We can't have a nuanced discussion about energy system training or 'conditioning' more broadly without first discussing bioenergetics. Bioenergetics are the stimulation of metabolic processes that result in the supply, transport, and utilization of energy in the body. Contrary to popular belief, energy systems do not create energy. Instead, energy is transferred from one state to another, both inside and outside of our bodies. This harkens back to the law of energy conservation which states, "energy can neither be created nor destroyed - only converted from one form of energy to another." For example, we get electrons from the sugars in the food that we eat, and we breathe in oxygen. Electrons and oxygen meet in the mitochondria to transform free energy into a form we can use in our body. The better our 'energy system' capacity, the better we can transform this free energy into a usable form, which in practice means restocking ATP, the energetic currency of the cell. For what sounds like a simple process on paper, the details and intricacies of this process are incredibly complex and are often poorly understood by athletes and coaches.
You'll find a NIRS measurement recorded from an athlete performing a 60-second sprint on an air bike in the picture above. Notice that the second they begin their sprint, oxygen is utilized instantaneously in the working muscle. In fact, oxygen is consumed at a much greater rate than it is supplied to the active muscle, which is why muscle oxygen saturation is declining. As soon as the athlete stops pedaling oxygen supply supersedes oxygen utilization, and oxygen saturation rises rapidly.
Having worked with many physiologists, researchers, and performance-oriented physicians, it's always interesting to me that they're not surprised in the least when they see these types of measurements. "Of course, oxygen is utilized immediately upon load" they'll say. Yet, this fact is often lost among coaches. After all, a max effort sprint is an 'anaerobic' event, right?". I write that facetiously, yet this is still the dominant paradigm. If you pick up any given training book you'll see terms like 'anaerobic a-lactic capacity', 'lactic endurance', and 'aerobic power' thrown around liberally. Yet, we can easily pick holes in this framework. Of course, some coaches will acknowledge that these energetic processes do overlap in time. Still, few appreciate the speed at which these different energetic processes occur or the fact that they are overlapping on the millisecond time scale. As a result, I assert in this article that all training is aerobic and all training is lactic. I mean this literally. In vivo oxygen is always part of the energy production process, whether direct or indirect, and lactate is always present as well.
You may already ascribe to this new paradigm of bioenergetics, or you may feel like the rug has just been pulled out from under your feet. If you're in the latter camp, that's perfectly okay. You don't need to have any of this committed to memory to grasp the concepts that i’ll be presenting in the coming weeks. I present this material because it opens the door for a more nuanced take on training where we think in terms of limiting systems rather than thinking in terms or what 'energy systems' we need to train.
What Is Wrong With the Old Paradigm Of Bioenergetics ?
Often, in discussions about energy systems, coaches and people will make a hard distinction between two modes of energy production: anaerobic and aerobic. The former is broken into the phosphagen, and the glycolytic pathways, which occur in the absence of oxygen , and the latter is called the oxidative pathway meaning oxygen is needed for its function. This model proposes that aerobic and anaerobic processes occur independent of one another — that is to say, that at any given time we are either operating aerobically or anaerobically at a given point in time. The issue is that there are some flaws with this framework.
Many of you are probably familiar with this chart directly above you see it in a lot of coach manuals, textbooks, and courses. When we look at this model, there are some good things about it — for example, it shows that all of the energy systems are working simultaneously with different contributions, but past two seconds is where it goes off the rails. For example, this model says that from two to ten seconds, we are primarily using phosphagen stores with a bit of contribution from the glycolytic and oxidative system. Past ten seconds up to two minutes, we’re relying on the glycolytic system until we eventually transition to solely relying on the oxidative processes to keep us going. This model is also not in agreement with contemporary scientific literature. For example, this model, which we’ll call the conventional view of bioenergetics, says that phosphocreatine supplies almost all energy needed for a sustained burst of contraction lasting less than ten seconds, after which it is replaced by glycogenolysis.
But, this isn’t supported by the biochemical evidence. In Robert Shulman and Douglas Rothman’s paper titled, The “glycogen shunt” in exercising muscle: A role for glycogen in muscle energetics and fatigue, they report the presence of the enzyme glycogen phosphorylase in its active form under conditions where glycogen concentrations are constant. Glycogen phosphorylase’s role is to break down muscle glycogen to release glucose and it’s the key enzyme needed for utilizing both muscle and liver glycogen stores. Thus, the only way for it to be found in its active form while glycogen concentrations are stable is for glycogen synthesis and breakdown to be occurring in tandem. This would only make sense if the support of continued muscle contraction requires continual phosphocreatine breakdown and glycogen phosphorylase rapidly increases activity to restore phosphocreatine, and in turn ATP. Through the classic lens of bioenergetics this would seem paradoxical since it is believed that phosphocreatine consumption falls after ten seconds. However, in Yourgran Chung and colleagues' paper titled, Metabolic Fluctuation During A Muscle Contraction Cycle, we see that phosphocreatine consumption is approximately forty times greater than values reported by dividing the drop in phosphocreatine after minutes of contraction by the number of twitches. This was made possible by using new measurement techniques, specifically P-NMR imagines which demonstrated that the traditional method of calculating phosphocreatine consumption per muscle twitch underestimates the high energy consumption that arises from the drop and subsequent restoration of the phosphocreatine pool during millisecond contraction cycles. Chung and colleagues' experiments also show that phosphocreatine cannot be the ultimate energy source in contracting muscle. At a cost of three millimolar of phosphocreatine per twitch a muscle would rapidly deplete its energy supply unless PCr were replenished between contractions.
Additionally, in Kevin McNully and colleagues’ paper, Simultaneous In Vivo Measurements Of HBO2 Saturation and PCr Kinetics After Exercise In Normal Humans, we see that phosphocreatine and oxygen kinetics are tightly coupled during exercise and following exercise. There are also reports by Paul Greenhaff and James Timmons where they state, “However, that PCr hydrolysis and lactate production do not occur in isolation, and that both are initiated rapidly at the onset of contraction.” This leads us to the contemporary model of bioenergetics.
The New Paradigm Of Bioenergetics
In the picture above, you’ll find a visual representation of a contemporary model of bioenergetics. You’ll notice that we still have the phosphagen, glycolytic, and oxidative pathways, but (a) they are all overlapping and (b) the time frame they are operating in is 0–100 milliseconds. Already we see some major departures from the conventional [classic] model of bioenergetics. Getting into some of the minutiae of this model, we see that…
The support of muscle contraction requires rapid non-oxidative ATP production on the millisecond time scale. So, within 0–15 milliseconds of contraction phosphocreatine is broken down to restore ATP. This isn't all that different from that old model except for the fact that it’s occurring on a much faster time scale, but what happens next is where things start to get interesting.
In order to sustain contractions we need a non-oxidative energy supply. However, we run into an issue given that glycolytic intermediates like glucose within a muscle are limited. This is where we need to lean on the biochemical evidence, which shows that glycogen phosphorylase can rapidly increase its activity, and as a result glycogen can be broken down to restore the phosphocreatine needed to sustain contractions. The question then becomes how we maintain glycogen stores.
Between contractions the ATP needed to re-synthesize glycogen, phosphocreatine, and re-establish ion gradients comes from the oxidation of lactate. However, only a fraction of the lactate produced needs to be oxidized to restore these energy pools. So, lactate accumulates in muscle cells which is not due to it being a fatigue by-product, but rather it’s due to an inefficiency in this process.
This is all important because it shows us that...
Oxygen is always present — therefore, all training is aerobic. In-Vivo oxygen is always a part of energy production, whether direct or indirect.
Lactate is always present — therefore, all training is ‘lactic’. This is one that’s kind of confusing to people. Contrary to popular belief, lactate is not a fatigue product, rather it’s a fuel source. Bruce Gladen said it best in his 2004 paper titled Lactate metabolism: a new paradigm for the third millennium where he stated “Lactate can no longer be considered the usual suspect for metabolic ‘crimes’, but is instead a central player in cellular, regional and whole-body metabolism.”
All energetic processes overlap in time, and the time frame is in milliseconds vs. seconds to minutes — it occurs far faster than classically believed.
There are no contradictions with observed Smo2 trends in the contemporary model. However, the classic model is in direct contradiction with what we can observe using NIRS and P-NMR imaging.
Finally, oxygen utilization responds immediately to load. When muscle oxygen saturation reaches a nadir performance stalls. Period. Low muscle oxygen saturation means that oxidative metabolism is compromised, which leads to an increased reliance on glycolysis to replenish PCr, and subsequently ATP. Glycolysis, though always active, is much less efficient than oxidative metabolism and has a greater cost. This leads to the onset of fatigue and the employment of compensatory movement strategies in order to try and maintain force output.
More Cracks In The Old Models Foundation
“Many physiologic testing protocols in use today are based on poor science, and do not make use of the available evidence, which has proven fundamental errors in the protocols themselves.” — Dr. Andrew Sellars
Knowing what we know now, a natural set of questions arises when we start to put pen to paper and write energy system training sessions for our athletes. “If all training is aerobic, and all training is lactic, how the heck can we do a-lactic anaerobic training?” Excellent question. The simple answer is that we don't, or rather we can’t. When people perform ‘alactic’ power training, they assume lactate isn’t generated because it doesn’t show up on blood lactate test results. Does this mean there was no lactate generation or that it was being consumed for fuel? When you take a lactate sample from the ear or finger, there is a time lag since the sample isn’t taken at the source of the working muscle. In reality the measured lactate level equals lactate production minus lactate consumption. The reading on a lactate analyzer tells us the difference between how quickly lactate is being produced and consumed, not whether it is being produced in the first place. In actuality, lactate production is incredibly high during ‘a-lactic’ training intervals, but it’s being consumed at an incredibly fast rate.
All training is aerobic, and all training is lactic. When we can speak in these terms and ditch a lot of the classic ideas about energetics, we can come to new conclusions that are better informed. For example, let's take maximal effort sprint performance. Traditionally, this type of work would be classified as a test of ‘a-lactic’ power or endurance. For example, in the picture above we have a chart from a popular energy system training course that says, “As a rule of thumb, the closer the event's duration is to one minute, the lower the aerobic contribution to overall performance will be. The opposite is also true: the longer the duration is, the more dominant the aerobic system will be.” Oh really? That’s odd given that oxygen is utilizing immediately upon load, even during a 30 second max effort sprint, and that muscle oxygen saturation begins to recover as soon as the sprint ends, as seen in images earlier in this article.
When we think in these confined terms and categories, like ‘alactic-anaerobic’ or ‘lactic power’, it leads us to natural conclusions as to how we get better at certain types of events. But, when we can look past these outdated models of bioenergetics we can approach training through a new lens. Knowing that oxygen is utilized immediately upon the start of activity, we can think in terms of ‘limiting factors’ in oxygen delivery and consumption rather than what ‘energy systems’ are limiting us. So, rather than saying “my athlete needs to get faster on a 200m sprint so we need to improve her lactic power” we can identify the individual's rate limiting factor, whether that’s the rate of oxygen utilization or the ability to supply oxygen to the working muscle.
If you enjoyed reading this article please consider subscribing to the Emergent Performance Lab Substack or sharing it with a friend.
Free subscribers can expect content short form posts sporadically throughout the month, as well as previews of my long form paid content.
Paid subscribers will receive 1-2 content pieces per week including one long form exclusive content piece every Sunday, as well as bonus content including mini-articles, case studies, monthly Q&A’s, and podcast.
If you enjoyed this article you may enjoy the following articles as well: