Durability, Fueling and Hydration: Why I Measure Them Together
- Mark Turnbull
- Aug 21
- 10 min read
Most endurance testing tells us how good an athlete is when they are fresh.
FTP, critical power, VO₂max, lactate threshold and a power-duration profile are all useful. I use these measures myself and with athletes I coach.
But there is an obvious problem if your target event lasts eight hours, 24 hours or several days:
You don't compete fresh.
The important question becomes not simply how much power can you produce?, but:
How much of that performance is still available after several hours of work — and what is causing any deterioration?
That thinking is behind what I am developing as the Sparks Into Life Durability Index.
But I've become increasingly convinced that durability should not be measured in isolation.
If an athlete's power deteriorates after four hours because they have consumed too little carbohydrate, become progressively dehydrated or simply failed to drink because the weather turned hotter than expected, that is not necessarily a failure of aerobic conditioning.
It may be a failure of fuel delivery, hydration strategy or race execution.
So I want to measure three things together:
Durability + Fueling + Hydration.
Why durability deserves its own measurement
There is now good evidence that durability is not simply another way of measuring FTP or VO₂max.
A 2025 study of well-trained amateur cyclists compared 5- and 20-minute performance fresh and after 1,000 kJ of prior cycling. After the preload, performance fell by approximately 10% in both tests. More importantly, the size of that deterioration was not significantly related to either relative FTP or VO₂max.
In other words, two cyclists with similar fresh fitness can respond very differently once fatigue accumulates.
We see the same thing at the top of professional cycling.
Analysis of 112 professional riders found progressively greater reductions in their record power profiles as accumulated work increased. WorldTour riders were generally better able to preserve performance than ProTeam riders, and the difference between the groups became much clearer under fatigue than when they were fresh.
Data from riders competing in La Vuelta showed something similar: WorldTour and ProTeam riders could look relatively similar early in the fatigue curve, but differences became increasingly apparent as work accumulated during stages and across the three weeks.
That is highly relevant to ultra-endurance sport.
The physiological ability that wins a 20-minute fresh test isn't necessarily the same ability that allows someone to climb strongly after 300 kilometres.
But there is a major problem with durability testing
Suppose I give an athlete this test:
Four hours endurance riding followed immediately by a maximal 12-minute climb.
Fresh 12-minute power = 330 W
Fatigued 12-minute power = 300 W
Performance decline = about 9%.
It would be tempting to say that athlete has relatively poor durability.
But imagine that during those four hours they consumed only 35 g carbohydrate per hour and drank 300 ml per hour on a warm day.
Now repeat the same test several weeks later.
This time they consume 85 g carbohydrate per hour and 650 ml fluid per hour.
They produce 318 W after the same workload.
Have they suddenly developed dramatically better mitochondrial durability?
Possibly.
But improved fuel and fluid availability could also explain part of the difference.
That distinction matters.
Hydration can affect the fuel you are able to use
A particularly interesting 2026 study brings these variables together.
Nine trained cyclists and triathletes completed 160 minutes of cycling while ingesting the same 60 g carbohydrate per hour in two conditions.
In one trial they maintained hydration reasonably well. In the other they were deliberately allowed to dehydrate.
Mean oxidation of the ingested glucose fell from approximately 0.61 to 0.51 g/min in the dehydrated condition, and peak exogenous glucose oxidation was also lower.
That's important.
We often discuss fueling and hydration as separate topics:
How many grams of carbohydrate per hour?
How much fluid per hour?
But physiologically they interact.
An athlete can have a perfectly calculated carbohydrate strategy on paper and still compromise fuel availability if fluid delivery, gastric emptying or environmental conditions are wrong.
For an ultra athlete, these problems don't normally occur independently. They compound.
A little dehydration increases cardiovascular strain.
RPE rises.
Power becomes harder to sustain.
The athlete stops eating.
Carbohydrate availability falls.
Pacing deteriorates.
And several hours later we describe what happened as "fatigue".
My aim is to understand what created that fatigue.
What elite sport science is already showing us
Professional cycling research is increasingly analysing performance under accumulated load rather than relying entirely on fresh laboratory values.
One field study took professional riders with a VO₂max around 83 ml/kg/min and compared a 20-minute time trial when fresh with the same test following approximately four hours and 40 kJ/kg of previous work.
Average power only fell around 3%, but individual responses ranged from an 8.5% deterioration to a slight improvement. Traditional laboratory measures did not explain those differences.
Another study compared U23, ProTeam and WorldTour cyclists by examining maximum power after accumulating increasing amounts of work above critical power.
The U23 riders began losing significant performance sooner. Professional riders tolerated more accumulated high-intensity work, with WorldTour cyclists demonstrating the greatest high-intensity durability.
And this is another important development: total kilojoules alone aren't enough.
A 2026 systematic review concluded that high-intensity work creates greater durability-related performance deterioration for a given quantity of accumulated work than low-to-moderate intensity exercise.
Two rides containing 2,000 kJ therefore aren't necessarily equivalent if one contains repeated climbs, attacks and prolonged work above critical power.
That has obvious implications for events such as the Transcontinental Race.
Four hours sitting steadily in Zone 2 is not the same physiological load as four hours containing repeated steep climbs, accelerations, heat and poor road surfaces, even if total energy expenditure looks similar.
How I'm approaching the Sparks Into Life Durability Index
I don't want the Durability Index to become one arbitrary number.
I see it more as a performance dashboard built around several related measurements.
The central measurement is performance retention:
Durability retention = fatigued performance ÷ fresh performance × 100
A rider producing 320 W fresh and 304 W after the standardised preload retains:
95% of fresh performance.
That is useful.
But alongside that I want to understand:
Measure | What I'm looking for |
Power/pace retention | How much actual performance remains? |
HR response | Is cardiovascular cost increasing for the same workload? |
RPE | How much harder does the same workload feel? |
Fuel intake | What was actually consumed in g carbohydrate/hour? |
Fluid intake | What was actually consumed in ml/hour? |
Body-mass change | Useful context for fluid balance, interpreted cautiously |
GI comfort | Can the athlete actually tolerate the planned intake? |
Environmental conditions | Temperature, humidity and airflow |
Accumulated work | Duration, kJ/kg and importantly intensity distribution |
Work above threshold/CP | How much high-intensity fatigue was accumulated? |
This means I can start separating performance durability from the things affecting it.
That makes the measurement far more useful for coaching.
A practical field test
For most of my cyclists I don't need a metabolic cart or laboratory to get useful information.
The key requirement is standardisation.
I would establish a fresh benchmark first. Depending on the athlete that might be a 5-, 12- or 20-minute effort, or another performance test appropriate to their event.
That test is then repeated on another day after a standardized endurance preload.
For an ultra cyclist, that might eventually look something like:
Start normally fueled and hydrated, after a normal recovery day.
Record morning body mass under repeatable conditions.
Complete approximately 2½–4 hours of controlled endurance riding, gradually progressing the workload as the athlete develops.
Record total work, kJ/kg and any meaningful work above threshold or critical power rather than looking at duration alone.
Use the athlete's intended race-fueling strategy throughout and record actual, not planned, carbohydrate intake.
Record fluid consumed and, where useful, sodium intake.
Note HR, power and RPE periodically through the ride.
Record GI comfort and environmental conditions.
Complete the standardized performance test immediately after the endurance preload.
Compare the result with the fresh test and with previous durability tests performed under similar conditions.
This isn't something I would perform every week.
It is a profiling tool.
Once we understand the athlete's starting point, we can repeat it at key stages of an event-specific build.
Measuring hydration without turning every ride into a laboratory
We can also establish a useful approximate sweat rate.
A straightforward field method is to weigh the athlete immediately before and after a controlled ride, while recording fluid consumed and any urine produced.
Very approximately:
Sweat loss = pre-exercise mass – post-exercise mass + fluid consumed – urine produced
Dividing that by exercise duration provides an estimated hourly sweat rate.
I prefer doing this initially over perhaps 60–120 minutes in reasonably controlled conditions rather than attempting to calculate sweat rate from a six-hour ride.
Over very long exercise, body-mass changes increasingly reflect carbohydrate use, glycogen-associated water, food consumption and other factors, so weight loss should not automatically be interpreted as pure dehydration.
I'd also repeat the assessment in different conditions.
A cool Peak District ride in March tells us very little about fluid requirements during six hours of European summer heat.
Fueling becomes part of the training intervention
Exactly the same principle applies to carbohydrate.
An athlete targeting 90 g/h in competition shouldn't discover during the race whether they can tolerate 90 g/h.
Long training sessions provide an opportunity to develop both physiological durability and nutritional durability.
If an athlete currently manages 55–60 g/h comfortably, I might progressively move that toward 70, then 80 and eventually 90 g/h or higher where the event demands it.
But more isn't automatically better.
The target is the amount the athlete can consume, absorb, tolerate and use while still riding effectively.
This is why recording what was actually consumed matters far more than writing "90 g/h" in a TrainingPeaks description.
What this looks like in my own ultra preparation
In preparation for an event such as the TCR, I wouldn't try to create durability simply by doing enormous rides every weekend.
I want progression.
Early in the build, most endurance riding can remain relatively controlled. I can build aerobic volume while maintaining one or two high-quality sessions elsewhere in the week.
As the event approaches, the long rides become increasingly specific.
That might mean finishing a four-hour ride with sustained climbing.
Later it could mean five or six hours with controlled tempo sections late in the ride.
Eventually there are selected back-to-back long days where the second day matters as much as the first.
At the same time, I'm practicing the fuel and hydration strategy I intend to use in the event.
The goal isn't simply to arrive home tired.
The question is:
How well can I still perform after doing the work, while fueling and drinking in a way that is realistic for the race?
That gives me much more useful information than simply adding another 300 TSS to the training diary.
What about female athletes?
This is an area where I would be particularly cautious about applying simple assumptions.
A 2025 field study analysing 42 female and 42 male professional cyclists found greater deterioration in female riders after equivalent accumulated work at higher workloads.
But a controlled 2026 laboratory study found trained women preserved power at the gas-exchange threshold and respiratory compensation point better than men following 90 minutes of heavy cycling.
Those findings aren't necessarily contradictory. They used very different exercise protocols and different definitions of durability.
For me the coaching message is straightforward:
Don't assume. Measure the athlete in front of you.
And fueling deserves particular attention.
A case study from the Tour de France Femmes recorded a WorldTour rider consuming an impressive 13.7 g/kg carbohydrate per day and around 84 g/h during stages, yet she was still estimated to be running an energy deficit of more than 2,000 kcal per day and lost 2.2 kg during the race.
That illustrates an important distinction for multi-day athletes:
On-bike fueling and total energy availability are not the same thing.
A female athlete can execute an excellent 90 g/h race-fueling plan yet still fail to replace the enormous total daily energy expenditure of an ultra event.
The same is true for men, but the health and performance consequences of prolonged low energy availability mean it deserves particular attention in female athletes.
How an everyday athlete can use this
You don't need to call it a Durability Index or perform maximal testing every weekend.
Start with three questions after your longer sessions:
1. Did my performance change?
Look at power or pace, HR and RPE early versus late.
2. Did I execute my fueling strategy?
Don't say, "I ate quite well."
Calculate it.
If you consumed 280 g carbohydrate during a five-hour ride, that is 56 g/h.
Now you have something useful to work with.
3. Did I execute my hydration strategy?
Record approximately how much you drank, the conditions and how you felt.
Over several sessions patterns begin to appear.
Perhaps your power always deteriorates after three hours when carbohydrate intake slips below 60 g/h.
Perhaps HR begins drifting rapidly on warm days when fluid consumption drops.
Or perhaps fueling and hydration are both excellent and performance still falls substantially.
That last scenario is particularly interesting.
Now we may genuinely be looking at a durability limitation that training needs to address.
Measure the athlete who finishes, not just the athlete who starts
Fresh testing still matters.
I want to know an athlete's aerobic ceiling, thresholds and power-duration characteristics.
But for long-distance racing those values are only the starting point.
If two cyclists both have an FTP of 320 W but after five hours one effectively has 310 W available and the other has 280 W, they are very different endurance athletes.
And if the second rider's deterioration largely disappears once we correct their fueling and hydration, they are different again.
That is why I think the next step in endurance profiling is not another fresh-state metric.
It is understanding how physiology, fueling and hydration interact as fatigue accumulates.
That is the rationale behind the Sparks Into Life Durability Index.
Ultimately I'm trying to answer a very simple question:
What version of this athlete is going to turn up after six, twelve or twenty hours — and what can we do in training to make that athlete better?
If you're preparing for an ultra-distance cycling, running or triathlon event and want to understand how well your performance holds up under real fatigue, this is something I can integrate into your testing and coaching rather than relying solely on fresh FTP, pace or threshold numbers.
Sources:
#CyclingScience #EnduranceTraining #SprintIntegration #TrainingStrategies #FatOxidation #PerformanceEnhancement #CyclingResearch #FitnessInsights #AIAssistedTraining #MetabolicProfiling #CyclistWisdom #PersonalizedTraining #EliteCyclists #TrainingInnovation #CyclingGoals #AerobicAdaptations #MuscleRecovery #CyclingStrategy #OptimalTraining
Comments