How to Build Durability That Lasts 100 Miles (#5)

For decades, endurance performance has been defined by VO₂max, lactate threshold, and efficiency, but there’s a fourth variable that often gets overlooked called durability. This episode breaks down what durability actually is, why athletes with similar fitness can perform very differently late in races, and how to measure it through real-world data like heart rate drift. We also cover how to build durability through training, so you can maintain your performance when fatigue starts to take over.

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HOST

Cliff Pittman is the Coaching Development Director at CTS, leading the Ultrarunning and Cycling Coaching staff with a specialty in guiding athletes from first-time ultrarunners to elite competitors at races like Western States 100, Leadville 100, and the Triple Crown of 200s. A competitive trail and ultra athlete himself, Cliff brings firsthand experience and a rare ability to turn complex training science into simple, actionable coaching.

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Resources:

• https://onlinelibrary.wiley.com/doi/10.1002/ejsc.70073

Transcript

The Missing Fourth Performance Variable

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If you've spent any time learning about endurance training, you've probably heard the same three variables come up over and over again. VO2 Max, the ceiling of your aerobic engine, lactate threshold, the upper edge of sustainable effort, and running economy, how efficiently you convert effort into movement. Now these three have dominated exercise physiology and training conversation for decades. For really good reason. They're real, they're measurable, frankly, they matter. But there's a fourth variable that doesn't get nearly the same amount of attention, and an ultrarunning specifically may matter more than any of them. And that variable is durability. I'm Cliff Pittman, pro ultrarunning coach and director of coaching at CTS. I coach athletes from first-time ultrarunners to professional and world-class competitors. Today we're talking about what durability actually is, why it's defining performance variable in long distance racing, and how to measure it in your own training. So let's start with the problem that durability is actually trying to solve. VO2 Max, lactate threshold, and running economy are all measured in the lab during very short, controlled protocols that tell you what your physiology looks like when you're fresh. And that's very useful information. But it has significant limitation. It doesn't tell you what happens to those values after three hours of running, or five or ten, or twenty plus. Two athletes can walk into a race with identical aerobic profiles, the same VO2 max, identical threshold pace, nearly identical running economy. And by mile 60, they can be in completely different physiological states. One of them has held their aerobic qualities close to their starting baseline. The other has drifted significantly. Their heart rate is climbing at the same pace, their breathing is heavier, and their form is breaking down. Now the athlete who has drifted less is the more durable athlete. And in a sport decided over hours and not minutes, that quality is often what separates a well-executed race from a slow, painful death marching collapse. Now that's durability, the ability to preserve your aerobic qualities deep into a long effort when fatigue has been accumulating for hours and everything starts to cost more. Now, durability has two dimensions and is worth being very clear about both. The first is aerobic durability, how well your cardiovascular and metabolic systems hold their function as effort accumulates. Can your threshold pace stay where it started? Is your heart rate staying proportional to your effort? Or is it climbing while your pace stays flat? Now the second is muscular durability, how well your legs continue to produce force and maintain efficient movement mechanics under hours of accumulated load. And that's a separate video. Are you still moving efficiently in mile 50 or have your quads given up and taken your form with them? Now both matter. And most athletes, one tends to fail before the other. And knowing which one is your limiter is genuinely useful information when it comes to building a training plan. But today we're focused primarily on the aerobic side. So what does aerobic durability look like in practice? Well, there's a phenomenon called physiological decoupling that sits right at the center of this. And under normal conditions, heart rate and pace maintain a predictable relationship. Your run at a given effort, your heart rate reflects that effort, and that relationship stays relatively stable. Now, as fatigue accumulates during a long effort, that relationship starts to break down. Heart rate starts to climb, breathing gets heavier, but pace either holds steady or even degrades. Now the internal cost of maintaining the same external output is going up. That drift or that decoupling is not random noise. It's a direct signal of aerobic durability loss happening in real time. Now, a 2025 study in the European Journal of Applied Physiology examined exactly this. Researchers tracked how much an athlete's aerobic threshold degraded after approximately two and a half hours of sustained effort. And what they found was that the athletes whose heart rate and breathing rate drifted the most showed the greatest degradation in aerobic threshold. The athletes who drifted the least, unsurprisingly, were the most durable. Now that connection between decoupling and durability validates what coaches have observed for years and gives athletes a concrete, field-based way to measurement. So how do you actually measure your own durability? Well, the gold standard is a lab assessment, a pre and post-aerobic threshold test built around extended effort. And if you have access to that, use it. Most athletes don't though, and they don't need it to get useful information. The most practical tool many of you already have is sitting in your training peaks account. It's called aerobic decoupling or pace to heart rate, the ratio pace to heart rate in the first half of a run compared to the second half. A decoupling percentage under 5% on a long aerobic run generally indicates really good aerobic durability for that effort. Consistently high decoupling across your long runs is a signal worth paying attention to. If a temperature rise during your run and you're not keeping up with fluid losses, then that's going to show up as decoupling too. See, it's not always a fitness signal, and context always matters. The most important thing to understand is that decoupling is the signal, and durability is what produces or prevents it, which brings us how to build it. And this is why and where I want to be direct with you, because the answer is not complicated and it's not what most people want to hear. Alright, you build aerobic durability through volume, accumulated over time at the right intensity, specifically through your long runs done consistently on terrain that resembles what you're training for. So there is no shortcut, no clever protocol, supplement, hack, or wearable that builds it for you. The long run is the most specific durability stimulus available. And you cannot adapt to three hours of sustained aerobic effort without doing three hours of sustained aerobic effort. Now, for newer athletes, frequency comes first. How many days per week you run. And then it's total volume, how much you run per week. Then the long run as the primary durability stimulus after that. Now, for more advanced athletes who have built that foundation, training density becomes a very useful tool, which is back-to-back long runs or concentrated volume blocks running on fatigue legs. The intensity piece matters too. Easy days need to be easy enough to actually be easy. Hard days need to be hard enough to actually count. If you want to understand how intensity distribution maps to your training zones, the zones video covers that in detail. But the foundation under all of it is volume. Run consistently, run long, and run specifically. Pay attention to what your body tells you on those long aerobic efforts. Whether that's a number on your watch, a metric in your training software, or simply the awareness that your breathing has shifted while your pace hasn't. Okay, here's the bottom line. VO2 Max tells you how high your aerobic ceiling is, lactate threshold tells you where your sustainable edge is. Running economy tells you how efficiently you move through space. And durability tells you how much of all three you actually have left when the race gets hard. And ultra running, that's often the whole game. Now, if you want coaching that builds durability as a deliberate part of your training plan, visit trainwright.com to connect with the CTS coach and subscribe for more coach driven ultra running education.