Durability, Resilience, Fatigability: What Sports Science Is Actually Arguing About

If you've read anything about "durability" in cycling and triathlon lately, you've probably absorbed a tidy origin story: scientists noticed a fourth quality beyond VO2max, threshold and economy, gave it a name, and built a test for it. That's true as far as it goes, and we've told a version of it before. What's happened since is less tidy and more useful: the people who study this have spent 2025 and into 2026 arguing, in print, about what to actually call the thing — and the argument isn't academic throat-clearing. It changes which number you should trust when your watch says one thing and your legs say another.

This article assumes you already know the basic idea of durability as a late-race concern. It covers the live terminology dispute, the practical split hiding inside it, and an honest look at the claim — that durability is independent of your engine size — that gets repeated more confidently than the evidence supports.

Why this matters if you're not a sports scientist

You might reasonably ask why an age-grouper training 6–10 hours a week should care what a paper calls this quality. Here's the practical answer: which term a study uses tells you which metric it measured, and different metrics can disagree without either being wrong. If your heart-rate decoupling number looks fine on a long ride but your legs feel hollow in the final 20 minutes, that isn't a contradiction — it's two different constructs giving you different information. Understanding the split lets you stop treating a single number as the whole truth about your fatigue resistance.

Where the idea came from — briefly, because we've covered this

The short version, which "Fresh Watts vs. Hour-Four Watts" already told in more depth: endurance performance has long been modelled on three pillars — VO2max, the sustainable fraction of it, and economy or efficiency, a framework attributed to Joyner. Andrew Jones argues that because all three are measured in a rested state, they miss something that matters for anyone racing longer than an hour: "there is a 'fourth dimension' which might be termed resilience or durability", because "measurements made in [a fresh] condition will be incomplete given that the athletes will be in some state of fatigue for a high proportion of their endurance sports event."

The word "durability" is credited to Maunder and colleagues in a 2021 Sports Medicine paper. That paper is blocked to automated readers, so we're citing it as reported by two reviews that quote it verbatim: Jones and colleagues state durability was "first coined by Maunder and colleagues in 2021", and reproduce it as "durability, or resilience to prolonged exercise-induced deteriorations in physiological profiling variables during prolonged exercise", with a fuller version — "the time of onset and the magnitude of any deterioration in physiological profiling characteristics over time during prolonged exercise" — in a second review. Notice the two parts: onset and magnitude. Almost every test you'll encounter, including the one in our earlier article, measures only magnitude; nobody has really operationalised onset yet.

None of this is new to science, even if the label is. A 2025 review states plainly that "the concept of what we now call durability has been known for decades and is described in various forms", and that coaches have long noticed riders who hold power late without owning the biggest engine — an old observation getting formal definitions and formal tests, not a new one.

The argument nobody outside sports science is watching

Since mid-2025, four groups of researchers have published, in direct response to one another, genuinely different positions on what to call this quality and whether it's even one thing — an active dispute, with named authors responding to named authors in the same journals.

Position A: just use the words interchangeably

A 2025 review of race-simulation protocols takes the practical route: it defines the construct as "the ability to withstand the functional decline following acute and/or chronic stressor," or "to limit a decline in an individual's performance after accumulated work compared to the same performance conducted in a fresh state," and states it will use "physiological resilience, durability, fatigability or fatigue resistance" interchangeably throughout.

Position B: resilience is the better umbrella term

Jones and colleagues disagree with that flattening. Their preference: "'resilience,' defined as 'the ability to resist functional decline following acute and/or chronic stressors,' provides an accurate definition of the concept" — resilience as the right umbrella precisely because it isn't limited to one metric.

Position C: these are four different things and conflating them is a mistake

The most forceful position comes from a 2025 Viewpoint in the Journal of Applied Physiology, which argues that using these words interchangeably "risks obscuring the conceptual distinctions between them" and proposes explicit, separate definitions (full text):

• Durability — "deterioration in physiological characteristics over time during prolonged exercise."
• Fatigability — "the (in)ability to repeat efforts at workloads corresponding to or near mean maximal power output," elsewhere described as the rate and magnitude of strain and performance loss within a single continuous task.
• Repeatability — "capacity to recover and reproduce high-intensity performance across multiple bouts, stages or heats with passive recovery in between."
• Resilience — "the capacity to maintain or regain physiological and mechanical function under conditions of fatigue, environmental extremes, or other perturbations" — the broadest term, because it stretches beyond pure fatigue resistance into adaptability to environmental and psychological stress.

Position D: that four-way split doesn't survive real racing

A 2026 letter in the European Journal of Applied Physiology pushes back directly on Position C, by name. In real, variable-intensity racing, the authors argue, "repeated high intensity demands and the progressive decoupling of internal and external load occur simultaneously, making a clear conceptual separation between fatigability and durability difficult," and repeatability overlaps so heavily with performance durability that it "may therefore be better viewed as a context-specific manifestation or extension of performance durability rather than as a fully distinct construct." They also flag a caveat worth remembering the next time you see "durability" applied to a sprint distance: the framework is "limited to a specific subset of endurance events (i.e., those > 60 min) and may not be generalizable." Meixner and colleagues have since published a formal reply — bibliographically confirmed, dated 2025 — but its full text wasn't accessible to us, so we won't characterise its content beyond noting the exchange is ongoing.

Four positions, two journals, 18 months, no resolution. When a coach or an app tells you flatly "this measures your durability," the scientists who built the concept can't currently agree on what durability, as distinct from three neighbouring terms, actually is.

The split that actually matters to your training data

Buried inside the dispute is a distinction that survives all four positions and is directly useful. Restated in the 2026 letter, it runs like this: "Physiological durability refers to physiological demands and is characterized by the magnitude and timing of the decoupling between internal (e.g., heart rate, oxygen consumption) and external load (pace, power) during prolonged exercise", while "performance durability pertains to external load metrics and is operationalized by maximal mean power or pace, or the corresponding power- or pace-duration profile, assessed after a defined amount of work has been completed."

A separate group, Zanini and colleagues, draw essentially the same line: durability as "the ability to resist deterioration of performance speed/power," and physiological resilience as "the ability to sustain physiological functions (e.g., VO2max and RE)" — reported independently in a 2026 European Journal of Sport Science paper that itself flags the terminology as "under scientific debate," and again in a 2026 Scandinavian Journal of Medicine & Science in Sports paper. The primary letter (Journal of Applied Physiology, 2025) is bibliographically confirmed but inaccessible to us, so we rely on these two secondary sources.

In plain terms: physiological durability is what your heart-rate-decoupling app or your drift number is trying to catch — the gap between what your body is working at internally and what you're actually producing externally. Performance durability is simpler and blunter: how many watts, or how fast a pace, can you still produce after accumulating work, full stop.

These two things don't have to move together. Pooled data across four studies of trained cyclists found heart-rate decoupling correlated strongly with the fall in power at the first ventilatory threshold — a physiological-durability signal. But that decoupling number tells you nothing about W′, the finite reserve above threshold that lets you surge, which the same broader evidence base shows can drop by roughly half after a comparatively modest dose of hard work while VO2max and lactate threshold stay untouched. A rider can have textbook-flat decoupling and still find they have nothing left to close a gap in the final kilometre — a performance-durability failure the decoupling number was never built to see. If you've been treating one number as the full verdict on your fatigue resistance, this is why it sometimes contradicts how you actually felt.

One more fork worth knowing: "durability" vs "fatigue resistance"

A 2025 systematic review draws a distinction most coaching content quietly erases: "Unlike fatigue resistance, which focuses on the ability to maintain performance despite fatigue, durability emphasizes the onset and magnitude of performance deterioration over time during prolonged exercise, often assessed by measuring performance after different accumulated work quantities." The same review admits "there is a lack of standardised terminology in the literature, leading to the often interchangeable use of fatigue resistance, durability and physiological resilience," and reports that roughly 30% of the studies it eventually included had to be found by manual screening, because inconsistent language defeated a standard keyword search — a real, measurable cost of the terminology confusion.

Is durability really independent of your engine size? Probably in elites, unclear in age-groupers

The claim you'll see repeated most — that durability is a genuinely separate, fourth parameter, unrelated to VO2max, threshold or efficiency — is the load-bearing justification for calling it a "fourth dimension" at all. The evidence for it is moderate, and more mixed than the confident framing usually lets on.

On the supporting side: in 12 male professional cyclists (VO2max 83.0 ± 3.6 mL·kg⁻¹·min⁻¹), 20-minute power fell from 386 ± 29 W fresh to 375 ± 28 W after roughly 4 hours and 40 kJ·kg⁻¹ of work, and "no significant associations were found between the power-output decay and any of the analyzed indicators" — not ventilatory threshold, not peak power, not VO2max, not recent training load. A study of 12 elite cyclists found the same pattern after a 4-hour intermittent protocol: "correlation analyses showed no strong associations" with fresh-state lab measures. A 2025 amateur study concluded outright that "fatigue resistance does not appear to be dependent on traditional parameters of endurance performance."

But it isn't unanimous. Jones's own review notes a conflicting finding — that cyclists with less critical-power decline also tended to have better gross efficiency, lower carbohydrate oxidation and higher traditional aerobic-fitness values — and concludes "additional studies are required to resolve whether resilience can be predicted from standard laboratory- or field-based measurements of physiological status." A season-long study of semi-professional cyclists adds a wrinkle: durability and fresh power don't track together across a year, with durability improving more over a season than fresh performance parameters did — observational, not a controlled test of independence.

Here's the honest point we haven't seen made elsewhere: almost every study supporting "independence" was run in professional or elite male cyclists with VO2max values between 73 and 83 mL·kg⁻¹·min⁻¹ — a narrow band. A sample that homogeneous will tend to suppress any correlation with that variable almost by construction, whether or not a real relationship exists more broadly, and none of the papers we reviewed raise that caveat themselves. When researchers looked at a more varied, amateur sample, the picture changed: VO2max differed significantly between more- and less-successful amateur racers (56.6 ± 8.1 vs 47.4 ± 2.3 mL·kg⁻¹·min⁻¹, p = 0.02), as did weekly training volume (11.1 ± 1.7 vs 8.2 ± 1.7 hours, p = 0.01), according to the same amateur research group. The one dataset that looks anything like TriForward's readership suggests durability and classic fitness plausibly are related in age-groupers, even if they decouple at the elite end where everyone's VO2max already sits in a narrow, high band.

That's not a reason to dismiss durability as a concept — it's a reason to be careful about repeating "durability is independent of your engine" as settled fact for an audience that looks nothing like a professional peloton.

What to actually do with this

You don't need to resolve this dispute to train sensibly. Three takeaways:

• Stop expecting one number to tell the whole story. If you track HR decoupling on long rides and also do an occasional post-fatigue power test, treat them as measuring two different things — not two readings of the same fact. A flat decoupling trend and a disappointing late-ride power number can both be true at once.
• Don't assume durability makes your engine irrelevant. The evidence that VO2max and training volume don't matter to durability comes almost entirely from athletes who all have similarly enormous engines. Building your aerobic capacity and threshold isn't wasted effort just because a professional-cyclist study found it uncorrelated with late-race power.
• Ask which half of the elephant a durability claim is describing: drift in heart rate and thresholds, or watts still on tap. The two questions have different answers and different training implications.

Common mistakes

The first mistake is treating durability, fatigue resistance, resilience and fatigability as one word with four spellings, because the practitioners you follow probably do — several of the reviews we cite openly admit doing this for simplicity. That's a reasonable podcast shortcut, but it will confuse you when you try to compare two apps quietly measuring different things under the same label.

The second mistake is repeating "durability is a fourth, independent parameter, unrelated to your engine" as a blanket truth. That claim rests almost entirely on elite, VO2max-homogeneous samples, and the one amateur dataset that exists points the other way.

The third mistake is assuming a durability test tells you both onset and magnitude of decline. Maunder's original definition includes onset — when decline starts — as well as magnitude. Nearly every test you'll encounter, including the ones in earlier TriForward coverage, measures only the end-state magnitude; you have no good way to know whether your decline starts at kilometre 60 or kilometre 140.

How to apply this week

This isn't a workout-prescription topic, so the practical step for someone training 6–10 hours a week is a data-interpretation habit, not a session. On your next long ride, note your HR-decoupling trend and, separately, how your legs felt in the final 20–30 minutes when you pushed. If the two signals disagree — decoupling looks fine but your legs felt empty, or vice versa — don't treat it as noise; write down which one it was. Over a training block, that log tells you whether your limiter is physiological drift or raw power availability, a more useful diagnostic than either number alone, for 30 seconds of note-taking after a ride you were already doing.

References

• Jones, The fourth dimension: physiological resilience as an independent determinant of endurance exercise performance, Journal of Physiology, 2023
• Jones & Kirby, Physiological Resilience: What Is It and How Might It Be Trained?, Scandinavian Journal of Medicine & Science in Sports, 2025
• Hunter, Maunder, Jones, Gallo & Muniz-Pumares, Durability as an index of endurance exercise performance: Methodological considerations, Experimental Physiology, 2025
• Sánchez-Jiménez et al., Is intensity the most important factor in determining the amount of prior work accumulated that affects cyclists' acute durability? A systematic review, European Journal of Applied Physiology, 2025
• Barsumyan, Soost, Shyla, Graw, Bliemel & Burchard, Durability as an independent parameter of endurance performance in cycling, BMC Sports Science, Medicine and Rehabilitation, 2025
• Peeters, Barrett & Podlogar, What is a cycling race simulation anyway: a review on protocols to assess durability in cycling, European Journal of Applied Physiology, 2025
• Meixner, Joyner & Sperlich, Durability, Fatigability, Repeatability and Resilience in Endurance Sports: Definitions, Distinctions, and Implications, Journal of Applied Physiology, 2025 (PubMed)
• Meixner, Joyner & Sperlich, full text PDF
• Schwalm, Schaffarczyk, Fohrmann, Hollander & Gronwald, Terminology missing distinction and implication: Nuances of physiological and performance durability, European Journal of Applied Physiology, 2026
• Meixner et al., Last Word on Viewpoint, Journal of Applied Physiology, 2025 (PubMed)
• Zanini, Jones & Nybo letter, Journal of Applied Physiology, 2025 (PubMed)
• Malinen, Nuuttila, Matomäki, Uusitalo & Kyröläinen, Test-Retest Reliability of Physiological Resilience During and After Prolonged Moderate-Intensity Running in Well-Trained Runners, European Journal of Sport Science, 2026
• Norte et al., Scandinavian Journal of Medicine & Science in Sports, 2026
• Valenzuela, Alejo, Ozcoidi, Lucia, Santalla & Barranco-Gil, Durability in Professional Cyclists: A Field Study, International Journal of Sports Physiology and Performance, 2022/2023
• Ørtenblad, Zachariassen, Nielsen & Gejl, Substrate utilization and durability during prolonged intermittent exercise in elite road cyclists, European Journal of Applied Physiology, 2024
• Voet, Lamberts, Viribay, de Koning & van Erp, Durability and Underlying Physiological Factors: How Do They Change Throughout a Cycling Season in Semiprofessional Cyclists?, International Journal of Sports Physiology and Performance, 2024
• Barsumyan, Soost & Burchard, Enhanced durability predicts success in amateur road cycling, Frontiers in Sports and Active Living, 2025