Pushing the Limits
The Science of Fatigue
Kilometer 30.
The "man with the hammer" has struck.
You’re forced to walk.
You seriously doubt whether you’ll even reach the finish line today.
Your body is giving you a clear command: "Stop!"
You can't go on.
Kilometer 41.
Just one last kilometer to go.
Suddenly, you start running again.
In fact, you do more than that:
You pick up the pace, accelerate over the final meters, and sprint across the finish line.
How is that possible?
Clearly, it was still physically possible to run; you wouldn’t have had to walk at kilometer 30. So, what held you back? What mechanism shouts "Stop!"? Is it physical systems that are completely exhausted and failing, or is it a warning signal from your brain, trying to protect your body from serious damage?
To this day, science has no definitive answer to the question:
What exactly is fatigue, and where does it originate?
However, the history of this research is incredibly multifaceted. In the coming weeks, we want to dive deeper into exactly that. Because the better we understand what limits us, the more confidently we can perform in competition.
The "Physiological Model" according to Archibald Vivian Hill
In the early 20th century, Archibald Vivian Hill shaped the image of the human body as a purely mechanical machine. He assumed that fatigue was a purely physiological "problem"—like a car running out of fuel or an engine overheating.
At its core, his theory states that during maximal exertion, the muscles' demand for oxygen exceeds the heart's capacity. This leads to a lack of oxygen in the muscle and the accumulation of "toxic" lactate. Hill believed that the heart's pumping capacity was the sole factor determining human endurance.
Today, we have a more detailed understanding and know that oxygen supply is only one of many factors. Yet, the fundamental idea remains the bedrock of performance diagnostics: we measure physiological processes like $VO_2max$ or lactate thresholds to determine what speeds we can run. We are looking for the limit of the "engine."
The Logic Gap
However, the purely mechanical model reaches its limits when we look at real-world data. For example, if you analyze the finishing times of millions of marathon runners, there is an unusual clustering just before the "magic" time barriers of three, four, or five hours (Allen et al., 2016). According to Hill's theory, these runners' systems should be completely exhausted at the end of a marathon.
So why were they often able to accelerate significantly over the last two kilometers? What stopped them from choosing that pace earlier?
Psychology Beats Physiology: The Price of Effort
A variety of other experiments show that our limits are "elastic," as Alex Hutchinson describes in his groundbreaking work Endure. A classic example from France by Michel Cabanac (1986) illustrates this phenomenon:
Participants were asked to hold a wall-sit for as long as possible. In the first trial, they received 20 cents every 20 seconds—they lasted an average of two minutes. In the second trial, the researchers increased the incentive to 7.80 euros every 20 seconds. As if by magic, the duration doubled to four minutes.
Or consider the so-called "hysterical strength" of a desperate mother who, in an extreme situation, is able to lift a car to save her child.
The message is clear:
Our musculature, or our "engine," cannot be the only limiting factor. Fatigue is not a purely physiological construct. Our brain and our motivation play the decisive role in controlling our performance.
And that is exactly what the next two blog posts will be about…
References:
Allen, E. J., Dechow, P. M., Pope, D. G., & Wu, G. (2016). Reference-Dependent Preferences: Evidence from Marathon Runners. Management Science.
Cabanac, M. (1986). Money versus pain: Experimental study of a conflict in humans. Journal of the Experimental Analysis of Behavior.
Hutchinson, A. (2018). Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. William Collins.
Noakes, T. D. (2012). Fatigue is a brain-derived emotion that regulates the exercise behavior to ensure the protection of whole body homeostasis. Frontiers in Physiology.
Tucker, R., Lambert, M. I., & Noakes, T. D. (2006). An analysis of pacing strategies during men's world-record performances in track athletics. International Journal of Sports Physiology and Performance.