Physiology Friday #271: Is Durability the Most Underrated Trait in Endurance Performance?
Your VO₂ max changes after a few hours of exercise. Here's what it means for training and racing.
Greetings!
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Most runners are obsessed with the “big three” pillars of endurance performance: VO₂max, running economy, and lactate threshold. We test them, train toward them, and use them to help predict our race times. But we so often treat these markers like fixed physiological constants, when in reality, they aren’t stable across a race or a long training session—they degrade as the miles pile up.
How Resilient Are You?
Durability, in a physiological context, refers to how well you can maintain your fitness over time—how long you can preserve your running economy, lactate threshold, and aerobic power before they start to deteriorate. It also goes by the name of physiological resilience.
Think about it this way. If VO₂max is your ceiling, durability is how well you hold the ceiling up after 60, 90, or 120 minutes of exercise (or more). It’s the difference between two runners with similar lab metrics, one of whom fades badly in the final miles of a marathon, while the other holds strong. Durability is not about how fast you can go when fresh—it’s about how little you slow down when fatigued. In a 100-meter race, Usain Bolt might not win because he runs faster (as an absolute speed) than the other guys, but because he slows down the least during the race. We can think about resilience in these terms as it applies to endurance exercise.
The durability concept is gaining attention in sports science. Recent work by scientists like Andy Jones and others has highlighted how small changes in running economy or VO₂max—even just 5%—can have a massive impact on race performance, especially late in endurance events.
Yet most physiological testing still happens in the “fresh” state. VO₂max and lactate threshold are measured at rest and economy is captured before fatigue sets in. That’s a problem—because marathons aren’t run in a fresh state. They’re run in a state of progressive physiological deterioration.
That brings us to a new study that asks: what actually happens to these variables after 90–120 minutes of steady, submaximal running? The answer could reshape how you train and pace for long races.1
To test the idea of “durability,” researchers recruited 14 well-trained male marathoners, each with an average VO₂max of 63.1 ml/kg/min and recent marathon times under 2:50–serious athletes who were fit enough to make any physiological degradation meaningful and measurable.
The researchers brought each runner into the lab on three separate occasions: once for baseline testing of VO₂max (maximal oxygen uptake), lactate thresholds (LT), running economy (RE), and fractional utilization (FULT), or the percentage of VO₂max being used at lactate threshold.
Another visit involved testing the same variables after runners completed a 90-minute treadmill run at 79% of their VO₂max—a steady effort in the “heavy” intensity domain, designed to simulate marathon race effort without pushing into anaerobic territory.
The final visit was the same as above, but this time the steady-state run was extended to 120 minutes.
Importantly, the researchers didn’t just measure performance outcomes—they looked at what was happening under the hood. They recorded oxygen uptake (VO₂), respiratory exchange ratio (RER), heart rate, blood lactate levels, and perceptual effort. And they tracked how these changed over time, both within the long runs and in the follow-up testing immediately after.
This was one of the first studies to measure all three “determinants” of endurance performance in both a fresh and fatigued state in trained runners.
Here’s what they found.
After 90 minutes, runners showed a 3.1% drop in VO₂peak. After 120 minutes, it was down 7.1% from baseline. That’s a massive shift. Simply running at a steady effort reduced how much oxygen these runners could consume at max effort.
What caused this? The researchers noted corresponding reductions in heart rate max, ventilation, and blood lactate at exhaustion—all signs that the limiting factor wasn’t muscle oxygen demand, but rather central drive and cardiovascular strain. In plain terms: their engines didn’t rev as high.
RE, measured as oxygen cost at a fixed submaximal speed, worsened by 4.2% after 90 minutes and 5.8% after 120. This deterioration was steady and linear, likely driven by subtle biomechanical changes, muscle fiber fatigue (especially in type I fibers), and possibly neuromuscular recruitment, causing runners to burn more fuel to go the same speed.
FULT increased over time, not because the lactate threshold improved, but because VO₂max dropped. After 120 minutes, runners were working at 82.8% of their now-lowered VO₂peak, compared to 78.6% when fresh. That means the relative effort required to maintain their marathon pace increased significantly—even though the absolute pace stayed the same.
The speed at lactate threshold (sLT) also declined by 6.6% across the 2-hour run, dropping from 14.0 km/h (8.7 mph) to 13.0 km/h (8.1 mph). sLT was also lower after the 90-minute run (13.5 km/h or 8.4 mph)—a 3% drop from the unfatigued state.
Perhaps most telling, runners’ fractional utilization during the run (FUrun) climbed from 79% to 91%, pushing several into the severe intensity domain (>95%)—a zone that’s unsustainable over time. In other words, the pace that would feel manageable early on became physiologically expensive by the end.
These changes represent measurable reductions in aerobic capacity, efficiency, and threshold. In racing terms, the cost of maintaining one’s early pace keeps rising until it likely becomes unsustainable. If you’ve ever raced a half or full marathon, this might sound familiar.
Finally, it’s worth noting the large individual variability in the changes in the physiological determinants. From unfatigued to 120 min, large between- participant differences were found for changes in each performance determinant, ranging from 0%–16%, and for sLT, ranging from 1%–14%). The individual differences found despite the similar marathon performance capabilities of the participants highlight the importance of measuring fatigued physiology to inform training and racing strategies.
This study gives us a detailed physiological map of what’s happening during a collapse, bonk, or whatever you want to call it—it’s not just a matter of willpower or muscle glycogen levels running too low. And it paints a far more dynamic picture of endurance physiology than we’re used to seeing. Most training and pacing strategies still rely on static metrics. You test your VO₂max, threshold, or critical speed in a fresh state, then extrapolate race pace from there. It’s what all of us do.
But this study shows that those numbers don’t hold under prolonged effort. If you’re planning your marathon based solely on what you can do at mile 3, you’re probably overestimating what you’ll be able to do at mile 23. It’s perhaps one reason why most race prediction calculators—which fail to factor in durability—prove to not be as accurate as we’d like them to be.
A few minor things to consider when interpreting the results of this study. One was that the participants ingested minimal carbohydrates during the 90- and 120-minute runs—just 30 grams per hour which is half of the lower end of the recommended 60 grams or more per hour. Second was that they didn’t wear advanced footwear (i.e., “super shoes”), which confidently improve running economy and likely enhance durability. I bet that with adequate fueling and super shoes, the deterioration observed in this study in all parameters would be somewhat less severe.
Durability can really become a difference-maker. VO₂max is important, but so is how long you can hold onto it. The same goes for your economy and lactate threshold. What I think is most exciting about the concept of durability is that we can improve our performance by training it—even if we don’t improve our aerobic capacity, lactate threshold, or other performance measures outright (if your ceiling stays the same).
Rather than train for fitness, train for resilience. You might be surprised where it takes you.
Thanks for reading. See you next Friday.
~Brady~
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The big question for me is how can you train your durability? Is it just running longer and faster or is there any other recommendation?
Also, how does this translate to an ultra endurance context like an ultra marathon? I guess the deterioration is slower because the effort is lower but I wonder if we can do some sort of test offer 12 hours.
Good stuff! I think when I train to optimize… I want to be as fresh as possible. When I train for resilience or toughness… I really want to pre-fatigue.
It’d be interesting to know how much of durability is physical vs mental.