Trail Research Weekly #6: Defining the Fourth Dimension of Endurance Performance

Endurance performance has previously been explained through three pillars: VO²max, lactate threshold (LT), and running economy (RE). Between them, they do a reasonable job of predicting how fast someone can run on fresh legs, in a lab, on a treadmill, and over short distances. What they don’t explain particularly well is what happens on hour six of a mountain ultra, when your legs are trashed, the sun is down, and you are not even halfway done.

Sport scientists have proposed several new constructs to fill that gap: a so-called ‘fourth dimension’. Four terms have been used interchangeably: durability, fatigability, repeatability, and resilience. A recent article by Meixner, Joyner, and Sperlich (2025) argues that they are not interchangeable. Using the incorrect terminology is causing problems in how we assess and measure an athletes performance, as well as addressing those gaps with the appropriate training.

This paper is worth unpacking in detail because the confusion it addresses shows up constantly in trail-running conversation. We say an athlete is “resilient” when we might mean they are “durable.” We say they “fatigue easily” when we might actually mean they “don’t repeat efforts well.” These aren’t just semantics, so let’s go through each one and define what they actually describe.

Durability: the slow leak

Durability refers to the deterioration of physiological characteristics over the course of prolonged exercise, such as marathon running, ultra-trail running, or long-distance triathlon. It captures the athlete’s ability to keep their power output, speed, or economy roughly constant as hours accumulate, resisting drift in markers such as critical power, VO²max, and heart rate.

Mechanistically, durability diminishes through cardiovascular drift, neuromuscular fatigue, and shifting substrate use. Picture a continuous, steady-state effort where the line on the graph slopes gently downward from start to finish.

Practically, durability is assessed through prolonged exercise protocols, generally over 90 minutes, tracking drift in heart rate, oxygen uptake, and lactate or ventilatory thresholds over time. Encouragingly, the evidence so far suggests durability responds to training. Heavy strength training and a mix of high- and low-intensity endurance training have been shown to impact durability.

Fatigability: how fast you break down under intensity

Fatigability is related to durability but conceptually different. It’s defined as the rate and magnitude of physiological and performance losses that accumulate over the course of a continuous task, without the benefit of recovery. Where durability is about the size of the decline over the whole event, fatigability is about how quickly that decline sets in once the intensity gets stochastic and demanding, think surges, breakaways, and climbs without rest.

The paper stresses that fatigability reflects the vulnerability to decline under load, largely independent of how well an athlete recovers between efforts. An athlete can have low fatigability (they hold up well within a hard, continuous push) while still having poor recovery capacity between separate efforts. Those are two different weaknesses, and they need two different training responses.

Fatigability is best evaluated by measuring how quickly performance decays, in power, velocity, or heart rate, during continuous or repeated tasks with no recovery, ideally tested against both fresh and pre-fatigued states. The training implication is fairly intuitive: time spent at low-to-moderate intensities builds the oxidative capacity and metabolic efficiency that blunts fatigability.

Repeatability: how well you bounce back between efforts

Repeatability describes an athlete’s capacity to recover and reproduce high-intensity performance across multiple bouts, stages, or heats, with passive recovery in between. Where fatigability is about within-task decay, repeatability is about between-task recovery.

This is the construct that separates an athlete who can nail one brutal climb from one who can nail the same climb again tomorrow, and the day after that. It depends heavily on energy depletion, neuromuscular recovery, and hormonal and metabolic recovery processes between bouts. In multistage events, this is often the deciding factor: not who has the fastest single stage, but who can produce a strong stage on day four.

The paper notes that repeatability appears to improve with maturation and accumulated training experience, likely through better fatigue resistance and metabolic efficiency built up over several years, not weeks.

Resilience: the umbrella term

Resilience is presented as the broadest of the four constructs. It’s described as the capacity to maintain or regain physiological and mechanical function under conditions of fatigue, environmental extremes, or other perturbations, extending beyond pure fatigue resistance to include an athlete’s adaptability to heat, altitude, dehydration, and psychological stress.

This is the construct that best explains why some athletes fall apart in the heat or at altitude despite being prepared, and why others seem to find a way to hold on. It’s assessed by deliberately exposing athletes to controlled stressors (heat, hypoxia, cognitive load, pain) and measuring the extent to which performance is preserved relative to optimal conditions.

Why the distinctions matter for training

Here’s where the paper gets useful for coaching.

• An athlete can have high durability but low resilience, and that combination will show up specifically in events where heat, altitude, or psychological adversity is important.

• An athlete can have high durability but poor repeatability, performing well in a single long time trial while struggling badly in a multistage event that demands repeated high-intensity days.

• Even small degradations in RE or LT under fatigue can meaningfully shift performance in extreme, sustained efforts, underlining that durability and resilience aren’t marginal factors; they can be decisive ones.

For trail runners, this maps onto race-day performance. Someone who trains on a treadmill in a cool gym might be very durable, and then unravel entirely on a hot, technical, multi-day stage race, not because they lack fitness, but because their resilience and repeatability were never challenged.

Key takeaways

• Durability, fatigability, repeatability, and resilience describe four distinct physiological capacities. Durability is decline over a continuous, prolonged effort. Fatigability is how quickly you crack under stochastic, high-load conditions. Repeatability is how well you bounce back between separate efforts. Resilience is how well you hold up under added stressors like heat, altitude, or mental fatigue.
• An athlete can be strong in one of these and weak in another, and that combination determines which race formats suit them.
• None of these constructs yet has a single, standardised test, so field-based, sport-specific assessment is currently more useful than trying to force a lab number onto them.
• All four appear to be trainable.

What this means for runners and coaches

The takeaway from this paper is that a single VO2max or LT test tells you relatively little about how an athlete will hold up on the back half of a mountain ultra, or on day three of a stage race. If you want a realistic view of an athlete’s capacity, test the specific quality that matters for their target event.

Training for a single long, steady mountain ultra with minimal course variability, prioritise durability work: long, continuous efforts that build resistance to cardiovascular drift and economy decline. Training for an event with repeated surges and technical terrain, prioritise fatigability work under stochastic, unrested loads. Training for a stage race or a back-to-back race weekend, prioritise repeatability, meaning structured high-intensity sessions separated by realistic recovery windows. Training for a hot, high-altitude, or psychologically brutal event, prioritise resilience through heat and altitude acclimatisation, and deliberate practice under discomfort.

The clearest practical message from the authors is this: stop treating these words as interchangeable descriptions of “toughness,” and start treating them as separate training targets. Coaches who define and prioritise these qualities based on what actually matters for their athlete’s specific race, not generic toughness, are more likely to get it right.

Reference

Meixner, B.J., Joyner, M.J. and Sperlich, B. (2025) ‘Durability, fatigability, repeatability, and resilience in endurance sports: definitions, distinctions, and implications’, Journal of Applied Physiology, 139(1), pp. 1703–1709.

FAQs

What is durability in endurance sport?

Durability in endurance sport is the ability to maintain speed, power, running economy, and physiological stability during prolonged exercise. A durable runner experiences less decline in performance markers such as heart rate, oxygen uptake, lactate threshold, and running economy as fatigue accumulates over long races.

What is the difference between durability and fatigability in endurance sport?

Durability describes how much an athlete declines across a long, continuous effort, while fatigability describes how quickly performance breaks down under demanding load. In simple terms, durability is the slow drift during prolonged endurance exercise; fatigability is the rate at which speed, power, or efficiency deteriorates when intensity becomes hard and repeated without recovery.

What is repeatability in endurance performance?

Repeatability is the ability to recover between efforts and reproduce strong performance across multiple bouts, stages, or days. It is especially important in stage races, back-to-back race weekends, and training blocks where success depends on backing up hard efforts rather than producing one isolated peak performance.

Is resilience the same as mental toughness in endurance sport?

No. In this framework, resilience is not just mental toughness. It is the capacity to maintain or regain physiological and mechanical function under added stressors such as heat, altitude, dehydration, technical terrain, sleep loss, or psychological strain. Mindset may help, but resilience also depends on measurable physical adaptation.

Can durability, fatigability, repeatability, and resilience be trained?

Yes. These endurance performance qualities appear to be trainable, but each needs a different emphasis. Durability can improve through long, continuous endurance work and strength training; fatigability through improved aerobic efficiency; repeatability through repeated efforts with realistic recovery; and resilience through controlled exposure to heat, altitude, fatigue, and event-specific stress.