1. Introduction
Running performance has generally been defined by three key factors: maximal oxygen uptake (VO2 max), the fraction of VO2 max that can be sustained indefinitely (lactate threshold), and running economy. What is interesting about ultramarathons is that a fourth dimension of performance becomes increasingly critical as the event duration goes past about 2 hours, this dimension has been called physiological resilience, or durability. Unlike races with shorter durations (4-60 minutes), where peak performance is dictated by those three physiological metrics, success in ultramarathons depends on how well an athlete resists and delays physiological decline over hours or even days of running. Durability has been defined as an individual’s capability to resist and delay deteriorations (magnitude and time of onset) in physiological profiling (Nuuttila et al., 2024, Maunder et al., 2021). In practical terms this means that after long durations of submaximal exercise your pace slows while physiological metrics like running economy deteriorate. In pro cycling, teams target durability through training, raising the question: can ultrarunners develop this same durability through targeted training?
2.Jargon, and physiological concepts:
In this article I will need to introduce some jargon, so I am including this chart to explain the different intensity domains. Primarily I will stick to the classic 3 zone model of exercise, with zone 1 being anything below the aerobic threshold or LT1. This LT1 is characterized as the first zone where lactate starts to accumulate in the bloodstream. You can still exercise at this intensity for a while, but most well-trained athletes can hold this for about 3 hours, this is why it is commonly referred to as ‘marathon pace’. Zone 2 is the area between LT1 and the second lactate threshold LT2. LT2 is an intensity where lactate accumulation in the bloodstream rapidly outpaces your ability to remove it, and becomes unsustainable after about 40 minutes to an hour. The speeds at which you enter these various physiological zones determines how well you will perform at various race distances. The faster you can run without accumulating lactate the longer you can sustain that pace, and the faster your race times will be. Above LT2 you enter zone three which is an unsustainable zone ranging from about 30 minute all out pace to sprinting. Most refer to any pace above this zone as VO2 max pace. For the sake of this article, we primarily care about LT1 and LT2 as durability is referring to how these two zones change based on being in a fresh or fatigued state. With fatigue these will deteriorate and your LT1 and LT2 will occur at slower and slower paces.
3. Insights from research on durability
Most research on durability occurs in cyclists, as studies requiring participants to run for several hours are much harder to do. Still, these studies provide insights into durability and its role in endurance performance. One study shows shown that while maximal power output and repeatability of efforts do not significantly differ between professional (World Tour) and semi-professional (Pro-Continental) cyclists, the durability of the professional cyclists is far better (Muriel et al., 2021). I wonder whether this is a chicken or an egg scenario. Is there something about high training loads, 3-week long stage races, and cutting-edge nutrition that makes the world tour cyclist superior in these metrics or is it that the world tour cyclist was inherently more durable and are naturally selected for the world tour because of this trait? Anyways, this shows that while the original physiological metrics (VO2max, Economy, and Lactate Threshold) do not differ between these athletes durability does and is the single most important metric for professional cyclists. There is some suggestion in the literature that durability is trainable, with polarized training emphasizing easy efforts below LT1 or above LT2 proving most effective in improving durability (Spragg et al., 2022). This same study showed that easy training below the aerobic threshold has even been shown to enhance 2-minute maximal power output in a fatigued state, which I speculate is due to improvements in submaximal efficiency not necessarily by increasing maximal power output. There is an additional study that shows both low intensity and polarized training increases durability, however this study was done in sedentary and recreational (training <5 hours/week) cyclists so I would speculate that any training would have improved this metric (Matomaki et al 2023).
What causes this decrease in performance after submaximal exercise? Well the causes are still not decided upon, however it has been shown that it is not due to VO2 max decreasing and in fact the magnitude of deterioration does not seem to be related to any “traditional” endurance indicators (VO2max, Economy, and Lactate Threshold), or to total training load (Valenzuela et al 2022). Interestingly, Carbohydrate availability appears to be a major factor in preserving power at lactate threshold during prolonged efforts. A study by Clark et al. shows that consuming 60 g of Carbohydrate per hour during 2-hour heavy-intensity exercise maintained power at LT2 significantly better than a placebo, with CHO supplementation leading to higher muscle glycogen levels, elevated RER, and increased carbohydrate oxidation, all of which correlated with an improved fatigued-LT2 (Clark et al., 2019a, 2019b). Maintaining a higher level of carbohydrate oxidation enhances efficiency and performance when glycogen is available, as the lower oxygen cost of ATP production from carbohydrate compared to fat increases efficiency. This is supported by findings that decreased carbohydrate availability correlates with reduced work output and efficiency (Krogh & Lindhard, 1920; Burke et al., 2019; Passfield & Doust, 2000). This is especially relevant today where we see professional cyclists consuming up to 150g/carbohydrate an hour. This is speculated to be the reason behind record breaking performances on climbs in the Tour de France after 4-5 hours of hard riding. These findings suggest that durability is possibly trainable but more closely tied to fueling strategies, which is good news for us ultrarunners who love carbs!
4. Training Interventions
Cycling and ultrarunning share many similarities in terms of event duration, hard repeated climbing efforts, and decisive moments taking place late in races. Unfortunately, that is where the similarities end, cyclists can coast down hills, and fueling is no problem as they are not constantly jostling the contents of their stomachs like us runners. Lastly, there is no muscular breakdown like there is in long ultra-events, what I believe to be a key component of the durability equation in ultrarunners. In fact, muscle damage induced by downhill running can increase the energetic cost of running by 4-7% which is a greater effect than running in a super shoe vs. non-super shoe (Chen et al., 2007). Luckily this is trainable through something termed the repeated bout effect. Essentially the more frequently you run hard downhills, the easier it becomes metabolically, and the more resilient your muscles become to breakdown (Tallis et al., 2024).
So, we know about the phenomenon, and what some causes of it may be, but how do we train this? First let’s look at training to resist muscle breakdown. This becomes as simple as run more miles, run more frequently and run more hard downhills in your training. What does this look like in practice? Here is some training from yours truly, as well as Kilian Jornet perhaps the most elite ultrarunner in the world. Let’s look at an example of a fairly difficult long run with plenty of elevation gain. In an ideal world the elevation per mile would mimic that of the race you are training to do, however I do not know whether this was the case with Kilian.
Notice how the pace is consistent throughout this run. It might even be advantageous to try and negative split a run like this where you are running faster the later you get into the run. Here is my version of a progression run mimicking the race elevation profile that I would be encountering. I fell off a little bit at the end, but overall, the stimulus was the same.
Another idea is to mix training to resist muscular fatigue with training to resist central nervous system (CNS) fatigue. CNS fatigue is brought about by long durations of fast running, typically in between Lt1 and LT2. I look to marathoners for wisdom here as that is the type of fatigue they are trying to combat most. Elite marathoners like Connor Mantz routinely do ‘fatigued mile repeats’ where they run several miles of tempo just above LT1, a difficult intensity, and at the end of the session they add on 4-5 mile repeats at or above LT2 to practice running fast on tired legs. The ultra-version of this is another workout from Kilian coined the Hill-Flat combo workout. The idea is to get a hard aerobic stimulus up a steep climb essentially fatiguing yourself metabolically, running down that steep incline to tenderize your legs and fatigue your muscles then pick a flat section of road and do some fast running. This fast running could be mile repeats, kilometer repeats, or in his case here a 5K somewhere above LT2.
Or my humbler version here in Utah, where I paired a local peak with 3x2k at threshold. I did these the day after my long run for the week and really felt it improved my durability, time to fatigue, and running economy in a fatigued state.
Lastly, train with high carbohydrate intake on any run over 90 minutes. Push this up and try to hit at least 90g/hour and it will make a difference in your durability and performance in long duration events.
5. Conclusion
In summary, both cycling research and ultrarunning case studies reveal that durability goes beyond traditional metrics like VO2 max, economy, and velocity at LT2, rather it hinges on building muscular and central nervous system resilience. Training interventions such as long runs with challenging descents, or a progression element, to CNS-focused workouts like fatigued repeats and hill-flat combo workouts, enhance your muscular endurance and capacity to resist fatigue and ultimately become more durable. Coupling these interventions with increasing carbohydrate intake during long sessions will collectively boost late-race performance, a key component of ultrarunning.
My final point is where I might lose some of you. Given what we know about durability I think we need to rethink pacing strategies in long ultramarathon events. Traditional wisdom says to go out slow, however muscle damage is not only a function of pace but of duration. Even if you are running at a metabolically sustainable slower pace your muscles will fatigue over the same duration that they would if you ran a bit faster. The only durability study I found in runners showed that even 90 minutes spent at 90% of LT1 (a fairly easy intensity!) induces significant decline in durability (Nuuttila et al., 2024). Do we really believe that starting out at 60% of LT1 will really reduce the magnitude of this decline? I would argue no. I think we should go out faster in ultras expecting this decline and save time early in longer races. Looking at recent elite ultramarathon performances most of them were run in a positive split. For example, Rod Farvard’s 2024 WS100 (7:28 GAP first half vs. 7:55 GAP second half), David Roche’s 2024 Leadville 100 (7:39 vs. 8:51 GAP), and Kilian Jornet’s 2022 UTMB (7:15 vs. 8:24 GAP). If the elites are not durable enough to negative split these races what makes us think we are?
Hopefully you have all found something that you can apply to your own training, and if you have any feedback feel free to reach out to me sky.mcdaniel@gmail.com
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