[SeaHodg]

(I was going to post this as a comment on the article. But, given its length, I figured this forum may be a more productive venue for it.)

Scott,

Thanks for writing such an interesting and engaging piece on aerobic base training. I particularly enjoyed the section on the origins of the heart “zones” related to lactate concentrations. I had no idea that’s where they came from! However, there are some omissions from the model of aerobic training in the post that I’m curious about. In particular, I’m curious why there was no mention of the creatine-phosphate (CP) system, the Cori cycle, or the other elements of the Fick equation.

I understand that the CP system may be even less relevant than glycolysis for the sports Evoke is concerned with, and so there seems little reason to focus on it. Nevertheless, it is an integral component of the anaerobic energy production and there’s some indication cycling CP work with active recovery may elicit an overall “aerobic” stimulus (I believe both Ed Coyle at UT Austin and the folks at Strongfirst have been experimenting with this in different fashions). Even if a minimal focus of training, it seems important to acknowledge.

The vacuum analogy of lactate clearance within the muscle is effective. But, at least in this discussion, it ignores the role of the liver for regenerating glucose from circulating lactate (i.e., Cori cycle) or other destinations for lactate (e.g., heart muscle). To my knowledge, cardiac muscle prefers lactate to an even greater extent than “slow-twitch” skeletal muscle. Furthermore, more recent work from George Brooks (well, if 2000 is still recent) showed that endurance training decreased both lactate appearance (i.e., in the blood) and clearance. There are a variety of reasons this may be the case (which Dr. Brooks may have revealed in subsequent work I don’t recall), but one of them is enhanced distribution of lactate between tissues, skeletal muscle and others.

Finally, the Fick equation (VO2 = Stroke Volume * Heart Rate * arterial-to-venous oxygen content different) outlines that oxygen consumption is a function of both oxygen delivery and  extraction. The current post emphasizes extraction over delivery but, to my knowledge, the primary adaptations to endurance training are delivery-related for most people (highly trained athletes may be different). So, the primary benefits to endurance training (including zone 2) are improvements in cardiac output and vascularization. While intramuscular metabolic adaptations certainly contribute improvements in endurance and aerobic capacity (i.e., VO2max), the mitochondria can’t generate ATP if the oxygen isn’t delivered.

Once again, I really enjoyed this post, generally agree with the proposed model of aerobic training, and support the perspective that important nuances of aerobic training have been lost in the current hype around “zone 2.” However, I think the model could be further strengthened by integrating or acknowledging the topics I’ve highlighted. There may be good reasons these topics were omitted and, if there are, I’m eager to discuss them.

 

 

[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.

 

• This reply was modified 10 months, 2 weeks ago by Scott Johnston.

[SeaHodg]

Scott,

Thank you for taking the time to provide such a thorough reply. Overall, I accept your point that there is a necessary reduction in detail to generate any useable model. You’re right, the Cori cycle and lactate oxidation by the heart are probably unnecessary details for the purpose of endurance training. I think that stuff is interesting. But, is it terribly useful as part of a rationale for training? Probably not. I will maintain, however, that acknowledging the creatine-phosphate system as a component of anaerobic metabolism and the capacity for aerobic oxidation of carbohydrate via glycolysis (i.e., without producing lactate) are important distinctions include. I don’t believe I mentioned the latter in my previous comment. Nevertheless, I think it could explain discrepancies between the cross-over point of %Carb vs. % Fat oxidation and VT1/Aerobic threshold (something I’ve observed in myself and seen discussed on the forum), and be useful to understanding the nature of someone’s aerobic development or lack thereof. That’s my own speculation though.

I’ll also accept that for endurance, particularly in the time domains of most mountain adventures, the cardiac adaptations may be of slightly lesser importance than the metabolic ones. In one of your book club lectures you mentioned that endurance is a metabolic trait (I’m paraphrasing). I found this position challenging at first, but I’m coming around to it. Whether in a 100m sprint or a 100 mi race, the ability to slow down less (endure) will be largely determined by the efficiency of energy production. I’d still like to acknowledge the vascular adaptations because O2 delivery will affect that efficiency and the ability to shuttle lactate. But, to your point, does that really change the objective and the nature of training? Probably not. Getting sufficient volume of lower intensity aerobic activity remains the prescription.

Anyway, thanks again for the reply and the content you and Evoke team generate. I’m relatively new to your training methods, but I’m enjoying learning (even if it triggers some disagreement). I hope at least some of the points I’ve raised were constructive and I look forward to future exchanges as additional questions arise.

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