Most endurance athletes I come across are familiar with terms such as “VO2 max”, “threshold”, and “movement economy”. These are all physiological characteristics of an athlete that can be measured in a laboratory (or estimated in the field), monitored over time to assess progress, and used to inform decisions related to training prescription and race pacing. There is a plethora of research that has accumulated in the past few decades demonstrating that higher VO2 max, greater lactate thresholds, and better movement economy are all associated with better performance as an endurance athlete, both among elites and amateurs alike (1). However, there is much more to an endurance athlete than just these three physiological characteristics. The concept of “durability” is a relatively novel one that proposes that endurance athletes experience a degradation in their VO2 max, threshold, and movement economy over time during prolonged exercise (2). For example, if you have a threshold run pace of 7:00 min/mile, your threshold run pace is likely not still 7:00 min/mile two hours into a marathon as you have accumulated fatigue and your original threshold was set when you were fresh.
Have you ever noticed on a long bike ride, a long run, or during a long race like an Ironman or a half-Ironman that your heart rate changes over time? Usually, heart rate and perception of effort increases over time even if speed/pace or power output remains constant. This is because there is really no such thing as true “steady state” exercise. The cost of a given exercise output will increase over time as glycogen stores are depleted, as body temperature rises, and as fatigue sets in. This is essentially an athletes durability. How long can an athlete go before fatigue sets in and the physiological cost of maintaining their current pace or power increases?
It is important to know that there are huge differences in durability between athletes. For example, two athletes might have the exact same threshold power output on the bike (250 watts), but one athlete might be able to hold 225 watts for 3 hours before reaching exhaustion whereas the other athlete might only be able to hold 225 watts for 75 minutes before they reach exhaustion. This is a huge difference between athletes that both might look equally “as fit” on paper if one only looked at their threshold power. This scenario is applicable to all endurance activities, not just cycling. Typically, as an athlete gets fitter and gains more years of training experience, their durability usually improves and they are able to hold paces and power outputs for longer periods of time before fatiguing, including paces and power outputs across all ranges of intensities. This is one physiological characteristic that separates professionals from most amateurs. Typically, professional endurance athletes can hold high percentages of their VO2 max or threshold for very long periods of time when compared to amateurs. This is related to greater durability!
Most athletes over-obsess with their functional threshold power on the bike, or their threshold pace on the run, or their 100, 500, or 1,000-yard time in the swim. While these metrics are an important piece of the overall puzzle that makes up a complete athlete profile, these are not everything. Let’s say you have a current threshold run pace of 7:15 min/mile (the pace you could hold for approximately an hour, give or take). You train for 12 weeks and it improves to 6:50 min/mile. You would be happy with this, of course, as would a coach. Now let’s say you train for another 12 weeks and you don’t see any improvement in your threshold run pace, but you can hold a 7:15 min/mile pace for 30-45 minutes longer than you ever could before as demonstrated by setting a new half-marathon best of 1:35:00 whereas before you could only hold 7:15 min/mile pace for a 10K. This improvement would obviously make you very happy, as would it make any coach happy as well. The improvement you saw in being able to hold 7:15 min/mile for a half-marathon distance run as opposed to just a 10K run is likely indicative of an improvement in your durability. In other words, you could hold a 7:15 min/mile pace for longer without fatiguing and slowing down. This is improved fitness even without an improved threshold run pace!
What Contributes to an Athlete’s Durability?
What makes an athlete more durable? Well, usually the things that end up fatiguing us and cause us to slow down are related to our ability to regulate our temperature, our muscles’ ability to contract repeatedly for prolonged periods of time, our ability to move through space with as little energy consumption as possible, our glycogen storage capacity, among other factors. So, an improvement in any of these arenas would likely lead to an improvement in durability.
For example, an increase in blood plasma volume due to regular training and exposure to training in hot conditions would give you an improved ability to regulate your core temperature and dissipate heat, thereby allowing you to hold a constant pace or power output for longer in warm and cooler conditions.
Another example, an improved capacity to metabolize fat for fuel at submaximal exercise intensities would allow you to spare glycogen utilization and allow your body to exercise for longer periods of time without running out of glycogen to fuel exercise. This would ultimately mean you could exercise for longer without fatiguing and slowing down.
One more example, an improvement in running economy from years and years of consistent low-intensity training would allow you to run at a given exercise intensity with less oxygen consumption and a lower utilization of calories to fuel that exercise. This has a myriad of physiological effects that ultimately allow you to run for longer with less effort, therefore accumulating less fatigue and delaying the need to slow down.
How Does an Athlete Train Durability?
The research surrounding the concept of durability is still relatively, somewhat vague, and many factors relate to an improvement in durability. However, training for improved durability is essentially training to resist fatigue for longer, and there are many approaches we know that help us in resisting fatigue for longer.
There are essentially two types of durability that an endurance athlete might want to train:
the ability to hold steady or constant paces or power outputs for prolonged periods of time (e.g., steady state races like in a non-drafting triathlon, a marathon, ultra-marathon, marathon swim, etc.)
the ability to perform repeated bouts of very high-intensity exercise over and over again (e.g., stochastic races like in a bicycle criterium race or a mountain bike race)
So, in order to train durability, you have to know which type of durability you want to train for. Are you a distance runner looking to run a fast marathon? Or are you a cyclist looking to win a local criterium?
First and foremost, all endurance athletes will want to train their body’s ability to move efficiently (movement economy), their body’s ability to metabolize and use fat for fuel, their body’s ability to regulate its’ internal temperature, their body’s ability to store glycogen, and their body’s ability to intake and deliver oxygen to the working muscles. This is not an exhaustive list of the qualities athletes will want to train, but it captures some of the big ones. How do you train to improve these characteristics? We know that training consistently, training for long periods of time, and spending a lot of time training at easy/aerobic exercise intensities can improve many of these characteristics mentioned above. One of the biggest mistakes most endurance athletes make is not training enough at lower intensities and training too much at moderately-hard intensities. A lot of durability comes with training at low intensities and training a lot as this improves so many physiological and metabolic qualities within an athlete that makes them more durable and better able to resist fatigue. In order to train a lot, you have to manage your intensity as an athlete, and it is for this reason that a polarized training approach is so critical for any endurance athlete. I have written on the topic of a polarized training approach previously in many of my posts, but if you are unfamiliar with this evidence-based method of managing intensity distribution in training, I would highly recommend reading the published paper by Hydren and colleagues from 2015 (3) or listen to a podcast I did on this topic.
Next, and as I mentioned previously, you will then have to consider what type of race or event you are targeting. If you are training for a steady-state race like an Ironman or a marathon, you will want to spend more time training at your target race paces, power outputs, or efforts as your body will become more efficient at this pace/power/effort the more you train at it. This is essentially what “competition-specific” preparation is within a periodized training plan. During this phase, an athlete shifts their focus from more general fitness development to more specific fitness development to match the demands of race day, including spending lots of time at and around target race pace, power, and effort. Example training sessions might look like the following:
a 90-min run comprised of: 10-min warm-up, 20-min @ steady pace, 20-min @ goal Ironman 70.3 pace, 20-min @ faster than goal Ironman 70.3 pace, 10-min cool-down
a 4-hour long ride comprised of: 1-hour warm-up, 2 hours Ironman power (50-min @ Ironman power, 10-min harder than Ironman power, x2), 1-hour easy cool-down
If you are training for a more stochastic event in nature, like a mountain bike race or a criterium bike race, you will want to train your ability to do repeated high-intensity efforts or power outputs over and over again even under fatigue. This usually requires, again, specific training during the competition-specific phase of your plan. Example training sessions might look like the following:
a 120-min bike ride comprised of: 15-min warm-up, 30 rounds of 1-min @ 400 watts with 2-min recovery spin (90-min), 15-min cool-down
a 3-hour long ride with 10 x 30-sec sprints w/ 150-sec of recovery at the end of the ride
Managing training intensity distribution and training consistently for months and years is likely the biggest training-related factor that contributes to improved durability. However, other training-related factors can also yield improvements that make you a more durable athlete. For example, durability might also be improved by training your ability to thermoregulate and dissipate heat as well as your ability to use fat as fuel, both of which I have written about previously here (heat on training/racing) and here (fasted training). It is plausible that an appropriately designed strength training program can improve durability due to strength improvements and efficiency gains. I have also written on the topic of strength training for endurance athletes here and here.
Again, the research on the topic of durability is still relatively new and a little vague, but essentially, anything that improves your body’s ability to tolerate exercise intensities for longer periods of time or to do repeated high-intensity bouts of exercise for longer could be thought of as being related to an improvement in durability.
How Does an Athlete Monitor or Track Changes in Durability Over Time?
Monitoring improvements in durability is a little tricky as there is not necessarily a single measure or metric that we can test for to tell us how durable you are as an athlete. More broadly speaking, if you can hold power outputs on the bike, paces on the run, or paces in the water for longer periods of time than you used to be able to with the same heart rate or a lower heart rate, then that would be indicative of an improvement in your durability. On a more granular level, however, it becomes more difficult.
Dr. Stephen Seiler, one of the leading researchers on the topic of durability, typically quantifies durability with a “decoupling” metric within a training session (3). He tracks internal load (% heart rate reserve) and external load (% of 6-min maximum power on the bike or pace on the run) over the course of a training session or race. As heart rate rises over time, there can be a noticeable decoupling of the heart rate and power output or heart rate and pace. The more these “decouple” from each other, the less durable that athlete was in that session or race essentially. To get a better sense of how this looks, I would encourage you to check out the paper published by Maunder and Seiler from 2021 (3) as there are graphical depictions of this decoupling metric using some athlete case profiles.
For those of you that use TrainingPeaks, they generate a metric they call “aerobic decoupling” for bike and run sessions that is similar to the one Dr. Stephen Seiler uses. The “aerobic decoupling” metric essentially shows you how much your heart rate drifted in comparison to your pace or power output over the course of the training session or race by taking your average pace/power and heart rate from the first half of your session and comparing it to your average pace/power and heart rate from the second half of your session. The higher the number, or the higher the percentage as it is expressed in TrainingPeaks, the greater the decoupling between your heart rate and power/pace. The more durable an athlete is, the less their heart rate will decouple from their pace or power output in sessions or in races.
With this aerobic decoupling metric in TrainingPeaks, one might look to this metric over time to see if heart rate decouples less and less from power or pace. The key here is to make sure you compare similar training sessions (both in intensity and duration) that were performed on similar terrain and in similar environmental conditions. For example, if you do 90-min aerobic long runs on a similar loop pretty often, these could be great sessions to look at the aerobic decoupling metric for signs of improved durability. The same could be done for a standard long bike ride you do frequently. Just remember, environmental conditions and terrain play a large role in influencing your heart rate response to typical paces or power outputs. So, when conditions change and get more challenging (i.e., it gets warmer, you go to higher altitude, etc.), know that these sessions may not be best to compare with sessions done in easier conditions (i.e., cooler temperatures, lower altitudes). However, in general, the aerobic decoupling metric can be a great metric to look at over time to see changes in your durability. It is also a great metric to look at for races as well because a very high decoupling metric may indicate a weakness in your durability. If you are going to look at this metric often in training sessions and races, just be sure you use an accurate heart rate monitor (chest strap ideally) and you gather pace and power data accurately (GPS watch and valid and reliable power meter).
To provide a practical example, I have included two figures below showing half-marathon run splits for a single athlete from Ironman 70.3 Arizona in 2017 and then Ironman 70.3 Arizona 2019. The run split in 2019 was at a faster pace (7:41 min/mile compared to 8:17 min/mile), done at a similar average heart rate (170 bpm compared to 173 bpm), and completed with a lower aerobic decoupling metric (Pa:HR or 6.52% compared to Pa:HR of 7.87%). You will also notice that the athlete walked many more times (primarily through the aid stations) in 2017 and far fewer times in 2019. The weather conditions for these races were comparable (it was a little warmer in 2017) and the run courses were only slightly different (run course changed in 2019 but was very similar). The athlete also got fitter in terms of their threshold pace on the run and accumulated two years of consistent training. Taken together and comparing these two race files, the athlete likely improved their durability, alongside other fitness improvements, between 2017 and 2019 due to a faster half-marathon at a similar heart rate and a lower aerobic decoupling metric. However, the aerobic decoupling metric from 2019 was still a little high at 6.52% (TrainingPeaks usually suggests <5% indicates decent aerobic durability and aerobic fitness; https://www.trainingpeaks.com/blog/aerobic-endurance-and-decoupling/, indicating that this athlete could spend more time improving their durability and aerobic endurance.
Figure 1. 2017 Ironman 70.3 Arizona Run Split
Figure 2. 2019 Ironman 70.3 Arizona Run Split
As endurance athletes, there is no doubt that physiological characteristics such as VO2 max, lactate threshold, and movement economy are important factors determining success. However, durability is an often overlooked and under-appreciated physiological quality that has recently been getting more attention in the research literature as an important determinant of endurance success. Herein, I highlighted what durability is, why it matters, how to train it, and how to monitor it over time. Next time you are out on a coffee shop ride with your buddies, push the usual conversation of functional threshold power aside and have a chat about durability! After all, functional threshold power doesn’t matter much if you don’t have the durability to actually sustain that power output without fatiguing!
1. Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. The Journal of physiology. 2008 Jan 1;586(1):35-44.
2. Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The Importance of ‘Durability’in the Physiological Profiling of Endurance Athletes. Sports Medicine. 2021 Apr 22:1-0.
3. Hydren JR, Cohen BS. Current scientific evidence for a polarized cardiovascular endurance training model. The Journal of Strength & Conditioning Research. 2015 Dec 1;29(12):3523-30.
Happy training and racing!
-Ryan Eckert, MS, CSCS
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