You start a race or training session as strong as the proverbial ox. But as time rolls by, your speedometer rolls back. Fatigue has struck. It’s a powerful physical phenomenon that leaves you struggling for speed and dignity. Or is it? Is it really your mind playing tricks? We dip into the evidence to see if your brain can turn things around and lead you to a new PB… 

Who’s wished for an extra lung or thrice the size of quadriceps to tame your local 1-in-4 hill? Try as you might, you can’t cling onto the wheel in front of you, plunging you back into your metaphorical broom wagon. But why? What’s stopping you if not mimicking Lotto Dstny’s indefatigable Thomas de Gendt, at the very least clinging onto your mate’s drop tail?

Of course, it’s that you’re drowning in fatigue. But what exactly is that? What’s happening to you that’s turning an unpleasant bonk into a reality? 

Physical factors

Hands up: Yes, you at the back. Lactic acid, you say. There’s certainly evidence that acidosis in the blood, caused by a build-up of this by-product of carbohydrate metabolism, is uncomfortable... but no, try again. Yes, you over there? Low glycogen levels? Again, close but no cigar. Though your carbohydrate stores impact power output, we’re afraid you’re potentially looking in the wrong place.

You see, while physiological parameters like these were once thought to dictate when you either slow down or even stop, an increasing body of evidence suggests it’s the brain not the brawn that prevents us all going higher, faster and stronger.

Which, when you think about it, makes sense because while your legs, heart and lungs might be your active components, it’s your brain that feels the pain, that feels whether you’ve burnt one too many matches.

It comes down to your central nervous system, which comprises your brain and spinal cord, and your peripheral nervous system, which connects the central nervous system to your organs, limbs and skin. We could dig right down into CERN levels of detail but for now it’s enough to just touch on the impact of afferent and efferent neurons.

Afferent neurons are sensory neurons that carry nerve impulses from sensory stimuli towards the central nervous system and brain, while efferent neurons are motor neurons that carry neural impulses away from the central nervous system and towards muscles to cause movement. These feedback and feed-forward systems, plus the brain, are at the heart of why many scientists feel it’s the brain holding you back. And it all started with the Father of Fatigue, Professor Tim Noakes of Cape Town University in South Africa… 

Stopping from within

Many moons ago, Noakes suggested that deep within the brain is a subconscious mechanism that ensures your organs are protected from serious injury. Noakes called this the ‘central governor’, which seemed to explain why during hot sessions you’d start off at a slower pace than you would on cool days. In an effort to not boil and instead reduce the threat, your brain would lower muscle recruitment and power output.

This makes sense, but how exactly does it work? Well, Noakes concluded that it’s from feedback – from the temperature of the skin or your core temperature. Or, in the case of riding at altitude, from your blood because it’s less oxygenated. If you deoxygenate the brain, you reduce muscle recruitment and activation, and you slow down. 

The model proved ground-breaking. It wasn’t torturous swathes of lactic acid or depleted energy sources that forge a cyclist’s limits – it’s the brain preserving itself. It was also controversial for many reasons. One, being the subconscious, it was damn hard – nay impossible – to prove. Second, as Noakes later confessed, was that his original model lacked a motivational component. He’d later reveal that the central governor was only ever meant to provide a framework to build on.

What is fatigue?

Kyle Smith, IRONMAN 70.3 Taupo winner

"For me there’s a sliding scale of fatigue. There’s acute fatigue and chronic fatigue, and they are the polar opposites in feeling and response. Every day I feel acute fatigue at the end of a session. It’s a good feeling and something I strive towards. I seek this feeling and go after it.

“Quite the opposite applies to chronic fatigue. This is the feeling of being unable to get out of bed. Feeling sluggish, lethargic, unmotivated and injury prone. Something’s wrong when this happens, whether sickness, overtraining or underfuelling. Something has gone wrong and I try and avoid this at all costs. I can now recognise when my body’s reaching the first stages of this and I can correct accordingly to avoid sinking down into the rabbit hole and not being able to get out. 

“The worst feeling played out in 2022 when I had a lingering virus. I felt hungover every morning. I had to sleep twice every day while attempting to line up at two of the biggest races of my career, PTO US open and Kona. It was a total mind f\*k and I did my best to convince myself otherwise, but this eventually ended up being two of the worst days of my career.”*

Motivation matters

Cue the work of Professor Samuele Marcora of Bologna University and his psychobiological model of fatigue. The Italian agreed with Noakes that fatigue is no physical phenomenon. But Marcora says that the decision to slow or quit is a conscious decision, deriving from two key areas – motivation and perception.

The evidence of such fatiguing heresy comes down to the labs. Marcora had 10 rugby players cycle at a power output based on 90% of their VO₂ Max. They pumped their powerful limbs until they could no longer maintain that wattage. Then, sadist Marcora ‘encouraged’ them to ride five more seconds as ferociously as they could. Despite ‘fatigue’ seemingly taking grip, they generated 731 watts, thrice the 242 watts that had nearly finished them off. 

Where did this shot of energy come from when seconds ago they were puffing out of their derrieres? Well, Marcora says it’s down to the tandem of perception and motivation, that the switch that’s flicked to stop isn’t through physical implosion but a conscious decision.

It’s why one Sunday you might feel great on a four-hour ride and sail home ahead of your mates with an ease that borders on arrogance. But on another ride, for whatever reason you’re just not ‘feeling’ it and spend the whole 240 minutes telling anyone who’ll listen (no one) that the meaning of life is no longer found on the bike but at the bottom of a bottle of beer. What’s changed? Apart from your outlook, your perception and motivation to nail that ride, nothing.

Specifically, this altering outlook comes down to a part of the brain called the anterior cingulate cortex. It’s associated with effort perception, and research showing rodents with damaged anterior cingulate become lazy when seeking food.

So, how can you avoid morphing into an apathetic and hungry rat? Marcora says the occasional evening session, when your mind’s tired from a day of information overload, delivers more bang for your buck because your mind is already fatigued. It’s like an extra gym session for your brain and, in turn, could lead to greater physical performance.

Does that mean you don’t have to train your body, but simply your mind? Should Arnaud de Lie replace cycling shoes for Descartes’ ‘Mediations on First Philosophy’? In a word, no. You see, if you have weaker muscles, your brain cranks things up to 11 to compensate. This is perceived as an increase in effort and will ultimately slow you down. However, this is an indirect, rather than direct, reason for stopping. In other words, you need to train the physical to strengthen the mind to help the body race faster. It’s a never-ending fatigue cycle to boost your carbon cycle.

What is fatigue?

Angela Naeth, 19-time IRONMAN 70.3 winner

“Real fatigue for me as a professional triathlete is often experienced after prolonged exertion, whether it’s from intense training sessions or competitive races. It’s a combination of physical strain from pushing my body to its limits and mental fatigue from staying focused and motivated throughout the effort.

“Fatigue is both a physical and mental struggle. Physically, it’s the exhaustion in my muscles from the intense effort; mentally it’s the constant battle to keep pushing forward despite the challenges. It’s the combination of both that makes fatigue so challenging to overcome.

“My worst experience of fatigue was during a solo day of training after six hours. I’d underestimated the distance and didn’t fuel properly, leading to a bonking experience where my energy levels plummeted and I struggled to pedal. It was a tough lesson learned about the importance of nutrition and pacing in endurance sports.”


Like Noakes’ central governor model, Marcora’s psychobiological model of fatigue has detractors, who accuse it of being overly simplistic and biased toward motivation. Critics argue that Marcora’s experiments don’t account for psycho-pathologically motivated athletes like Lance Armstrong and Bernard Hinault. Studies have shown that some Olympians would literally sacrifice their lives to win gold. That’s extreme, but illustrates that clearly it’s not motivation preventing them from success.

Instead, maybe your physical and mental brakes stem from the ‘complex systems model of fatigue’. This is specific to the situation and says that sometimes fatigue’s down to the physical, sometimes the mental. So, the pain you feel from maxing out at the squat rack, which leads to very focused pain in the quads but can be relieved by offloading the weight, is very different to the fatigue kicking in from riding for hours on a turbo trainer in your garage. Boredom and motivation are unhappy bedfellows here.

Ultimately, if we had to settle the debate, we’d say it's context dependent. What can you do with that information? Like many things stalling at the research phase, very little (apart from consistent training punctuated with harder sessions, which is proven to be good for body, mind and performance). We could have, of course, stated that in the intro. But hey, where’s the fatiguing fun in that?

What is fatigue?

Jocelyn McCauley, 4x IRONMAN champion

“Fatigue to me is a combination of both the physical and mental struggle. The physical fatigue makes it harder to control the mind and mental outlook, so then it all compounds with each other. To that end, I’ve passed out in two races. You’d think that those would be my worst experiences with fatigue, but I passed out because of mismanagement of hydration not fatigue. 

“My deepest fatigue happens during every block of Ironman-distance training. My coach used to say, ‘When you meet more than one bastard in a day you need a day off.’ That’s a sign and trigger that I’m too fatigued. In a race it happens during the last 10km of every race, no matter what – probably because it’s partly mental that you’re almost done and physical because you’ve pushed for so long.”

Further reading