minuscule. Yes, I understand that in intermittent high intensity exercise there may be some aerobic adaptations that will help restore CP stores more rapidly.  But aerobically generated ATP is going to supply  99.999% of the energy for long duration exercise.  It is pretty basic: I want to help people improve their ATP recycling ability because that’s what going to propel them in long duration events.

Lastly regarding oxygen delivery vs extraction:  You might want to read https://evokeendurance.com/understanding-max-vo2/

The reason I emphasize the extraction end of the oxygen transport is that this is the area where we can have the most impact with training athletes.  A conservative estimate would put 90% of the people who come to us for coaching and training advice in the “Aerobically Deficient” category. This article might interest you https://evokeendurance.com/aerobic-deficiency-syndrome-ads/ . Those with ADS have poorly functioning aerobic metabolisms in the propelling muscles. The only fix for that is a high volume of Z2 training. With the thousands of athletes with ADS we have dealt with over the years we typically see 20-30% performance gains (measured as running speed at aerobic threshold) just with Z2 training over the course of a few months. These sorts of gains by focusing on the delivery side are just not possible. In the process of curing their ADS there will of course be cardiac adaptations in response to the high volume of low intensity training that will result in a stroke volume increase.  Whereas a focus on improving stroke volume (oxygen delivery) would normally entail a low volume of high intensity training that would only exacerbate their ADS.

As you probably know, stroke volume is a first wave response to exercise that peaks relatively early in those new to training. Children being the most trainable in this regard and adults the least trainable.  It is unusual to move the VO2 needle much in adults with a significant training history.  IN some cases elites will show a decline in VO2max while increasing performance.

It is our mission here at Evoke to provide the most actionable information to the mountain endurance athlete community to help them improve their performance.  To do this does means some oversimplifications in our explanations.  But we’ve been using these methods for over 30 years with everyone for Olympians to rank beginners with excellent results.