Fresh Fitness Isn’t Enough: What New Endurance Research Says About Durability, Fueling and Smarter Training
- Mark Turnbull
- Aug 28
- 7 min read
Most endurance athletes still judge fitness in fresh conditions.
FTP. Threshold pace. VO₂max. Critical power. A strong set of intervals. A good race when legs are fresh.
Those metrics still matter. I use them myself and I use them with the athletes I coach.
But if you are preparing for an ultra-distance ride, a multi-day event, a long-course triathlon, a marathon or any event where fatigue accumulates for hours, fresh fitness only tells part of the story.
The more useful question is this:
What version of the athlete turns up after three, five or eight hours?
That is the thinking behind the Sparks Durability work I’ve been developing.
Durability, in simple terms, is the ability to resist performance deterioration as fatigue accumulates. It is not just how good you are when fresh. It is how much of your usable physiology still remains when the work has already been done.
And the latest research supports an important point that sits right at the heart of this:
Durability cannot be separated cleanly from fueling, hydration and the way training is prescribed.
Durability is not just another word for FTP
One of the most useful lessons from the durability literature is that two athletes can look similar on paper when fresh and perform very differently later in a session or later in a race.
That matters because many endurance events are decided not by who starts strongest, but by who retains the most useful performance once fatigue, rising strain and fueling demands begin to bite.
This is exactly why I don’t think a fresh FTP or threshold test is enough for long-event athletes.
For a rider targeting something like the Transcontinental Race, a mountain ultra, a long sportive or an ultra-distance triathlon, I want to know more than their ceiling. I want to know how stable that ceiling remains under load.
That is where the Sparks Durability approach comes in.
I am interested in:
how much power or pace is retained after a prolonged endurance preload;
how heart rate changes for the same workload;
how perceived effort drifts;
how much work was done in total;
how much work was done above threshold or critical power;
and critically, what happened with carbohydrate intake, hydration and gut tolerance.
Because if performance falls late in a session, I want to understand why.
One of the biggest mistakes in endurance coaching: treating durability, fueling and hydration separately
A poor late-session result is easy to misread.
If an athlete rides strongly for two hours, fades badly in hour four and then fails to reproduce meaningful power late in the session, the temptation is to say they lack durability.
Sometimes that is true.
But sometimes the problem is not a physiological durability limitation at all. Sometimes it is poor carbohydrate delivery. Sometimes it is under-drinking. Sometimes it is heat. Sometimes it is a simple execution problem.
That distinction matters.
Recent research strengthens that point.
A new study showed that after three hours of exercise, fatigued critical power was better preserved when athletes consumed more carbohydrate. In practical terms, fuel availability influenced how much sustainable performance remained late in exercise.
That is a big deal.
It means a durability test done at 40–50 g/h carbohydrate and another done at 90–120 g/h are not really testing the same thing. One may be measuring training status, while the other is partly measuring substrate availability.
Another recent study showed dehydration reduced exogenous carbohydrate oxidation, even when carbohydrate intake was held constant. In other words, hydration and fueling are not separate systems running alongside one another. They interact.
That fits what many athletes experience in the real world:
fluid intake slips;
heart rate drifts upward;
perceived effort rises;
eating becomes harder;
carbohydrate delivery falls;
and performance deteriorates.
We often just call that “fatigue”.
But from a coaching point of view, I want to know whether that was:
a durability problem,
a fueling problem,
a hydration problem,
or some combination of all three.
That is why I believe these variables should be measured together.
What the newer training research means in practice
Alongside the durability and fueling work, the most recent endurance studies also help clarify how training should be prescribed.
1. More training is not automatically better training
A randomised study from this month found that adding a lot of moderate easy volume to a short six-week HIIT block did not produce extra performance benefit compared with HIIT alone in moderately trained cyclists and triathletes.
That does not mean easy riding is useless. It does mean that for a time-constrained athlete, simply adding more volume is not always the most effective lever.
That fits a principle I use often:the right stimulus only creates progress if the athlete can absorb it.
2. There is still no single “best” intensity distribution
The latest synthesis comparing polarized and pyramidal training still does not give us a clear universal winner.
That’s useful.
It means we should stop treating training models like belief systems.
For some athletes, a more pyramidal structure makes sense. For others, a more polarised setup fits better. The real question is not which label wins, but which distribution best suits the athlete, the phase of training, the event demands and their ability to recover.
3. Longer HIIT intervals still look very effective
Recent reviews suggest that intervals of two minutes or longer, and in some cases variable-intensity intervals, are very effective for accumulating high oxygen uptake time.
Again, that doesn’t mean short intervals are obsolete. It does reinforce the idea that session choice should serve a purpose.
I don’t want athletes doing a session because it is fashionable. I want them doing it because it develops something they actually need.
4. Glycolytic traits still matter
Another new paper showed that markers of glycolytic power help explain threshold-related performance independently of VO₂max.
That matters because two athletes with identical fresh VO₂max values may still have different threshold characteristics and very different event suitability.
And once fatigue accumulates, those differences can become even more meaningful.
How this ties into Sparks Durability work
My aim with the Sparks Durability framework is not to invent a magic number and pretend it explains everything.
I want something more useful than that.
I want a repeatable field-based performance profile that helps answer:
how well an athlete resists deterioration;
what is deteriorating first;
whether the limiting factor is physiological, nutritional or execution-based;
and how training should change as a result.
A practical Sparks durability assessment might include:
a fresh benchmark effort;
a standardized endurance preload;
controlled carbohydrate intake;
controlled fluid intake;
tracking of HR, power/pace and RPE;
and a repeat performance test under fatigue.
From there, I can look at:
performance retention
HR drift
late-session RPE
carbohydrate intake
fluid intake
GI tolerance
total work
work above CP or threshold
conditions
body-mass change
That gives a far better picture than a single fresh-state test.
It also makes the result actionable.
If the athlete’s late-session performance is poor despite appropriate fueling and hydration, we likely need more event-specific durability training.
If late-session performance improves dramatically once fueling and hydration are fixed, then training wasn’t the first problem.
That is useful coaching information.
How I would use this in a real build for a long event
For an athlete preparing for a very long day or a multi-day event, I would not just chase more threshold or a higher FTP.
I would build the athlete’s ability to hold useful performance after accumulated work.
That might look like:
one structured quality session each week, often threshold or VO₂-focused depending on need;
one progressively longer endurance session;
occasional long rides with controlled harder work late in the ride;
specific fueling practice during long sessions;
hydration practice in different conditions;
and, closer to the event, selected back-to-back days.
The goal is not simply to accumulate fatigue.
The goal is to improve the athlete’s ability to:
fuel while riding,
maintain stable output,
tolerate the intake required,
and keep performing when freshness has gone.
That is durability in the real world.
What my audience can implement now
If you are a cyclist, runner or triathlete training for longer events, you do not need a lab to start using this approach.
Start with these five practical steps:
1. Stop assessing long-event readiness only when fresh
Fresh FTP, pace or threshold is useful, but it is not the full picture.
2. Compare early-session and late-session performance
Look at power, pace, heart rate and RPE after meaningful accumulated work.
3. Record your actual intake
Don’t write “fueled well”.Write:
carbohydrate per hour;
fluid per hour;
what you tolerated;
and what happened.
4. Practice race fueling in training
If the event requires 80–100 g/h or more, don’t leave that to race day.
5. Ask what caused the fade
Was it lack of durability? Poor pacing? Under-fueling? Under-hydration? Heat? Gut issues?
That question alone can improve training quality.
The bigger coaching message
The latest endurance research is useful not because it gives us a new trend to copy, but because it sharpens how we interpret performance.
Fresh-state physiology still matters.
But for long events, the athlete who finishes strongly is not always the athlete with the best fresh numbers. More often, it is the athlete who:
loses the least,
fuels effectively,
hydrates appropriately,
and executes the event with control.
That is why I think durability work is becoming increasingly important.
And that is why, in my own coaching and in the Sparks framework, I want to measure durability + fueling + hydration together.
Because the question is no longer just:
How fit is this athlete?
It becomes:
What is left of that fitness when it matters most?
If you want help understanding how your performance holds up under fatigue — and how fueling and hydration influence that — that is exactly the kind of work I can build into your testing and coaching.
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