Podcast, Science and Physiology

Critical Power and VO2 kinetics with Mark Burnley, PhD | EP#257

 October 26, 2020

By  Mikael Eriksson


Mark Burnley

Mark Burnley, PhD, is an exercise physiologist at the University of Kent. Mark's main research interests include oxygen uptake (VO2) kinetics, where an important application is how to optimally use warm-ups, as well as Critical Power, which forms the transition point (or transition area) between the heavy and severe exercise domains. Because of the non-steady-state nature of exercising above CP, it has significant importance in a large number of training and racing applications.

In this Episode you'll learn about:

  • VO2 kinetics - what does it mean? 
  • The fast and slow components oxygen uptake
  • Heavy priming exercise and its effects on VO2 kinetics (or the importance of a good, specific warm-up)
  • Critical Power (CP) - what is it and how does it compare to other "threshold concepts"
  • How to assess CP and use it in training
  • How fatigue develops differently in the different exercise intensity domains (moderate, heavy, severe) and interventions to minimise fatigue

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04:20 -

  • My name is Mark Burnley I am an exercise physiologist at University of Kent.

    Very early on I got involved into the science behind VO2 kinetics and critical power.

    During my under- and postgraduate years I was very much into cycling, running and triathlon and I wanted to apply the science to my interests in these sport.
  • In 2012 I started to focus more on neuromuscular fatigue and how it is related to different metabolic threshold, which is what my research has been focused on in the last 10 years or so.

VO2 kinetics 

06:25 -

  • Very basically, the VO2 kinetics is the curve of how the oxygen uptake responds to an increased level of exercise (can be any kind of modality), the science behind VO2 kinetics is trying to determine what affects the shape of the curve, which phases it goes through and how it is related to fatigue and ultimately performance.
  • When we engage in an activity, the metabolic demand for oxygen rises instantly, however, the oxygen uptake cannot respond in an instant manner and hence there is a delay until the oxygen uptake reaches a steady state, which usually takes around 2-3mins.

    The phase until the oxygen uptake reaches a steady state is called ”the fast phase”.

    There are situations in which the oxygen uptake does not reach a state state, the most obvious example of such is work at an intensity above the aerobic threshold, when the oxygen uptake increase until maximum oxygen uptake is reached.

    At intensities above critical power, the oxygen uptake continues to rise, but does so in a much slowly manner (around 10 times slower than the fast phase), and this phase is consequently referred to as ”the slow phase”.

    A few theories exist about why the oxygen uptake continues to rise above threshold intensity, and one such theory is that new muscle fibers are recruited in order to keep up with the high intensity and these muscle fibers require oxygen.

    The muscle fiber composition is probably also a factor that plays into the kinetics of the slow phase, the larger your pool of fast twitch muscle fibers you have, the bigger the slow component is likely to be.
  • Plenty of research has been conducted on how to improve the fast phase of the VO2 kinetics curve, but the only practically viable way of speeding up this component is by endurance training and it does not seem to matter what kind of endurance training (high intensity or low intensity).

    The opposite applies to the slow phase of the curve, which is very logical since fatigue resistance is improved by endurance training/increased fitness and hence a decreased need to recruit new muscle fibers that pushes the oxygen demand up.

    This would also explain why people with a higher composition of slow twitch fibers express a less prominent slow phase, since the slow twitch fibers are very fatigue resistant by nature.

Warm up and priming

23:50 -

  • Research on warm up or exercise ”priming” was the subject on my dissertation back in the early 2000:s.

    In summary, what we did find was that heavy exercise, i.e. work above the anaerobic threshold, performed as a bout 10mins prior to a time trial or ”time to exertion” test does indeed increase both performance and the time to exhaustion.

    However, it seems like the work has to be performed on a rather high intensity (above threshold), work at a medium intensity does not seem to effect VO2 kinetics and hence also performance.

    We could show these results both in cycling and middle distance running, and to my knowledge the British athletics are still implementing this strategy as part of their race warm up.
  • Mechanism wise, we could show that the duration of the fast phase (until the oxygen uptake reaches a steady state) becomes shorter after a heavy exercise priming that is conducted 10-12mins before the actual race or test.

    This means that less energy in the start of the race or test is coming from anaerobic energy sources and hence the ”anaerobic debt” is less at the start of the race, which enables the athletes to go longer on a given power or maintain a higher power during a given duration.

    Shortly one can say that the heavy exercise warm up basically is ”priming” the aerobic system to react more quickly to greater energy demands.

    It seems like the sweet spot to ”hit” intensity wise on the warm up/priming is between 3-5 mmol/l, the intensity that get you to this kind of lactate levels seems to be the kind of intensity to aim for during the warm up.

    For those unaccustomed to blood lactate levels, this should represent 5-10mins of work at around threshold pace or power.
  • However, for the Ironman (or even Ironman70.3) distance, VO2 kinetics is not too big of an issue since the slow phase is basically non-existent during these long distance event and the fast phase represent such an incredibly small percentage of the total race.

Critical power

41:00 -

  • In VO2 kinetics, critical power is the final intensity at which the oxygen uptake can reach a steady state, above critical power, the oxygen uptake will continue to rise until VO2max is reached.

    From doing several all out tests for different durations, one can also extract a power duration curve that in theory represents the maximal power that you can sustain for any given duration.
  • An important parameter determining critical power is W´, which is the amount of work (defined as kilojoules) that an athlete can produce above his or hers critical power.

    Critical power and functional threshold power (FTP) tend to come out as fairly similar when they are analyzed, however, when maximum lactate steady state is measured, it normally does seem to come out 20-30w lower than the critical power or FTP.

    However, I am of the opinion that this probably merely reflects a degree of error in measuring the different parameters.

    When it comes to using ones critical power, FTP or maximum lactate steady state during competition for pacing etc., I use to recommend people who race at fairly short distances (up to 1h) to look more at the critical power or FTP, while athletes racing at the half Ironman or the Ironman distance would probably find it more beneficial to use their maximum lactate steady state as their starting point as a precautionary measure in order not to risk over pacing oneself.

Adapting critical power in training

55:05 -

  • If you want to be sure to get a physiological training response from training above the critical power, one should target an intensity around 20-30w above the calculated critical power, as you then have eliminated most calculation errors and you will be certain of that you’re doing training above threshold.
  • When it comes to threshold training, my recommendation would be to start slightly below your calculated threshold and then try and titrate the intensity up until you find that balance of what feels sustainable and what’s not.

    However, one must point out that the physiological response to threshold training does not vary too much if you’re 10-20w below or above your threshold, it’s going to be rather similar in both cases.
  • In regards to estimating critical power, my advice for most athletes would be to do a somewhere around 30min close to all out time trial, which will get you very close to your critical power.

    This can then be combined with a three minute all out time trial in order to estimate W´, these two tests would get you a good idea of where you stand physiologically.

    However, only having two data points in a two parameter model creates a large room for error (to get a 97.5 % confidence intervals, the power range could be up to 60w), which is why I recommend athletes to do 4-5 different duration time trials, as you then can limit the certainty to a range of around only 15w.

Fatigue in different intensity domain

1:12:40 -

  • Most research on fatigue has been done in the high intensity domains (basically since it is easier to study).
  • When it comes to fatigue occurring at moderate intensities, the energy stores are mostly not an issue since the intensity is so relatively low, instead factors such as central fatigue, i.e. ”brain fatigue” which basically is lack of motivation as well as recruitment of muscle fibers and (especially in running) soft tissue and muscle damage that accumulate over time.
  • When one reaches the first lactate threshold, the energy starts to become an issue (it must exist enough energy substrates for this kind of intensity), together with heat accumulation (if the conditions are such), even blood glucose levels shrink and affect the ability to keep on exercising, also, central fatigue does play an important part at this intensity as well.
  • In the high intensity domain (above critical power), energy substrates is once again going to be a central mediator of fatigue, but here also muscle contraction hampering molecules such as inorganic phosphate, changes in calcium signaling, calcium phosphate, hydrogen ions (lactate acidoses) etc. start to build up and reduces the ability of the muscle to contract and produce power.

    Here it is important to point out that research has shown that one can expect a decrease in critical power of around 5-10 % after 1-2h of heavy exercise, an affect that interestingly seems to be able to become less prominent if the athlete takes in carbohydrate based sports drink.

Practical take aways

1:20:30 -

  • In my opinion, it can be very useful for athletes to know their critical power and also have an idea of what their W´ is as these parameters can help you achieve the kind of physiological responses that you wish to get.
  • In terms of interventions for reducing fatigue processes, I think that nutrition and hydration plays an absolutely integral part to be able to sustain a certain intensity as well as keep up motivation (”brain fatigue”), here caffein can also be an important substance to pay extra attention to and take in, in the right amount at the right time.

Rapid fire questions

1:26:45 -

  • What is your favorite book, blog or resource related to endurance sport? Endure by Alex Hutchinson.
  • What is a personal habit that has helped you achieve success? I would say that I am good at looking at things from different perspectives and ”shake” things around a little bit to see things from new angles.
  • Who is somebody who has inspired you or have looked up to? Brian Whipp, the man who basically founded VO2 kinetics, he was a phenomenally intelligent man who still inspires me to this day.


Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

Please contact me if you have feedback on the podcast or want to make suggestions for improvement or send in a question for a Q&A episode.

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