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Hi,
Over a decade ago, Megan and I developed a fatigue resistance test for coaching. The basic protocol mirrors what we see in lots of studies (it’s usually called “durability” or "physiological resilience" in the literature, because scientists can't let anything be easy).
Our protocol:
- A fatigue session (for trail runners, 90 minutes to 2 hours moderate with high Z2 or low Z3 average heart rate)
- A climb time trial (anywhere from 5-15 minutes)
Compare the fatigued climb time trial to a fresh time trial on the same route, and the offset gave a fatigue resistance metric that we could track over time.
I say “track over time,” but what I really thought back then is that we were revealing destiny. Every athlete I saw who was an ultra champion had little offset (sometimes even performing better after the fatiguing protocol. Strange!). But perhaps weirdest of all, the test seemed to have high predictive value about who would later become an ultra champion, even before they had their breakout races.
In an old article for Trail Runner Magazine, I used that test to predict that Drew Holmen and Katie Asmuth would have big days at the 2019 Western States 100 Mile (both went on to finish in the top-5 when the groupthink predictions had them 15th-20th). And I used that test to predict something much sadder too: I was destined to suck at ultras forever.
My score was swirling around the crapper. And it echoed my real-world experience. At the 2017 World Trail Championships, I finished 198th. In my first attempt at 50 miles, I DNF’d at mile 26. What were we seeing?
Since we were only testing athletes we coached, it was unlikely to be a fundamentally different training approach. We theorized that it was some mix of muscle fiber typology and central nervous system-mediated genetic (and possibly environmental/epigenetic) differences. Basically, the freaks were slow-twitch monsters with cool brains.
But something weird started happening around 2019. The predictive value of the test began failing more and more, until it was basically useless except as a snapshot in time for an individual athlete. Athletes were improving fatigue resistance by massive leaps.
The theory that you knew was coming: It was the high carb signal.
The most exciting revelation was not that carbs improved fatigue resistance on a single day. We knew that already, and higher fueling on test day (or race day) alone never resulted in fundamental changes to the expected outcomes. 40-60 grams per hour versus 90-120 grams per hour is usually a marginal change at first--definitely noticeable, but probably of a similar magnitude as sleep, short-term training status, whether Mercury was in retrograde, etc. That's why isolated studies are never going to reveal the 10-20%+ improvements we are seeing in many cases.
Instead, there seemed to be longer-term adaptation that happened when an athlete practiced better fatigue resistance from high-carb fueling on dozens (ideally hundreds) of longer efforts. Perhaps those adaptations were mediated by improved aerobic and muscular output, reduced stress, muscle fiber typology factors, or something obvious like that. The easiest explanation is that athletes were breaking down less from the sessions, allowing them to train better subsequently, adapting more and more.
But I think it’s so much, much weirder, and so much cooler. I think the nervous system itself adapts too.
Now, we’re venturing into a very strange world full of anecdotes. Let’s start with mine! I went from a fatigue resistance pumpkin into a freak, all while doing less weekly mileage. It took practice with high-carb, but suddenly I was being named #2 Ultra Runner of the Year after never receiving a vote in my previous 18 years of running. I also noticed an ability to process pain that just seemed different, including outside of running.
I was no longer running with my own brain, which dreaded hour 3 and beyond. I craved mile 60. What the F happened to me?! (potential confounders: maybe it was having children or getting hit by a car while biking)
We see it in coaching constantly, with marathoners no longer fading even when they are running threshold effort, and ultra runners who come out of nowhere to have international breakthroughs. Everyone is seeing it in cycling, where the entire peloton has right-shifted power output, especially at the end of long stages. That shift happened around the same time our fatigue resistance test lost its predictive value.
The reason I lean toward a primary influence of a central nervous system mediator is that the effect is happening at both shorter and longer events. The shorter event improvement points toward better training, adaptation, and health, which is definitely a big part of it. But we're seeing the long event improvement (including 6-10+ hour events) even in athletes who aren't changing their training approaches or doing higher volume work. In some cases, the endurance improvements are non-linear and seem independent from their raw training data, happening even when athletes are not having speed/power improvements when fresh or increasing total volume, and the fatigue resistance persists even after periods of detraining. They just don't seem to have that "off" switch in the same way that they would expect based on their history.
To me, that points toward an explanation in the nervous system.
At the very top end of endurance sports, we’re seeing what happens when freaks get freakier from fueling. However, freaks dominated in the old days too, likely selected partially because they can excel with any fueling protocol.
But what’s most interesting is the bigger part of the bell curve, where all endurance sports are seeing the same rightward shift in power, speed, and most of all endurance.
I think a relatively small part of that signal is fueling on a single day of racing, which we already know from studies has the benefit of improving nervous system function in long events. Most of it is something that’s wildly exciting for everyone who wasn’t immediately a freak: we are now casting a much wider net.
Fueling allows athletes who would have been relegated to the "untalented" bin to find out that they actually just needed to practice better fatigue resistance in a highly fueled state. Do that for a few years, and capabilities fundamentally shift. My theory is that the call is coming from inside the house--from adaptations in the central nervous system itself.
Here’s how to do it:
- On every long effort (90+ minutes for lower volume athletes and most female athletes, 2+ hours for higher volume athletes), fuel as if you’re doing a fatigue resistance test at the end, even when you’re not. The high-carb fueling funnel starts at 75 grams per hour, and ideally 90+ grams per hour for days when you are pushing.
- Also practice hydration with electrolyte ratios you will use on race day. Hydration is the gateway to high carb, and it’s essential for proper nervous system functioning. You want to finish feeling "good," or at least "better" than you have before. Exhaustion is not an adaptation signal, it's a breakdown signal.
- Periodically do hard efforts at the end of the long days. These efforts can be as short as 30-45 second hills.
Here are my favorite products that I recommend to athletes who we coach:
- Science in Sport Beta Fuel Gels (I’m currently loving the electrolyte options)
- Amacx Turbo Gels (for me, it's the fastest high-carb gel to consume mid-workout)
- Amacx Drink Gels (the Strawberry flavor feels like a berry took cocaine and has strong opinions about fatigue resistance it needs to share with you)
- Skratch Hydration Mix (Lemon Lime is my go-to for races like Leadville and Javelina 100 Milers)
- Tailwind Endurance Fuel (Orange rocks. How do they make it taste so good? Admit their chemists to Hogwarts because that can’t be Muggle science)
- Precision Fuel and Hydration Big A$$ Gels (90g per big gel, easy to transport and slurp).
- David
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