Aerobic vs Anaerobic Exercise: It’s Missing the Point

I recently saw a question on the 220 Triathlon twitter feed from The Swim Guru on whether anaerobic work should be included within training sessions.

In this Blog, I’ll do my best to answer the question from a personal perspective.

Working anaerobically
Working anaerobically?

Science and exercise intensity

As I’m a chartered scientist, it wouldn’t take Sherlock Holmes to guess that I love science. That’s because I love observing, quantifying and analysing to better understand phenomena.

In endurance sport exercise intensity is demarcated into different training zones using threshold intensities such as lactate threshold, Fatmax, Critical Power, VO2max and so on. All these threshold values are based on factors relating to aerobic or anaerobic metabolism. However, to think about exercise intensity in terms of only these metabolic factors may be missing the point. Rather, doing so may be limiting training effectiveness and I tell you why.

Like many things in my life, I have ‘light-bulb’ moments when things that were completely confusing become immediately obvious. One such moment occurred during the write-up of my Ph.D. when I read a very brief paper by Mark Hargreaves: which totally changed my view on endurance performance.

Hargreaves stated the obvious, that the body is a complex system not simply a metabolic one. Rather, the human psyche, neuromuscular activation and skeletal muscle are all implicated in fatigue processes, and thus endurance performance.

My conclusion was that fatigue (or training zones) should not be considered in metabolic terms alone. This is despite virtually every textbook and training manual making such an elementary error.  I was guilty too, focusing my complete thesis on a few tiny areas of science. Hargreaves highlighted how naive I was being. He changed my (scientific) life.

Working aerobically?
Working aerobically?

What is a threshold?

Aerobic and anaerobic metabolism occur on the same continuum with rest being at the lower end and surpra-maximal intensity being at the higher. Human beings like clear differentiation between things and threshold points are used for such demarcation.

Over many previous months in the laboratory and staring at excel spreadsheets, I’d been trying to identify the first and second ventilatory thresholds (VT1 and VT2), using the plotting method below. For all intents and purposes, this is the same as identifying aerobic and anaerobic thresholds. Occasionally, I’d see clearly distinguished thresholds in the data but more-often-than-not  it was messy.  The diagram below took many hours of ‘trawling’ my data to find the set that best represented what Wasserman et al.’s Principles of Exercise Testing and Interpretation was describing.

Data 1

Typically scientific data is presented in such a very clean way. After all messy data or stuff that is difficult to interpret rarely progresses beyond a journal editors desk. However, the more academic literature I reviewed around thresholds, the more I became confused. Researchers argued about the mechanisms and mediating factors of thresholds, using different terminology to describe similar phenomena and the same terminology to describe quite different phenomena!

My conclusion was that this lack of consensus was because the term “threshold” does not perfectly fit physiological processes. Rather, changes in physiological these processes occur on a sliding continuum, without a clear threshold but as a transitional or inflection point (supported by Meyer et al., 2004).

Furthermore, factors such as the the human psyche and neuromuscular activation processes would have to be wholly aligned for threshold intensities to adequately describe fatigue. I reached the conclusion that exercise intensity should not be described only in terms of aerobic or anaerobic metabolism.

More physiology

An area that I also studied was oxygen uptake kinetics, which took me a few years to get to grips. This is despite the subject being relatively simple. The diagram below illustrates virtually everything a coach needs to know about the subject. It shows the oxygen uptake response for a bike rider who’s initially pedaling at an easy 100W. He instantaneously increases power to Functional Threshold Power.      

Data 2

As the rider is relatively well trained, it takes around 2 minutes for him to reach the point where a steady aerobic energy turnover is achieved. The aerobic steady-rate requirement for this rider is 3500ml of oxygen per minute. However, for the initial two minutes the rider is in energy deficit, having to make up for this shortfall through anaerobic pathways. Of course, this is beautifully clean data, it was measured in a lab using £40k worth of equipment and I still had to spent an hour or so to ‘reduce the noise’ in the data by eyeballing then applying a statistical filter! In reality, data in the ‘real world’ would not look like this.

In reality we rarely maintain a constant power/speed (because of choppy water, other people, hills, wind and poor pacing) so our oxygen requirement is never truly constant. This means that our aerobic and anaerobic systems are both ‘switched on’ all of the time. This is particularly true for interval sets (including most swim sessions), fartleks and when running on hilly terrain. Any increase in power/speed, even below the ‘anaerobic threshold’, requires a contribution from anaerobic pathways. Therefore, there is no such thing as either aerobic or anaerobic exercise.  They are not mutually exclusive!!! 

Understanding this in a triathlon context

Imagine a swimmer is doing 10 x 100m in the pool at their fastest maintainable pace with minimal recovery (circa 20 sec per 100m faster than CSS pace with 15secs recovery). A swim coach may call this a ‘lactate tolerance’ set (although they would be talking nonsense…lactic acid is a fuel and tolerance infers suffering….we should love the burn!!!).

For the first 100’s in the set, this would require about a 60% aerobic contribution and a 40% anaerobic contribution to total energy turnover. Theoretically, the  ratio would change as the set continued because the swimmer would be depleting anaerobic stores and not be able to resynthesise this energy quickly enough in recovery. A further likely outcome is that technique falls to bits, pace drops rapidly, as does the anaerobic contribution. Sub-optimal training will result!

Many would describe such a session as an anaerobic workout even though aerobic metabolism dominates! However, I think having such a debate is immaterial to performance because of the demands of the sport.

The demands of endurance sport

There are three reasons why we train:

  1. To get faster (achieved by increasing the applied force and/or rate of force whilst minimising the forces acting to slow us down)
  2. To minimise/prevent injury
  3. To have fun (earning money doesn’t really count…….because the WTC etc have crap prize funds).

Now to use a Kirkland anecdote to explain more!

A few years ago I was at a testing session in a leading lab with one of the world’s best age-groupers….who I hope will read this. He’d just done an anaerobic test and we had an amusing conversation:

“Wow……..your numbers are equivalent to Chris Boardman’s on that test”

“Really” the athlete said with a grin on his face!

“Yep…….his were similar to a 12 year old girl’s too”

The bottom line is that this athlete who is good enough to finish on the age-group podium in Hawaii is unlikely to have his performance limited by the lack of ability to work anaerobically. The same can be said for any non-drafting triathlete in the world. It’s simply not a demand of the sport!

It terms of (1) to go faster no triathlete needs to undertake training to specifically improve anaerobic capacity. Never! However, this is semantics as ‘smashing it’ has its place. Here are my reasons:

  • Training hard and training fast hurts and if done right, it helps reduce the rating of perceived exertion at sub-maximal intensities…… i.e. training hard makes racing seem easier….thus it takes into account psyche!
  • Training above FTP/CSS taxes the aerobic systems more than training below it. Therefore, with adequate recovery, it promotes aerobic adaptations such as improved VO2max.
  • Maximal type training can up-regulate aerobic enzymes such as phosphofructokinase which enhances oxygen delivery and hence performance
  • It can be used to develop strength and speed
  • It makes us feel good, satisfies the ego and it can hit #3……to have fun!

I’m gonna stick to the 80% train low, 20% train high ratio as a rough rule of thumb. That’s because:

  • Training close to and above FTP/CSS relies heavily on carbohydrate metabolism (aerobic and anaerobic glycolysis) which reduces exercise efficiency when compared to fat oxidation. Doing too much high-intensity work compromises endurance
  • High-intensity training often compromises subsequent training sessions
  • Training should primarily be specific to the demands of the event that you’re racing in
  • Most triathletes are unable to maintain good technique when going fast!


So there we go! I believe there is a time and a place for triathletes to work above an FTP/CSS intensity but there’s very few circumstances where training should target improving anaerobic capacity. If however you’re racing ITU, gimme a shout and I’ll revise what I say to you!

All the energy is depleted!
All the energy is depleted!

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