Defining the Nebulous “Aerobic Base” 

Norwegian Bjorn Daehlie, The “King” of Aaerobic capacity, and arguably the greatest nordic skier of all time.

By Jeff Sonderegger


Among the most common terms used in fitness programming for endurance sports is “Aerobic Base.” This is always stressed as the most important factor in having a successful competition season, in preparing for a mountaineering project and in a myriad of other endeavors where covering long distances, preferably quickly, is the name of the game. The annals of the social media world and various communal training apps are littered with clichés like, “The (name that event) is won in the offseason,” or, “Every (insert event name) is won six months before it takes place.”

Despite the wanton abuse of such rhetoric, the sentiment is nevertheless absolutely true. Nobody who is serious about competing in or participating in endurance-based events can start training for an event just a couple of weeks before it takes place (excepting, of course, olympic-level athletes who have years of training to rely on in recovering from illness or injury). To do so only guarantees disappointment, frustration at one’s own inability and, depending on the event, puts the individual at personal risk. Even someone who goes on a technically mild backpacking trip deep enough in the backcountry could simply run out of gas and be in serious trouble of being stuck in an untenable position if they have no capacity for long aerobic exercise.



In an effort to ensure understanding of the importance of building Aerobic capability and, thus, the entire purpose for this conversation, I will give a very simplified explanation of what it means to have a high-level of Aerobic training:

Adenosine Triphosphate (ATP) is, essentially, the fuel for energy and, thus, movement. Other than red blood cells, everything in the human body runs off it. The body produces ATP through the metabolic process using either oxygen (aerobic) or glucose (anaerobic) processes. Given this fact, the two primary factors that determine the pathway by which ATP is produced are incredibly logical: intensity and the physiological trained-state of the individual. The same logical train leads one to understand how codependent these two factors are in the production of ATP:

If an individual is well trained, i.e. has developed an efficient capacity for ATP production by the Aerobic pathway, the physiologically-perceived output allows the body to produce ATP with oxygen as the primary pathway when operating at a higher intensity. Incidentally, this is also the means whereby someone burns fat, as that is the primary fuel used for this pathway. This means the individual’s glycogen stores won’t get burned, and they’re able to continue operating at this intensity for a greater duration (we have much higher energy-potential stored in the fat in our body than in the glycogen in our muscles). In short, once someone has built a high-level of Aerobic fitness, they can go fast for a long time.

Consider elite marathoners, triathletes, cyclists and nordic skiers: if you watch them racing, they appear, during most phases of a race, to be breathing through their noses in what looks to be a comfortable, controlled manner. Now at that pace, their heart rates are undoubtedly in the higher percentages of their max in order to provide the necessary ingredients to their muscles to allow for them to move at such a rate. But they don’t look it. And they’re able to maintain such a pace for hours, clearly indicating that they’re not consuming their glycogen stores. This is because these individuals have spent literally decades fine-tuning their body’s capacity to utilize the aerobic pathway for ATP production. So they look comfortable and controlled because they ARE comfortable and controlled.

There’s much more to the science of the entire process, to include the production and consumption of Lactate in slow twitch muscles, but for the purpose of this informative essay, I don’t feel a need to go any deeper. Simply stated, the goal of building an Aerobic Base is to make the body more efficient at oxygen-fueled ATP production to allow faster movement over longer distances/durations of time.



The real trouble with all the discussions about Aerobic Base is that the actual meaning of this endurance staple, which is almost categorically regarded as fundamental, remains wildly vague. Or, at the very least, a passable definition is difficult to find among the millions of articles and opinions that can be found in every medium. But so many training plans for cycling, MTB-racing, cross-country ski racing, running and so many other endurance events share the same roughly 8- to 12-week “base building” period at the beginning of the training season. This time period is often adhered to regardless of the length of the actual event (half-marathon vs. full triathlon), which further confuses the notion of what “Aerobic Base” actually means, given the vastly different overall-endurance demands of these events.

Now why is the 8- to 12-week base period so common that it’s almost doctrinal? It harkens back to the work of Arthur Lydiard, a coach for the New Zealand Olympic Running Team during the 1960s whose athletes saw such success that the entire world had to stand up and take notice. Beyond taking notice, the entire endurance training scheme changed in many ways to mirror what Lydiard and his athletes were using to achieve such results. But, as is so often the case, the meat of his training regimen was lost in favor of attempting to make it into a simple equation, so the training itself took a back seat to the general periodization. Presumably the lack in time-variation is additionally due to the assumption that an athlete training for an event already has the training background and general fitness capacity (as well as wherewithal) to maximize performance-increase during the time afforded for training in this timeframe, regardless of the specific demands of a specific event. Furthermore, the risk of suffering from overtraining syndrome or injuries likely contributes to a general desire to not extend beyond that 12-week mark. But such is simply bad programming and bad coaching, as a coach should be aware of those risks and build defenses thereto into the programming itself. And for what does an athlete need a coach if they already have all of the training experience and tools that they need?

To compound the problems of lazy coaching, rarely (if ever) do these base-building periods include any form of assessment to measure a baseline or to track progress throughout, rather, they are simply a long line of “time under stress” training sessions in which one should be moving, but not pushing too hard. That “not pushing too hard” portion is smart science when it comes to building aerobic capacity while also developing resilience against overtraining (as proven by Lydiard), but assessments are crucial to ensure that the given training plan is having the desired training effect on the subject. If no true aerobic assessment is conducted at the beginning of the training session, how will one know that their aerobic capacity has, indeed, increased over the base-building period? Furthermore, if no additional assessments are conducted prior to the completion of this training period, a potential net-negative training effect will not be detected early, meaning that entire 8- to 12-week period has been, in essence, wasted. So clearly assessments are crucial. The problem here is that, with regards to Aerobic Base, the concept is so vague that the question of “what is even being assessed,” becomes an impassable chasm in the logic of a training program. So the cycle of simply writing the first three months of a training plan as “base-building” continues.

Additionally, in this world, there are an increasing number of people entering the realm of endurance sports with little-to-no training background or experience. These people are reaching for whatever existing programs and material exist to help them in their training without having years and potentially decades of aerobic-tuning to rely on. These existing programs hardly afford such individuals the time to actually build the necessary aerobic capacity to be competitive and CERTAINLY not the same aerobic capacity as the individuals for whom the programming was done. Thus, the concept of “Aerobic Base” HAS to be considered a moving target depending on where an individual is in their endurance- and fitness-journey. When programming is created without the variability in experience considered, the relatively untrained who are attempting to follow such programming will ultimately fall further behind due to negative training effect and/or overtraining injuries/burnout. And the gap between the elite and the pedestrian widens every season in a world where, with proper training and focus, it would not necessarily have to.

All of this equates to a realm of training that’s far too difficult to actually grasp, causing the majority of people, both coaches and athletes, to seek the simplest possible solution; they want to be given a number. “Once I can run 7-minute miles at 60% of my heart rate, I have a good Aerobic Base,” or, “Once you’ve trained 12-weeks following this ‘time under stress’ regimen you have a good Aerobic Base,” or something along those lines. And though a benchmark akin to the former may be plausible in a “general fitness” sense, there is simply no catch-all that can be considered an adequate base mark across the spectrum of endurance events.



Though I’ve stressed how few real definitions for Aerobic Base there are, a couple can, in fact, be found. There is are two definitions that are generally accepted by their respective camps, among which many of the high-performing endurance community, both coaches and athletes, can be found. Though there’s no specific scientific study that has been conducted to prove either of these definitions, again, due to the variability of human genetics and physiology, the amount of anecdotal evidence backing both of them certainly lends credence to their veracity. I, personally, have found great success in implementing the first into my own training.

That equation, which is championed by Scott Johnston of Uphill Athlete, states that one must simply determine their Aerobic Threshold, their Anaerobic Threshold, and then determine the difference. If a person’s Aerobic Threshold heart rate is within 10% of their Anaerobic Threshold heart rate from their Anaerobic Threshold, “Aerobic Base” has been established. If the difference is greater than 10%, a concerted effort must be given to aerobic training (time spent at sub-aerobic threshold heart rates) to allow the body to adjust and increase the Aerobic Threshold to within 10%.

Okay, maybe that’s not so simple, but it’s also not incredibly difficult. Importantly, it allows for individualized results which, in turn, allows for a truly individualized aerobic/anaerobic training regimen. If you’re interested in testing your own Aerobic Base by this method, follow these links for descriptions on methods by which one can attain their Aerobic and Anaerobic Thresholds for the purpose of finding whether one has attained Aerobic Base by this theory.

Despite my personal successes with this methodology, it absolutely has its flaws. Chief among them is the timeframe of the assessments listed above, the longer of the timeframes being only 60 minutes. This is a period which hardly mimics the aerobic- and muscular-endurance of most endurance events. Furthermore I’ve found that most HR monitors that the majority of people own can be unreliable at best and absolutely worthless at worst, meaning that it’s difficult to attain reliable metrics and, thus, ensure that the subsequent training periods are planned with correct focus on pacing.

The second definition is propagated by Joe Friel, author of the “Training Bible” series of books and one of the most widely recognized coaches of endurance athletes. His concept of Aerobic Base is an incredibly simple definition: Aerobic Base has been achieved once an athlete can move by event-specific means (run, bike, swim, etc) at 70% of max heart rate for a period of four hours. The utter simplicity of this definition is incredibly appealing to an athlete at every level (for the reasons noted in “The Problem(s)” above), and the four hour time-period certainly addresses the primary issue with Johnston’s definition.

However, this same simplicity lends, unfortunately, to flaws in the theory and the duration of the time period increases the risk of overtraining injuries for the many individuals looking to enter the world of endurance who don’t have the years and years of training experience and general fitness to rely on. This concept also does not specify anything to do with pacing meaning that, theoretically, an individual could be walking briskly at 70% of their MHR for four hours, but incapable of running any form of competitive or tactically mandated (as in a PT test) pace for any sort of duration.

Furthermore, such a training methodology still relies on the use of a heart rate monitor. These are certainly getting better and better every year, but I would argue that a person is better served learning to read their own body. Personally, I’ve found myself during many training sessions frustratedly chasing heart rates, stopping me from simply finding a zone and enjoying the experience. But if I simply settle into a comfortable, nose-breathing (or conversational) pace, I can maintain pace longer and enjoy the training much more. Additionally, an individual who learns to listen to their own body is able to better perceive strengths, weaknesses and the onset of certain overtraining injuries before they become a problem. Thus, rate of perceived exertion (RPE) should be considered a valuable tool in Aerobic training, where the desired RPE is somewhere in the range of conversational, or nose-breathing, pace.



The notion of spending a great deal of time training at the pace mentioned above seems sinful and foolhardy to the majority of Americans who have done any degree of training in their life. For most, disciplined training ended with high school or prep school sports. Most Americans, regardless of their chosen sport, have had the training notion of “No Pain, No Gain,” pounded so firmly into their heads that at the thought of going on an hourlong movement at the pace required to maintain the aforementioned RPE makes them roll their eyes and chuckle. ‘This is the way geriatrics train, and we’re red-blooded Americans.’ Even the most physically untrained will put forth a Herculean effort on a weekend hike and spend the following week sick and unable to walk because of it to avoid the shame of being “slow.”

And even the very physically-trained suffer from the negative effects of this same mentality. CrossFit being exhibit 1A. Though there is certainly some smart science in the training methodology of CrossFit, the entire doctrine encourages the idea that every single training session should leave one soaked, beaten down and exhausted. Otherwise, it “wasn’t a good workout.” And the statistics for CrossFit injuries are proof of what this training mindset leads to.

In both cases mentioned above, the end result is an inability to train due to overtraining. Logically, the inverse can be viewed as incredibly valuable. For training to be effective, one must avoid overtraining so that training can continue. In this regard, Americans need to, in the words of Dr. Phil Maffetone, slow down to speed up. And Dr. Maffetone’s success in coaching Mark Allen to six Ironman World Championships should be proof enough of the truth of that statement.



So what is “Aerobic Base,” then? I briefly covered what the entire concept of “Aerobic” means, as well as the broad strokes of how training should be conducted, but what is the actual definition of Aerobic Base?  As mentioned, it has to be considered a moving target for every individual based on their training experience and physiological state. This makes it seem that no passable definition COULD be given, as the variability would be so great that a single definition is impossible to broadly apply. Especially given the fact that the definition for a single individual would change over the course of time.

But therein is exactly the answer. Aerobic Base MUST be considered event- and trained-state-dependent. Meaning, on a sliding-scale, as an individual graduates to a capacity for greater distances and faster paces, their target for Aerobic Base will adjust to accommodate. For example:

A 28 year-old male soldier has always struggled during his APFT run. He’s never considered himself a runner and genuinely runs twice a year, during his diagnostic and for-record APFTs, if he can help it. He has cut it close to failing several times and has decided he’s sick of being ill with worry for weeks leading up to his APFT twice per year. For such an individual, his training periodization should be programmed such that he spends significantly more time training in a low RPE than a high RPE for as long as six months. He should be assessing his progress every three- to four-weeks to get an idea of what needs to be adjusted in his regimen. One week before his APFT he completes the final assessment of his training regimen. He’ll know that his programming has helped him to achieve “Aerobic Base” for this event if, during this final assessment, he is able to run his two miles, maintaining a nose-breathing or conversational pace throughout, and finish 10-20 seconds faster than his “passing” time.

With this accomplished, this soldier’s training plan SHOULD continue to implement Aerobic efforts (RPE being comfortable nose-breathing/conversational pace) with a faster time being the goal for Aerobic Base. After two years of such training, with the lion’s share of training time being spent at an Aerobic pace, this same soldier would be capable of maxing the APFT run at this same low RPE. His body has adapted and evolved and he is capable of moving further, faster, longer and at a higher heart rate while still aerobically producing ATP. Or rather, BECAUSE he is still aerobically producing ATP.

This example is poignant, as most people would consider the two-mile run an anaerobic or mixed-pathway event. It’s short enough to be able to really give it gas. And for the majority of soldiers taking the APFT, that’s exactly what they’ll do. However, this does not take away from the veracity of the training methodology. Because the event itself is so short, soldiers satisfy themselves with the fact that they can complete it with relatively little time spent on training their aerobic pathway. However, they’re left gasping for air throughout the entirety of an event which, as stated, is very short, indeed. And still getting relatively slow completion times in spite of their perceived and physiological efforts. This is as plain and simple a sign of an individual being aerobically deficient as can be found, and should reiterate to the individual the importance of aerobic training and building the topical “Aerobic Base.”

So, again, what is Aerobic Base? It’s the ability to complete the desired time over distance for an event while maintaining aerobic ATP production. It’s the ability to go as far as necessary as fast as desired.



In wrestling with this topic, in spite of the fact that endurance (see: aerobic) training has become a keystone of the programming that I do for myself and for others, I will admit that I got my butt handed to me on a daily-basis. Aerobic Base is such an immensely important and heavily used phrase in every phase of endurance training and there is just so little concrete information that serves to define its actual meaning. Rob and I probably could’ve passed a Bill through the molasses-covered halls of Congress with all of the time we spent kicking ideas, experience and research back and forth while I slogged through this writing.

So if anyone has found any concrete definition, assessment, or theory with regards to Aerobic Base that isn’t mentioned above, or if you have any specific experience with the assessment methodologies listed, please reach out and let us know.



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