Podcast, Science and Physiology

Aerobic decoupling, durability and endurance performance with Daniel Muniz, PhD | EP#343

 June 20, 2022

By  Bernardo Gonçalves


Daniel Muniz - That Triathlon Show

Daniel Muniz, PhD, is a senior lecturer at the University of Hertfordshire. Some of his recent work involves big data analysis (from Strava) of marathon runners, where Daniel and his co-authors set out to explore how decoupling (changes in the ratio of internal to external workload, like heart rate to pace) can be an important parameter in marathon (and possibly broader endurance) performance.

In this Episode you'll learn about:

  • What does decoupling mean and how can it be calculated?
  • How a lesser degree of decoupling is associated with better marathon performance
  • What are the physiological underpinnings of decoupling?
  • What do we know about training interventions to reduce decoupling?
  • Is decoupling independent of other markers of performance (like Critical Speed) or can it be improved by improving more traditional markers of performance?
  • Using big data from Strava to calculate Critical Speed based on training data only

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Daniel's Background

03:01 -

  • I am a Spanish living in the UK and did my PhD at St. Maris University, Twickenham.
  • Now, I work as a physiology lecturer at the University of Hertfordshire, close to London.
  • My PhD was on aerobic capacity, where we tried to quantify anaerobic production.
  • During my PhD, I became interested in critical power and W'. (the anaerobic component of the power duration curve)
  • Since I finished my PhD, I have been working on applying critical power and W' to prescribe exercise and quantifying performance.

Decoupling of internal and external workload during marathon running

The background behind this study

04:56 -

  • The first author of this paper is a meta-analytic, but he also has an interest in marathon running.
  •  When we met, he had a database from Strava runners, and I hypothesised if we could apply the critical power concept to predict running performance only with running data.
  • We did that study and published that work a few years ago. One of the key findings was that the best athletes could complete the marathon at a pace close to the critical speed. The slower runners tend to perform their marathon at a lower percentage of their critical speed.
  • Critical speed and the percentage of critical speed at which runners perform their marathon are factors that influence performance.
  • I thought everyone would run at 90-95 % of their critical speed. A paper from Andy Jones found that elite marathoners run at 96-97 % of their critical power.
  • In our study, we found the average value was 85 %, but people that run a marathon in 2h30 run at 90 % of their critical speed, and that percentage goes down linearly to 75-78 % for runners that complete the marathon in about four hours.
  • With this in mind, we published this paper on aerobic decoupling with Stephen Seiler and others and discussed this topic with some PhD students.
  • We linked all data together, including the database we had from Strava, the information concerning critical speed, the decline in the percentage of critical speed as the marathon time increases, and the decoupling paper.

Research goals

08:53 -

  • We wanted to address if this phenom would happen in marathon runners and see if this would have implications for the average athlete.
  • We found that the database confirms our findings and that the elite marathon runners have a low aerobic decoupling, while the opposite happens with less-fit individuals.

Aerobic decoupling

09:45 -

  • Aerobic decoupling is the relation between the internal stress you face at a specific moment and an external output measure (speed/power).
  • We calculated that ratio early in the race and evaluated how that ratio changes over time. We evaluated the size of the decoupling and where it started.
  • It would be better to have access to VO2 or lactate measurements to evaluate this parameter. However, as we are evaluating data from Strava, we only have access to HR Strava data. 
  • HR is an accessible tool that most athletes use and can evaluate.

How Daniel calculated decoupling

11:23 -

  • We split the data we had into two phases: we have the marathon data and the six months before the marathon.
  • First, we estimated critical speed by setting the overall performance over specific distances. Then, we looked at the marathon data and the ratio between the internal and external workload over 5-km intervals.
  • We ignored the first 5 km because we assumed people were not racing at a constant pace, as they can try to run faster to be in a good position.
  • Afterwards, we normalised the 5-10 km split data and evaluated the ratio. To calculate the size of that decoupling, we looked at the last 5-km segment.
  • We also wanted to see when we would start to see this decoupling. We had to define a starting point, assuming an arbitrary 2.5 % value, and evaluate the point where the decoupling reaches and maintains above 2.5 %.
  • We expected the heart rate to rise slightly, but we saw that the heart rate remained constant, but the speed would decline.
  • The strength of this study is that we had an extensive database, which is regularly not the case in exercise physiology.
  • Of course, we had to make some compromises. For example, HR might not be the best tool, but it is a handy metric.  

How decoupling correlates with the marathon performance

16:21 -

  • We knew that faster runners could run at a higher percentage of their critical speed. So, we hypothesised that faster runners could run close to that critical speed and maintain it for the whole marathon.
  • When you start a marathon, we thought that your critical speed was a specific value, and that value would deteriorate as the race progresses.
  • If you can minimise the decrease in critical speed, you can run at a high percentage of the critical speed for the whole marathon.
  • We saw that athletes that experience a low decoupling could run faster than athletes with higher decoupling amplitudes.
  • Moreover, these athletes can run at a higher percentage of their critical speed.
  • There are some papers from Andy Jones's lab looking at this aspect. They calculated the critical power before and after a couple of hours of cycling at a decent pace.
  • They saw a 10 % decrease in critical power, which matches our data.

Relation between the onset of decoupling and performance

19:02 -

  • The onset could also predict performance based on the data we had.
  • There seems to be a linear rate of decoupling. If your decoupling starts early, you will have a more extended decoupling.
  • However, I believe the decoupling size is more critical than the onset.
  • We might have athletes that could see some decoupling, but the decoupling rate is not so high. Therefore, the magnitude of decoupling would be the primary factor influencing your ability to run at close to your critical speed.

Key takeaways from this study

21:20 -

  • First, do not assume the physiology at the beginning of the race will be the same as the physiology 2-3 hours into the race.
  • There will be some deterioration in your physiology. Therefore, we need to look at your physiology will change after a couple of hours of exercise.
  • Decoupling might not be the best way of measuring fatigue, but it is something athletes can use regularly.

Practical applications of this study

23:24 -

  • We must be aware of your durability during a prolonged bout of exercise. Therefore, I would recommend athletes evaluate decoupling in the long run.
  • Moreover, if you start to see some decoupling, it is something athletes should notice during the race.
  • Mark Burnley talked about incorporating critical speed, W' and those concepts and using all the new technology available.
  • I do think that in a few years, your Garmin will incorporate some metrics that link your physiological stress (HR, for example) and your speed.
  • We are only starting researching this point, so there will be much work on implementing this for athletes to use regularly.

Tips for athletes that want to use the information

25:33 -

  • I would try to measure decoupling in the weeks leading into the marathon, particularly in those sessions that will be similar to the event.
  • If you go for a 30-35 km run, I would pay attention to the workload (your HR) and your external workload (speed) and see how that ratio changes over time.
  • However, if we detect decoupling, we do not have data on what to do to minimise the magnitude of the decoupling. We do not know why it is happening and what training sessions would do to address that issue.
  • If you exercise at your "predicted marathon pace" and start seeing a decoupling of 10 %, it would be a red flag, and you would have to revisit the pacing strategy.

Mechanisms behind decoupling

29:02 -

  • We can speculate on things that might affect decoupling.
  • This process is complex, and I do not think there is reliable data on what determines the magnitude of decoupling.
  • We can address the muscle fibres and the amount of type I fibres that are more fatigue-resistant, the glycogen depletion you experience during the race, thermal regulation, mitochondrial function and density. All these factors are likely to contribute to the decoupling someone experiences.
  • In particular, muscle damage could be a factor that contributes to the decoupling. Your running economy and efficiency will deteriorate to a greater extent than during cycling because you have an eccentric component.

Correlation between decoupling and critical speed

31:33 -

  • We do not know if those runners that show low decoupling have that ability because of their training. Or if it is the other way around.
  • An athlete might have some characteristics that allow them to do specific training and run at a high percentage of critical speed and maintain it throughout the marathon.
  • I believe it might be a combination of your physiological straits and the training you do that will affect the decoupling.

Decoupling differences between men and women

33:21 -

  • Women have a low critical speed but run at a high percentage of their critical speed and exhibit low decoupling compared to male athletes.
  • Some research shows that female athletes have a threshold of a high percentage of their VO2max. Therefore, it shows that females are more fatigue-resistance than male athletes.
  • The reasons for this could be from the factors mentioned that affect decoupling.
  • Women typically have more type-I muscle fibres and a higher volume and density of mitochondria. These factors explain why women can better prevent the decline in critical speed.
  • It seems that the longer you go, the lower the gap between male and female athletes. I do not think female athletes will outperform male athletes, but they seem more fatigue-resistant.

Future research on this topic

36:43 -

  • One limitation of this approach was the way we calculated critical speed. We evaluated critical power by evaluating performance over different distances.
  • However, it does not mean that those were your best performances.
  • Therefore, we did a follow-up study where we grouped a couple of people, asked them to train regularly for six weeks, and asked them to make maximal intentional efforts.
  • The values we obtained from critical speed from regular training were lower than those obtained from the maximal efforts.
  • However, the difference was not so high. (about 1 km/h)
  • Based on the training data, we also want to predict the decoupling we saw in the marathon. In that way, we might get some information on what will be helpful to minimise the decoupling.

Three pieces of advice to improve performance

39:39 -

  • First, run a lot of miles. Then, run some at a pace faster than run speed and rest.
  • Do your best in every session and rest properly. If you follow the basic principles, it will take you a long way. 

Rapid-fire questions

40:38 -

What is your favourite book, blog or resource?

The Sports Gene: Inside the Science of Extraordinary Athletic Performance by David Epstein

What is an important habit that benefited athletically, professionally or personally?

Talk with people and discuss with anyone you come up with because people regularly lose to talk about their research.

Who is someone you have looked up to or who has inspired you?

Andy Jones has been a reference because of his work in critical power for me in the last few years. And Mark Burnley was someone that inspired me as well.


Bernardo Gonçalves

Bernardo is a Portuguese elite cyclist and co-founder of SpeedEdge Performance, a company focused on optimising cycling and triathlon performance. He writes the shownotes for That Triathlon Show, and also produces social media content for each new episode.

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