The Analytic Fitness™ Dictionary – Energy Systems

This entry of the Analytic Fitness™ Dictionary looks at yet another nugget of physiology often misrepresented in popular exercise science: the human body’s energy systems. (About 3.300 words, estimated reading time: 15-17 minutes.)

Muscles only use one source of biochemical energy: adenosine triphosphate (ATP) and effort depletes it pretty fast.

What is commonly referred to as the human body’s ‘energy systems’ are nothing else than the pathways our body uses to resynthesize ATP. If you’re a chem freak, don’t set your hopes too high: this blog is about Analytic Fitness™ and not analytic biochemistry, so I’ll handwave the details of chemical processes and refer to external resources a lot.

Even so, you won’t miss much in terms of practical consequences. Analytic Fitness™ is an application of the type of analytic philosophy that can actually be useful to make practical decisions. This type of philosophizing was for instance characterized and illustrated by the late David. K. Lewis (1941-2001) in his early career, and summed up as follows:[1]


“It is the profession of philosophers to question platitudes that others accept without thinking twice. [W]hen a good philosopher challenges a platitude, […] the philosopher [notices] trouble that one who did not think twice could not have met. In the end the challenge is answered and the platitude survives, more often than not. But the philosopher has done the adherents of the platitude a service: he has made them think twice.”

David K. Lewis, Convention: A Philosophical Study (1969)

In this post,  I’ll challenge the platitude that aerobic exercise burns fat. Steering clear of it goes a long way towards making better-informed choices about exercise protocols, including, but not limited to, exercise protocols for fat loss. As Lewis would predict, however, the platitude will eventually survive but it’s a stalking horse. There are more than a few surprises and some interesting consequences for almost everybody’s training when the goal is not ‘burning fat’.

Continuing with my obsession with health, I’ll mostly cover the relations between energy systems and improvements of cardiovascular fitness. How to optimize fat loss through exercise may be examined in details in the Diet entry of this dictionary, if I ever find the courage to write it.

One Fuel to Rule them All

Muscles only use one source of biochemical energy: adenosine triphosphate (ATP) and effort depletes it pretty fast.

An ATP molecule. The phosphate group is on the left, with 3 PO43-

Sounds familiar? Good. Bis repetita placent anyway. And now for the obvious: ATP is not fat. Hence, the notion of ‘torching fat’ through exercise should raise a few red flags from the get-go. Of course, there’s the second part of the sentence: muscular stores of ATP are depleted pretty fast, which in this case means ‘within seconds’.

That’s where bioenergetic systems kick in to synthesize ATP and/or shuttle whatever reactants are necessary for ATP synthesis to where these reactants are needed. That’s also where fatty acids will come into play, but they will share the central stage with a host of other players. But I’m’ getting ahead of myself, so for now, let’s stick to the basics. There are three pathways for ATP synthesis:

  • ATP-CP (adenosine triphosphate-creatine phosphate). Creatine phosphate (CP) is stored locally in skeletal muscle. During muscle contraction, CP breaks down in Creatine and a phosphate ion. This phosphate ion can be captured by adenosine diphosphate (ADP), the byproduct of ATP breakdown, to yield ATP all over again. The ATP-CP pathway is limited by the muscle storage capacity for ATP and for CP and is good for up to 30 seconds (tops). However, CP can be rapidly resynthesized and the ATP-CP pathway can be used all over again in a manner of minutes or even less (under the right conditions, I’ll come back to that).
  • Glycolysis. The second pathway, glycolysis, kicks in when (1) glycogen, the storage form of glucose in skeletal muscle, is broken down into glucose proper (that’s glycogenolysis); or (2) glucose is shipped directly to the muscle by the bloodstream (via the Cori Cycle, that I’ll come back to later). Glycolysis yields ATP via the breakdown of its next-to-last byproduct (phosphoenolpyruvate, which donates phosphate to ADP, see here). One molecule of glucose yields only 2 molecules of ATP that way. The pathway is limited by glycogen storage, efficiency, and accumulation of other byproducts. Some of the by-products of glycolysis (pyruvate and H+ ions) can be used by cellular respiration to resynthesize ATP, but in absence of said respiration, they accumulate and may impair muscle contraction (I’ll come back to that too).
  • Cellular respiration (also: Citric Acid Cycle, Krebs Cycle). The third and last pathway and the only one requiring oxygen uses the byproducts of glycolysis (pyruvate and H+ ion) to resynthesize ATP. The Krebs cycle runs twice per molecule of glucose (one for each pyruvate molecule that glycolysis yields as a byproduct) but the total yield of the cycle is a whopping 32-35 ATP molecules making it by far the most efficient of the three pathways.

Below is a nifty visual summary of the transition from glycolysis to cellular respiration, with proper CC BY-SA 4.0 attribution to ScienceGal4.0.

By ScienceGal4.0 – Own work, CC BY-SA 4.0,

And now for the first shot at the platitude: past the ATP-CP phase, the only requirement for ATP resynthesis is glucose and glucose is not fat. Local glycogen stores do the job at first, followed by liver glycogen and/or glucose shuttled from the liver, through the blood flow, to the muscles. After those stores are depleted, the liver needs to synthesize glucose from other sources, like amino acids, or the byproduct glycolysis (pyruvate). Notably, not fatty acids (although it may use one of the byproducts of fat breakdown, glycerol).

As for the second shot at the platitude, triglycerides (the other storage form of glucose in the body, colloquially referred to as ‘fat stores’) only enter the picture as yet another precursor of the Krebs cycle. Mitochondria in the muscles can convert fatty acids into Acetyl CoA (starting the Krebs cycle, as seen in the picture above) when needed, provided that they are shuttled to them through the bloodstream. Then again, this is only necessary if pyruvate metabolism is insufficient to maintain effort. Which is not always the case, in particular during high-intensity, short duration effort, as we’ll see.

A third shot at the platitude is that triglycerides must be broken down before fatty acids are shuttled to mitochondria which in turn requires the release of glucagon, a hormone that is produced in response to decreasing blood sugar levels. Glucagon causes the liver to convert liver glycogen to glucose and to release it in the bloodstream and is in a constant tug-of-war with insulin (produced in response to rising blood glucose levels). The breakdown of triglycerides releases glycerol, which the liver can use to synthesize glucose, ultimately re-fueling the glycolytic pathway but also up-regulating insulin and down-regulating glucagon (hence the tug of war metaphor).

And now for the parting shot: modern dietary habits typically result in abnormally elevated levels of blood sugar, and thus, of insulin, with an increased incidence of type-2 diabetes as a known consequence. Resulting, also, in fewer occasion for higher levels of glucagon. And thus, fewer occasions for mobilizing ‘fat for fuel’. A sustained effort may offset the consequence of dietary habits a little, but only if it lasts long enough to deplete glycogen stores and outpace the liver’s ability to send glucose around the body.

Do we need the middleman?

Muscles only use one source of biochemical energy: adenosine triphosphate (ATP) and effort depletes it pretty fast.

Sounds familiar? Good. Three times’ a charm. Now, from the above, it should be clear that there is no necessary connection between the aerobic pathway and the mobilization of fat stores for energy. Training your cardiovascular fitness won’t automatically make you a ‘fat burning machine’. Let alone cranking up the intensity. Quite the opposite, actually.

Logic alone suffices here. Consider what happens when cranking up the intensity and reducing the duration of otherwise aerobic exercise such as running, rowing, or cycling (Tabata, anyone?). As soon as VO2max is exceeded, pyruvate anaerobic metabolism must do the heavy lifting alone without the help of the Krebs cycle, leading to the accumulation of two by-products: lactate and H+ ions (aka protons).

To cut a long story short, lactate is released in the bloodstream, shuttled to the liver, converted to glucose, and shipped back to the muscles to keep glycolysis going (this is known as the Cori cycle), while H+ ions are responsible for muscle acidosis with its cohort of side effects (cf. the entry on Cardiovascular Fitness in this very same dictionary). The accumulation of H+ ions is what prevents anaerobic glycolysis from going on more than 2-3 minutes without dramatic loss of power output. Also, lactic acid, often blamed for this, plays no role (see that post by Chad Waterbury on lactic acid and lactate, or the short summary in the aside at the end of this section).

Subsequently, training strictly within high-intensity intermittent training (HIIT) parameters will make you a glucose-burning machine, not a fat-burning one. ‘Fat burning’ can only happen once the oxygen debt is paid, and provided that effort lasts long enough for fatty acids to be required during the Krebs cycle. Which, incidentally, is also how H+ ions are disposed of and acidosis gets corrected (as shown by the diagram below borrowed from the Wikipedia entry on Cellular Respiration, with proper CC BY-SA 3.0 attribution to it author, RegisFrey).

By RegisFrey – Own work, CC BY-SA 3.0,

A first interesting albeit well-known consequence is that aerobic exercise at a submaximal intensity (60%-80% of VO2max) is a better recovery option for paying off the oxygen debt than doing nothing, because it helps clear out H+ ions. A second interesting consequence (this one often overlooked) is that the need for clearing out H+ ions contributes to explain why adaptations to high-intensity anaerobic exercise include improvements in aerobic capacity (VO2max). In fact, the relation goes both ways: improving VO2max through aerobic exercise makes you efficient at long-duration anaerobic exercise (were ‘long duration’ is 2-3 minutes, as opposed to the 12-30s of the ATP-CP pathway) by helping dispose of the byproducts of glycolysis that limit performance (H+ ions).

In fact, there is no need to train above the threshold at which lactate accumulates to the point of impairing performance (see that post by Waterbury again about the importance of the lactate threshold, or the short summary in the aside at the end of this section). And there is more where that comes from. It has been known from 30 years that rapid regeneration of the ATP-CP pathway is dependent on oxygen (initial research here, more recent review there). In layman’s term, improving VO2max through aerobic exercise makes you efficient at short-duration anaerobic exercise, such as lifting heavy weights. The relation does not go both ways here, because short-duration maximal effort is usually accompanied by ‘bracing’ (through the Valsalva maneuver) which prevents venous return and forfeits cardiovascular adaptations. So maximal effort and aerobic effort need to be trained both. But an improved aerobic fitness would make you a better lifter.

What happened to lactic acid? Performance drop in anaerobic conditions is still often blamed on a mythical accumulation of lactic acid. The myth is sometimes fleshed-out by the statement that lactate is the metabolic form of lactic acid. The reasoning behind this statement is that lactate is the conjugate base of lactic acid, i.e. the base that is obtained when an acid donates a proton (H+) during a chemical reaction. Then again, muscle lactate is formed when pyruvate captures a proton, and the protons that accumulate during glycolysis are the by-product of the breakdown of glucose. Therefore, no lactic acid is actually produced in the muscles, the chemical reaction by which lactate is formed is thus alkalinizing (i.e. prevents pH to rise), and the notion that lactic acid is responsible for metabolic acidosis is a myth. You can read more about that myth in this excellent post by Chad Waterbury which has nifty copyrighted diagrams that I could not use in this post (but I’ll ask Waterbury’s permission to use them in follow-ups).

The lactate threshold. The lactate threshold of an athlete is the point at which lactate begins to rise faster than the athlete’s body can dispose of it. Crossing the lactate threshold results in losses in power output and can eventually lead to muscle failure. Both are the consequence of metabolic acidosis, of which lactate is not directly responsible (see above). You can read more about the benefits of lactate threshold training in this excellent post by Chad Waterbury (again) which illustrates them with a nifty copyrighted diagram that I could not use in this post (but I’ll ask Waterbury’s permission to use it in a follow-up).

Cutting the Middleman

Muscles only use one source of biochemical energy: adenosine triphosphate (ATP) and effort depletes it pretty fast.

Is this getting old? Good. The ‘fat-burning’ business can now take the back seat, and I can move on to more interesting consequences of the above, the most striking being that you do not need to train within glycolytic parameters to become efficient in the glycolytic zone. Let me demonstrate this with an example of general-purpose fitness program, before concluding about more sport-oriented programming.

Recently, fitness experts who actually care for science have been moving away from HIIT. The trend is spearheaded by former HIIT advocate Chad Waterbury (with his arguments here) and Pavel Tsatsouline. Tsatsouline’s relation to science has been rife with alternative facts at times (just as his early promotion of kettlebell training) but he has improved on both counts in recent years. Actually, Waterbury, whom I have never caught bullshitting or abusing science, credits Tsatsouline for changing his mind on HIIT and drawing his attention to endurance research.

No Soviet supermen were armed while testing this program. (Pun intended)

Increase your 100% and learn to use a lower percentage of it.

Pavel Tsatsouline

Pavel Tsatsouline

In 2012, Tsatsouline parted company with RKC® and the bullshiters at DragonDoor (cf. here and here, again, for the bullshiting I’m alluding to), founded StrongFirst®, cleaned his kettlebell act by dropping the ‘Soviet Supermen’ nonsense and began actively promoting ATP-CP/VO2max protocols instead of contributing to the promotion of HIIT.[2] Waterbury summed up the guiding principles behind these protocols quoting Tsatsouline in a recent seminar on endurance (for which I found this recap): “Increase your 100% and learn to use a lower percentage of it.”

Tsastouline’s injunction to “raise your 100%’ can be understood in obvious ways (VO2max, since the topic is endurance) and less obvious ones (n-repetition maximum effort with n ≥ 1, or lactate threshold). Case in point, the über-excellent Simple & Sinister, which would probably be my default recommendation to anyone interested in all-around fitness improvements with minimal time investment. The Simple & Sinister protocol cashes Tsatsouline’s 100%-advice out in multiple ways, by coupling:

  1. intermittent maximal effort, namely 10 heavy one-hand swings with maximal tension (abdominal bracing, etc.), every-minute-on-the- minute (EMOM), for 10 minutes; and:
  2. repetition effort with anatomical breathing, namely Turkish Get-Ups for 10 minutes, with a goal of 5/5 per side.

Part (1) is anaerobic by design and bears a surface resemblance to HIIT, while (2) is going to be initially anaerobic for most people (who tend to start too heavy, rush through the TGU, etc.), but that last part is self-correcting. In fact, the S&S progression cuts the glycolytic middleman:

  • improving speed with swings leads to a 1:4-to-1:5 exercise-to-rest ratio: with 10 swings in 12-15 seconds, this falls smack-dab into the top-end of the ATP-CP pathway; and:
  • recommended performance speed for TGUs results in 10 minutes of continuous effort, which is way past what glycolysis can provide without crippling acidosis.

The end-result is reliance on the ACP-TP pathway for (1) and on the cellular respiration pathway for (2), while glycolysis was tapped into for both at the onset. That’s genius. Also, not Tsatsouline’s genius, although he deserves the praise for acknowledging the genius while others have not.

Wrapping up: no middleman energy system training

Muscles only use one source of biochemical energy: adenosine triphosphate (ATP) and effort depletes it pretty fast.

Again? Yup. Why? Well, why not? Also, because keeping that in mind helps to keep one’s priorities in the right order: when it comes to physical performance training whatever improves ATP regeneration comes first and steering clear of everything that impairs it comes next.

Here, I can anticipate an objection: what if something that impairs ATP regeneration is required for sports performance? The answer is simple: that’s where the difference between training, sport practice, and sport performance comes in. There are no sharp boundaries here: sport practice ‘trains’ the skills necessary for performance, and on occasion, one will want to push practice arbitrarily close to performance parameters. But there is no conflation either, as the key phrase is ‘on occasion’.

While  Tsatsouline and Waterbury deserve heaps of praise for their advocacy of a-glycolytic training, they are no innovators: the ‘cut-the-middleman’ model of energy system training has been around for decades and was first proposed by Yuri Verkhoshansky in the late 1980s. Verkhoshansky is the genius I was alluding to, and if you are curious, you can read his arguments (with a very clear application to mid-distance running) here.

Yuri Verkhoshansky, on the official picture of TASS news agency for the 1980 Moscow Olympics

Verkhoshansky’s work has however suffered from a combination of factors. First, the model was highly theoretical when first proposed and, while being backed by science and evidence, relied on deduction more than on hard data. Second, the model is still in part theoretical, since the research and publication biases that followed the Tabata study favored HIIT research. Then again, training protocols are not always chosen based on hard evidence about training protocols, and fortunately so. Verkhoshansky’s name carries enough weight for sports coaches to actually follow his suggestions without waiting for the data.

So where does that leave us? Well, the ‘cut-the-middleman’ model of energy system training has slowly gained traction in the sports-and-performance world and the general public both, thanks to household names such as Verkhoshansky, Tsatsouline, and Waterbury. At the same time, it has trailed behind the HIIT wave, in the sports-and-performance world due to research and publication biases, and in the general public because of the get-fit-fast promises of HIIT. Then again, smart protocols like Tsatsouline’s Simple & Sinister show that the model is a viable alternative to the HIIT-yourself-in-the-face for those who are short on time resources.



[1]^Not all analytic philosophy is good or even remotely useful. In fact, analytic philosophy is rife with stuff that would not be considered serious subjects in any other scientific pursuits and yet get published, like that crap (which, ironically, is inspired David K. Lewis too). Then again, it may be an intentional reductio ad absurdum of meaningless analytic philosophy, like computer-generated postmodern crap is a reductio of the notion that non-analytic philosophy is actually meaning something.

[2]^ As far as I can tell, Tsatsouline himself has never seemed to be big on HIIT, but he was at least guilty by association: the other thinking half of RKC®, Dan John, was an early adopter of HIIT protocols in the mid-1990s (following the Tabata study) and still heavily promotes it. On a side-note in the side note, the HIIT-for-fat-burning myth, which is actually unsupported by any scientific data (as I mentioned in passing in my Tabata piece) is one of the few things I’d call bullshit on Dan John for, the other being: (1) conflating the onset of metabolic acidosis with reaching Tabata-levels intensity (discussed in same piece), or in simpler terms, the equation “feeling pukey after 4*(20:10)=170% VO2max”; and: (2) forgetting about the aerobic exercise performed as part of the Tabata protocol (again, discussed in same piece, with some other consequences discussed there).

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