The Physiology of Base Building

“Physiology of base training
There are two basic energy systems you use when training; anaerobic and aerobic. Unfortunately, you can not build both your aerobic and anaerobic systems at the same time very well. The idea behind base training is to train your aerobic energy system specifically and solely. Why is this important? The more work you perform aerobically, or in the presence of oxygen, the more efficient you are. Prolonged aerobic training produces muscular adaptations that improves oxygen transport to the muscles, reduces the rate of lactate formation, improves the rate of lactate removal, and increases energy production and utilization. These adaptations occur slowly over time.” – Matt Russ, Aerobic Base Training – Going Slower to Get Faster,

“Building a good aerobic base means training only aerobically. During the base period, no anaerobic workouts (including racing) should be incorporated. Anaerobic activity will jeopardize the efficient development of your aerobic base, so every workout is aerobic. That includes your long run on Sunday, your hilly runs in the park, and any other workouts where you’re heavily influenced by other athletes or the terrain…Anaerobic training can decrease the number of aerobic muscle fibers, sometimes significantly. This can happen in just a few short weeks of higher heart rate training.” – Dr. Phil Maffetone, Want Speed? Slow Down,

“…the best way to cause improvements in slow-twitch fibers was to run long and slow at 70% VO2peak (adaptation began from as low as 50% VO2peak pace). Faster was not better.” – Hadd, citing the research studies of Dudley & Hickson

“A strong aerobic base is essential if you want to remain injury free or run that fast race you are aiming for. Aerobic training involves running at 60-75% of your maximum heart rate. This translates to a pace that is 1½ to 2 minutes per mile slower than 10K race pace.” – Kirk West, Building an Aerobic Base,

Claims about base building:
1. You cannot build both your aerobic and anaerobic system at the same time.
2. Anaerobic training jeopardizes the development of aerobic base
3. Anaerobic training can decrease the number of aerobic muscle fibers.
4. Slow-twitch fibers are best trained at 70% of VO2peak
5. Aerobic training is running at 60 – 75% of max Heart rate

Standard Base Building Physiological Claims

The above comments and claims about aerobic base building are normal and usual. If you run a Google search of aerobic base building and read many of the cited articles you will find that the above claims are repeated often. Are the claims true? Does anaerobic training jeopardize aerobic base? Do aerobic adaptations occur best from low intensity training? It doesn’t matter if you believe base building to be an effective training method or not; the issue is if the physiological claims being made about aerobic base building are accurate. Let’s have a look at the research and see if it supports the oft-repeated claims made by base building proponents.

Dudley’s Research

In his internet-famous article on base building, Hadd cited studies by Hickson and, especially, Dudley in support of his claim that slow twitch fibers were best trained via easy paced running at 70% of VO2peak. However, a review of Dudley’s research shows this is not the case at all.(1)

Dudley examined changes in slow twitch, fast twitch oxidative, and fast twitch glycolytic fibers by training rats at intensities ranging from 62% – 116% VO2peak.(2) Dudley found that both slow twitch and fast twitch oxidative fibers showed significant adaptations even at the lowest training intensity of 62% VO2peak. Both fibers continued to adapt as training intensity increased, with adaptations peaking at about 94% VO2peak. In fact, at intensities of 94% or less, the mitochondrial adaptations in fast twitch oxidative were actually higher than those in slow twitch fibers.

Fast twitch glycolytic fibers began adapting at between 73% and 83% VO2peak and continued to adapt all the way to 116% VO2peak.

Not only does Dudley’s research not support the claim that running at 70% VO2peak is the best way to cause aerobic adaptations in slow twitch fibers, it showed that:
a) easy paced running caused significant adaptations in fast twitch fibers,
b) adaptations from easy paced running was greater in fast twitch oxidative fibers than in slow twitch fibers,
c) intensities between about 80% VO2peak – 100% VO2peak caused the greatest adaptations in both slow twitch and fast twitch oxidative fibers
d) adaptations in fast twitch glycolytic fibers began occurring between 73%-83% VO2peak

In short, this research shows that both slow twitch and fast twitch oxidative are active and producing force at paces of about 60% VO2peak – 100% VO2peak. Fast twitch glycolytic fibers become active at between 73%-83% VO2peak. At paces beyond about 100% VO2peak, the adaptation of the slow twitch fibers drops off, while the fast twitch glycolytic fibers continue to adapt.

Muscles and Marathon Training

Is there a human study supporting the findings of Dudley’s animal study? Yes, there is. A study was conducted on muscle adaptations from marathon training in novice runners.(3) The training program these novice runners followed was a 4 days-per-week, 18 week program of running at an intensity of 60%-75% heart rate reserve (an easy to moderate training intensity).

Researchers measured changes before and after training in many physiological factors, including changes in different muscle fibers types. They discovered major changes in the strength and power of both slow twitch and fast twitch oxidative muscle fibers as a result of the training. Since adaptations only occur in muscle fibers that are active during exercise this study confirms the findings of Dudley, showing that both slow and fast twitch fibers are active during easy to moderate intensity run training.

Let’s examine the results of these studies in relation to some of the claims noted above about base building.

Claim: Aerobic training is running at 60 – 75% of max heart rate.
Truth: Aerobic adaptations occur in both slow twitch & fast twitch oxidative fibers at intensities up to 116% VO2peak, with maximum adaptations occurring around 94% VO2peak. Adaptations in fast twitch glycolytic fibers begin to occur at around 73% VO2peak.

Claim: You cannot build both your aerobic and anaerobic system at the same time.
Truth: Slow twitch and fast twitch oxidative fibers are active and adapt during both easy and fast paced training. Fast twitch glycolytic fibers begin adapting at about 73% VO2peak.

Claim: Anaerobic training jeopardizes the development of the aerobic base.
Truth: Slow twitch fibers adapt to training up to an intensity level of 116% VO2peak, with peak adaptations occurring around 94% VO2peak.

Claim: Slow-twitch fibers are best trained at 70% of VO2peak.
Truth: Slow twitch fibers adapt during both easy and fast paced training. Maximum aerobic adaptation in slow twitch fibers occurred at around 93% VO2peak.

Let’s assume for a moment that easy paced, aerobic base building is not only an effective training method but that, just as base building proponents preach, it is THE most effective method for preparing an athlete for maximum performance.

If base building training is as effective as many believe then we are faced with an unanswered question – “Why is it effective?”  What, physiologically, is occurring during base building training that results in it being so effective? What adaptations are occurring that produce results that trump other training methods?

The review above of the physiology of muscle fiber activation and adaptation clearly shows it is NOT aerobic adaptations in slow twitch fibers that is producing the results that occur from base building.  Since greater aerobic adaptations occur in slow twitch fibers at higher training intensities it shows that something other than aerobic adaptations in the slow twitch fiber accounts for the results of base building.

For those who believe the cardiovascular/anaerobic model (i.e. aerobic capacity and changes in aerobic capacity explain/determine performance), the answer to the question is evident in the cited data.

The Answer from the Cardiovascular/Anaerobic Model

The answer to the question, from the standpoint of the cardiovascular/anaerobic model, is that significant aerobic adaptations are occurring in both slow & fast twitch fibers.  These adaptions explain the improved performance.

To expound, running at marathon pace and faster requires significant activation of both slow & fast twitch muscle fibers. You don’t use just slow twitch fibers when running – you use both slow and fast twitch fibers.  Fast twitch fibers are clearly active at running intensities of 60% VO2peak and above.  As the running pace increases fast twitch fibers bear an increasing amount of the workload (marathon pace averages about 75% – 88% VO2peak).

So, easy pace running is causing significant aerobic adaptations in both slow & fast twitch fibers. Since both slow & fast twitch fibers are heavily involved when running at marathon pace & faster, this explains why easy paced running improves performance. It is training the fibers that are used when running at marathon pace (75%-88% VO2peak).

This explains, from the perspective of the c/a model, why “aerobic” training results in faster race performance.

An Incomplete Answer

However, there is still a gaping hole in the above explanation.

The hole in the c/a model explanation is this – significantly greater aerobic adaptations occur in both slow twitch and fast twitch fibers occur at higher training intensities.

If improved performance were primarily due to aerobic adaptations, as preached by the cardiovascular/anaerobic model,  then it logically follows that the training that produces the greatest aerobic adaptations will produce the greatest improvements in performance. Dudley’s research showed that higher intensity training – not lower intensity training – causes the greatest adaptations in both slow & fast twitch fibers.  Base Building proponents preach that lower intensity training causes the greatest gains in performance due to greater development of the aerobic system but the research clearly shows this to NOT be the case.

Standard base building training theory proposes that easy paced running is the best method for developing the aerobic system and that slow twitch muscle fibers are best trained at an intensity of about 70% VO2max. Research contradicts this belief by showing that aerobic adaptations in slow twitch fibers occur at intensities ranging from 63% – 116% VO2peak, with maximum aerobic adaptations occurring at around 94% VO2peak. Research also shows that fast twitch oxidative fibers are very active during easy paced running and that fast twitch glycolytic fibers become active at about 73% VO2peak. In short, easy paced training involves the use of both slow twitch and fast twitch oxidative fibers. Moderate intensity and higher training involves the use of all three major types of muscle fibers.

This leaves us with our question unanswered. If we assume, for the sake of discussion, that base building is as effective as its proponents preach, why is it effective? Clearly the answer is not “greater aerobic adaptations”.

1. An Analysis of Hadd’s Approach to Distance Training, Part 2,
2. Dudley G., Abraham W., Terjung R., Influence of exercise intensity and duration on biochemical adaptations in skeletal muscle J. Appl. Phsiol 1982, 53(4), 844-850
3. Trappe S, Harber M, Creer A, Gallagher P, Slivka D, Minchev K, Whitsett D., Single muscle fiber adaptations with marathon training, J Appl Physiol, 2006, 101: 721-727.



The Physiology of Base Building — 12 Comments

  1. Training at low intensity allows one to significantly increase the running / riding mileage and time. Could the effect of aerobic base building be mental adaption for greater volume of training.
    The first thing to do in this case is to see if the aerobic base building works. If it works we can just say that yes it works but not because of the reasons that have been given up till now.

  2. Hi Rich,

    Just confirming the claim stated ” Claim: You cannot build both your aerobic and anaerobic system at the same time.” as being false? so you actually can build both at the same time yes?no?



    • Hi, Ronald.

      Yes, you can build both the aerobic and anaerobic system at the same time (though not necessarily during the same workout).

      • Hi Rich,

        So you aren’t completely refuting periodisation (which I believed to be wholely accepted); but simply that you can by all means, for instance, throw an anaerobic interval workout into the same period as longer aerobic steady state training? The periodisation theory still holding up because there’s only a certain number of workouts you can fit in a week, and at least 4 weeks of progression for a structured workout is needed to extract results? Hopefully that makes sense.

        It’s just throwing a bit of a curve ball for what I’ve basically learnt from Friel; i.e build up workload, and then build up intensity. I would much prefer to not have to decondition myself to higher HRs for half of the year but then you remove the theory of peaking surely? ‘Always fit’ is supposed to lead to burnout. It’s all a bit confusing!



  3. low output + long time = extended time in window of opportunity (easier, more relaxed, fewer injuries, illness, etc)
    high output + short time = less time in the window of opportunity (burnout, constant reaching for improvements without realizing them, injuries, high effort required, etc)

  4. Just remember that all leading scientists at one time believed the world was flat. Science is just a guess. Before looking at any results you should first ask the question of how the research was funded.
    This in itself will lead you to guess the conclusions before reading the results. I would like to suggest that no sports science is in fact science at all . For science to have any value you have to test like with like. No two human beings are alike. You also must not test one thing in isolation. For any test to have any semblence of science all the people taking part should have the identical diet, should all weigh the same,should all have the same ammount of sleep, suffer from the same ammount of stress and should be genetically identical. Why sports science is regarded as science at all I have no idea. It did not even exist until the universities had no room on their proper courses and had to invent courses for underqualified entrants.

    • Hi, Andrew.

      Wouldn’t the same argument apply to medical science too? No two humans are exactly the same, with identical diets, genetics, etc. Yet, medical science is accepted as a science.

  5. It strikes me that training at an easier pace simply allows one to train longer and more often which is probably why it is supported and encouraged by so many folks.

    I’d like to see an analysis and comparison of the costs of training at different intensities in terms of recovery time, fatigue and muscle fiber damage because in my opinion there must be a sweet spot for every athlete where the costs in terms of fatigue and injury versus the benefits in terms of adaptations will be maximized.

    For example I might improve fastest by training at 90-100% VO2peak but what about recovery and fatigue not to mention how often I would be injured in the process? Human beings are not machines.

    • The article does not talk about HR training as well. The more limes you accumulate, the better adapted is your heart to certain intensities and one doesn’t need to have a high heart rate to extract greater intensities.

      • Pavan,

        “The better adapted is your heart to certain intensities” is part of the cardiovascular/anaerobic theory of endurance performance. However, no proponents of that model have ever shown that a) the heart itself limits endurance performance (so why does the heart adapting improve performance?) or b) that limited ability of the heart to deliver oxygen enriched blood to working muscles limits performance (the heart is a muscle and if it couldn’t get enough oxygen rich blood to itself then intense exercise would always end with a heart attack).


        • The question I don’t see answered here necessarily, how does each intensity affect the metabolic pathway and the balance between aerobic/anaerobic respiration? Overactive anaerobic respiration will starve the aerobic pathway of the pyruvate needed by lowering concentrations available. Meaning the balance between glycogen burning and fat burning will change and the muscle will rely more heavily on the anaerobic pathway for energy. How do we shift the balance to the aerobic pathway? All out high intensity effort seems to increase the muscles aerobic capability the greatest as shown in the research but if it’s being starved by the anaerobic fermentation process it’s not being utilized to its fullest. How do we free up more pyruvate for this bigger aerobic engine? The answer to this question may be a base building period to lower concentrations of the molecules responsible for fermenting pyruvate until close to plateau then adding high intensity exercise to raise the ceiling again. There’s a lot of anecdotal evidence for this stepped process and it seems to me the research here agrees with that in part. You seem to think it means “no base building needed” but I believe you’re jumping the gun here.

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