Sunday, March 9, 12025 Human Era (HE)
Preamble Ramble
This post is a continuation on energy metabolism and its relationship to stand up paddleboarding (SUP) that covers the philosophy and physiology of energy systems training. For the first instalment from this theories series, on the philosophical and physiological underpinnings of endurance training, click here. For the second instalment on catecholamines and the exercise stress response, click here. And for the third instalment on tissue types and training, click here.
- Preamble Ramble
- Organelle Du Jour: Mitochondrial Hype
- A Counterview on Zone 2?
- We Talking About Practice
- Why Do We Fatigue?
- Few Principles Lead to Many Methods
Organelle Du Jour: Mitochondrial Hype
It is hard to have a deep conversation around health today and not have mitochondria mentioned. This article, “Why Mitochondria Is The Organelle Of The Moment,” by pharmaceutical and biotechnology giant Pfizer highlights the trend. More than just the powerhouse of the cell, mitochondria are being recognized for their role in crucial cellular processes like calcium regulation, iron/heme production in red blood cells, and programmed cell death or apoptosis (another buzz word). Not to mention that mitochondria also play a crucial role in some of the key pathways that are at the centre of generalized tissue regeneration.
A search of Google Trends corroborates the tendency toward rising (re)search rates regarding mitochondria starting around 12010 HE (i.e., the low point in the graph below).
Mitochondria are the secret sauce at the heart of endurance training. Whether HIIT or moderate-intensity continuous training (MICT), which is closer to the traditional view of LSD, the resulting endurance benefits are highly related to mitochondrial modifications.
As previously mentioned, endurance training results in increased mitochondrial respiration (i.e., efficiency) and content (i.e., density). The enhanced skeletal muscle mitochondrial density is a major factor contributing to decreased carbohydrate utilization/oxidation and thus lactate production, as well as increased fat oxidation and enhanced endurance exercise performance. Despite the decreased carbohydrate utilization, the capacity for carbohydrate oxidation increases, thereby enabling maintenance of a higher power output during exercise, ultimately enhancing performance potential. The current evidence suggests that moderate intensity continuous exercise (think traditional LSD training) is more suited towards increasing mitochondrial density. Whereas high-intensity training appears to improve mitochondrial efficiency. Specifically, HIIT increases the capacity for anaerobic energy liberation, enhances tolerance of metabolic acidosis due to increased muscle buffer capacity, and improves ionic regulation, notably potassium (K+) balance. Therefore, in the context of performance, it is important that a range of intensities are covered to optimize changes across the spectrum of adaptations. It also appears to be the case that this is necessary for general health.
A Counterview on Zone 2?
However, as is often the case, sometimes the message gets muddled. If you are keeping abreast of fitness and performance trends in the endurance space, you are likely already aware of the hype around Zone 2 training (if not, take my word for it, Zone 2 is all the hype right now). Podcasts (or perhaps better stated as “icasts”, i.e., internet broadcast – let’s face it, they’re no longer going through ipods), promotional prerecords, photoelectric print posts, and other propaganda that pontificate on the pros of petty-powered physical training are popular presently. A significant source of the hype is the recent success of professional cyclist Tadej Pogačar and team trainer Iñigo San Millán, who has been aversely appointed as the poster child of Zone 2 training. San Millán has championed his use of and success with Zone 2 training (in his version, he uses a 6-Zone metabolic framework). However, the internet Zone 2 zealots have missed that Millán promotes a high proportion of Zone 2 training, not its exclusive use. The more cynical view is that the zone zealots have deliberately misrepresented Millán’s approach with a more generous take being they genuinely misunderstood his training philosophies. In all the content I have encountered of San Millán, he highlights using Zone 4, or “turbo,” as he calls it, as an essential aspect of endurance athlete development. As previously discussed, an elite endurance training program contains a lot of volume. So much so that even if it fully follows the sangraal 80/20 split, the 20 percent of higher-intensity training is enough volume to crush most mere mortals. This is also the case if the less holy and possibly more practical (as in what is actually practiced) pyramidal protocol (e.g., 60/30/10 or 65/25/10) is followed. The high proportion of Zone 2 training is a hedge toward athletes avoiding overtraining as they flirt at the margins of volume and intensity.
Zone 2 is Overrated: Clickbait or Counterview?
In any case, a counterview caveated as clickbait is this video from the Global Cycling Network titled, “Zone 2 Is Overrated Says Norwegian Super Coach.” The caveat comes from the fact that coach Olav Aleksander Bu does not dismiss the use of Zone 2. Bu’s view is Zone 2 is important, but cannot be used as a standalone panacea. Bu notes that he does not believe that San Millán’s position is one of exclusive Zone 2, so it is likely that they have more similarities than differences in their approaches. Again the recipe for elite endurance performance is a whole lot of volume which necessitates low-intensity coupled with dash of high-intensity. How much is a dash is open to the interpretation of the chef.
We Talking About Practice
Practical Insights: Zone Mentions
The split between high – and low-intensity training is more of an imposed dichotomy as the true range of metabolism is spectral. A case in point is the domain model of exercise intensity, with four domains (moderate, heavy, severe, extreme). However, as we will see, the aforementioned differences in mitochondrial adaptation arising from the scaling of exercise intensity (or domain) from low to high drive the philosophical (and probably the practical) partitions of physical activity physiological practice parameters. Or put in proper prose rather than peremptory poetry, the range exercise intensities that are commonplace in trending training programs are there to drive specific adaptations. The focus on Zone 2 and Zone 4 (more on zones and domains to come) in endurance training programs arises from the specific changes to metabolism that predominate at each workload.
For a great, simple breakdown of training zones, check out this video from the Global Cycling Network titled “Training Zones Explained.” It is important to remember that training zones are a construct developed to help coaches and athletes better communicate their workload intensities to drive the desire physiological adaptations. As highlighted in the GCN video, after you bypass the basic 3-zone model, a thin-slicing of the 5, 6, and 7-zone models reveals they are essentially the same for zones below Zone 5. A thick-slice reveals more nuance, but you will have to deep dive on your own to discover the differences. Very simply, where the models differ is the degree to which they divide the 5th zone, either keeping it as one zone in the 5-zone model, splitting it into two zones in the 6-zone model, or three in the 7-zone model. These distinctions allow athletes and coaches to fine-tune their communication and prescription of training programs.
The current focus toward Zone 2 and 4 in endurance settings seems warranted, but it is important to realize that in both performance and health, a full spectrum of exercise intensities is beneficial. Zone 2 promotes more mitochondria and facilitates the utilization and exchange of oxidative substrate throughout the body without an accumulation of waste associated with high levels of energy conversion (e.g., H+, Pi). Whereas Zone 4 promotes glycolytic metabolic efficiency. Training at or near the lactate threshold (i.e., Zone 4) recruits more fast-twitch (i.e., glycolytic fibres) upregulating glycolytic enzymes and the expression of MCT-4 transporters. These changes improve glycolytic flux and the capacity of fast twitch fibres to shuttle lactate out to be used by slow twitch fibres.
But I think it would be foolish to think that athletes and coaches are sticking strictly to these two zones only. The reality is that all the zones are important to differing degrees at different times. That is, athletes need to train across the entire intensity spectrum to drive the metabolic adaptations that best suit their needs and goals.
Variety is the Spice of Life
A full spectrum of training intensities and volumes is essential for optimal health, but also for performance. Due to the spectral nature of low to high intensity training it is impossible to span the spectrum without transitioning through all intensities (i.e., rest to maximum intensity exercise). The nuance lies in where the bulk of volume or training focus resides. Depending on personal preference, level of training, training goals, and training circumstances, the ratio of SIT to HIIT to MICT/LSD should be individually determined. As per this schematic diagram of training intensity and volume on mitochondrial respiration versus content, different physiological adaptations are optimized under different conditions, and the emphasis of training should adjust accordingly. Always bearing in mind that this is not an all or none process. It is one of focus. Thus, the question of what the desired training effect is or the optimization goal. Central to that question, it is worth taking a step back and asking, “Why do we fatigue?”
Knowing the answer to why we fatigue can better guide our approaches to avoiding or at least mitigating fatigue. Spoiler alert, is that the answer is not so simple and is very much context dependent.
Why Do We Fatigue?
There are a whole host of reasons why humans fatigue. But in order to not get off track, let us narrow the definition specifically to ‘temporary muscle fatigue.’ Muscle fatigue is “a decrease in maximal force or power production in response to contractile activity.” In the sports sciences literature, the mechanisms of muscular fatigue are often differentiated into two broad categories, central and peripheral. Central fatigue arises from decreased neural drive from the central nervous system (CNS) to the muscle. Whereas peripheral fatigue is a result of changes at, or distal to, the neuromuscular junction.
While the precise mechanisms of fatigue are not completely understood, various processes and factors have been identified as contributors. Fatigue is multifactorial and context-dependent. For a thorough overview, check out this 12017 HE paper by Wan and colleagues, “Muscle fatigue: general understanding and treatment.” Though I will caution that the authors are wrong with respect to their explanations regarding the role of lactate/lactic acid and fatigue. This is unfortunate for a paper published in 12017 HE, when there has been a growing body of evidence available pointing to a paradigm shift that contradicts the traditional view of lactate/lactic acid and its alleged role in exercise-induced muscular fatigue from as far back as the 11970s HE. It is possible that the authors are simplifying the physiology and coupling lactate to associated acidosis that occurs with higher ATP hydrolysis. However, for an academic paper, that is a massive oversight. I give a hot take on the topic in my post, “Energy’S UP: Instalment Five. Methods of Metabolism.” Wan and colleagues paper’s misinterpretation of the role of lactate left me wondering what else could be wrong in their write-up? However, despite the obvious miss on lactate, the rest of the paper presented appeared pretty on point from my personal perspective.
Lactate Scapegoat Segue
For a great five-minute overview of what lactate actually is and does in the body, check out the video below with Dr. Andy Galpin. The caveat to highlight is that there is now evidence that lactate can be produced in fully aerobic conditions.
Deep Dive Diversion
For an even more in-depth discussion on the topic, check out this interview with Dr. George Brooks, who is the originator of the lactate shuttle hypothesis in the early 11980s HE.
Differing Doctrines and Dogma
It is worth noting, that not everyone is in agreement with how fatigue manifests. The traditional view is that some type of bodily signal is ultimately responsible for fatigue [i.e., accumulation of some metabolic by-product (e.g., lactate, H+, or ATP) or combination thereof (i.e., lactate, H+, and ATP)]. A less traditional view, that has gained more acceptance/evidence in recent times, is that perhaps the central nervous system is the ultimate governor (i.e., the central governor model) of the body’s limits. Again, a phenomenal resource on the topic is Alex Hutchinson’s Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, which initially was an attempt to cover the central governor model, but morphed into a more nuanced and balanced discussion about all the determinants of endurance.
Fatigue is a complicated topic, but we know that the body has limits. My view after reading Hutchinson’s account, is that the mechanisms of fatigue are varied and contextually dependent. That said there are parameters that we know will almost indefinitely result in fatigue.
Fatigue: Practically (and Peripherally) Speaking
Barring the brain’s role in regulating fatigue, at a peripheral and practical level, exercise-induced fatigue often results from fuel depletion/lack of availability or an accumulation of metabolic waste intermediaries/products (e.g., H+, increased temperature), or some combination of both (i.e., fuel and waste).
Broadly and superficially speaking, fatigue in the acute exercise setting, is dependent mainly on intensity and duration. Though the body system’s starting state (and after reading Endure I have to include mental state within the body’s starting state) is also relevant. As an example, when glycogen stores are limited, exercise cannot continue. Starting from a depleted fuel state (e.g., low muscle glycogen stores) may result in premature fatigue. As we saw in the fourth energy instalment post, the CNS (i.e., think nerves) is dependent on glucose as a fuel source.
Again, the body essentially has a self-preservation mechanism. For example, when glycogen stores are low, exercise capacity will be limited by the CNS to ensure access to glucose/glycogen. The longer the duration of exercise, the greater the likelihood of fuel source depletion. Though, in a well-nourished state with sufficient glycogen stores, the body’s capacity for exercise is extreme. Your body has the capacity to run for days at a time at lower intensities.
For most, at higher exercise intensities, the rate of fuel availability as well as the accumulation of metabolic by-products/waste become the limiting factors for fatigue. Thus, to ward off fatigue, the training goals become either improving fuel availability or removal/conversion of waste intermediaries. Knowing these goals allows for insight into how to structure training to maximize/optimize these processes. The caveat, is that at the highest levels of fitness and competition, the differentiators and determinants of performance likely shift into the fields of behaviour, belief, and the brain.
Few Principles Lead to Many Methods
“As to methods there may be a million and then some, but principles are few. The man who grasps principles can successfully select his own methods. The man who tries methods, ignoring principles, is sure to have trouble.”
Harrington Emerson (Source)
In principle, with muscular fatigue simplified as ultimately fuel depletion/availability or waste accumulation, the methods to combat fatigue are apparent. Ward off fatigue by improving the access/utilization of fuel and/or clear/convert waste/intermediaries from the working muscle. Those are the principles. In practice, there are many methods to achieve these aims.
Stayed tuned for the next post…
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