How Durable Is Your Body?

Traditional fitness tests assess an athlete’s ability to go fast when they’re rested, but this ability is distinct from the ability to go fast when tired - and that's what endurance athletes need most in order to race successfully, as Matt Fitzgerald explains.

Endurance and durability go hand-in-hand. In fact, both words derive from the Latin durum, meaning “hard”. In endurance sports, athletes are generally regarded as durable when they can handle large amounts of training without breaking down. But exercise scientists have recently found a new application for the term that may have important implications for your training.

Many if not most scientific advances emerge from the skeptical questioning of existing scientific assumptions, and durability is no exception. In this case, skeptical questioning of traditional approaches to fitness testing in endurance sports produced new insights about the specific type of fitness endurance athletes need most. Perhaps the best-known endurance fitness test is the functional threshold power (FTP) test, a 20-minute time trial used to assess cycling fitness. Protocols like this one can useful for tracking changes in fitness and for calculating intensity zones. But they’re almost always done with fresh legs a brief warm-up, a convention that caused some scientists to question their relevance to longer races and to wonder what difference it would make if such they were instead done in a fatigued state, after prolonged exercise.

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Turns out it’s a big difference. A 2022 study published in the International Journal of Sports Physiology and Performance reported the results of FTP tests done by twelve professional cyclists. Each subject completed two tests, one after a standard warm-up and another after nearly four hours of low-intensity riding. Average power output in the fatigued test was 2.9 percent lower than in the fresh-legged test, which is about what you’d expect. But the key finding was that performance decrements in the fatigued test varied significantly between individuals. Some athletes performed far worse when they were tired than when they were fresh, while others performance almost as well in a fatigued state. The authors of the study labelled this second group of athletes “durable.”

An interesting question is raised by these findings: Which is the better predictor of race performance—a time trial done with fresh legs or one that’s done in a fatigued state? The answer comes from a study led by Peter Leo of the University of Innsbruck and published in the Journal of Science and Medicine in Sport 2023. Thirty professional cyclists completed a pair of 5-minute power tests, one after a standard warm-up and the other after prolonged low-intensity pedaling. The results of these tests were compared against each cyclist’s race results during a competitive season. Performance in the fatigued 5-minute time trial was found to be a strong predictor of whether a given cyclist achieved at least one podium finish, whereas performance in the fresh 5-minute time trial was not.

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Traditional fitness tests like the FTP test assess an athlete’s ability to go fast when they’re rested. But we now know that this ability is distinct from the ability to go fast when tired (i.e., durability), and it’s durability that endurance athletes need most in order to race successfully.

Okay, but what makes one athlete more durable than another? This question was addressed in another study by Peter Leo’s research team. Ten professional cyclists completed a pair of critical power tests—one fresh, the other fatigued—to assess the durability of each individual. Separately, the cyclists underwent a battery of physiological tests, the results of which were compared against their durability ratings. Factors associated with greater durability included aerobic capacity (i.e., VO2max), fat-burning capacity, and efficiency. In other words, if you want to be durable, you need to strengthen your aerobic engine, get better at burning fat, and become more efficient.

Next question: How should athletes train to improve their durability and its physiological underpinnings? Once again we go to the science. In a third study conducted by Peter Leo’s group, durability and training characteristics were tracked in professional cyclists over the course of a full season. The strongest predictors of improvement in durability were time spent at low intensity and a shift in emphasis from moderate to high intensity as the season unfolded.

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I’m not sure what to make of the second finding, though I suspect it’s specific to professional cycling, where the competitive season is long and grueling and many riders are barely hanging on by the end. But the first finding is straightforward and applies to everyone: No matter what type of endurance athlete you are, or what level you’re at, doing a lot of training at low intensity (i.e., below 80 percent of maximum heart rate) is the key to maximizing durability.

Which is not to say there aren’t other ways to enhance durability. In my own coaching, I have athletes practice going fast when they’re fatigued. After all, that’s exactly what durability is. Fast finishes work well for this purpose. For example, I might have a runner in marathon training complete a half-marathon training run where the first 10 miles are run at low intensity and the last 5K is run at 90 percent effort.

Final question: How do you know if you’re becoming more durable? One option is to do the same fitness tests you’re already doing, except in a fatigued state rather than the usual rested state. For example, instead of warming up for 15 minutes and then completing an FTP test, do the test after a long ride at low intensity.

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I don’t believe that formal testing is strictly necessary to track durability, however. If you notice that you’re able to maintain a faster pace at the same heart rate after, say, 90 minutes of running, or your heart rate is lower at the same pace when you’ve been on your feet a while, you have all the evidence you need that your durability is increasing. At the end of the day, how you measure durability is less important than simply prioritizing it in your training.