The Dreaded DNF: Are You Actually Quitting, or is Your Brain Tricking You?

Your brain is very important in endurance performance.

If you have taken part in an ultra-marathon, you have almost certainly had a moment where you wanted to quit—maybe a few. Hopefully, you didn’t! But maybe you did.

The dreaded ‘did not finish’ (DNF) is a real possibility at every race. Statistically, around 5% to 15% of runners will drop out of a 50-miler, a figure that climbs to between 30% and 40% at the 100-mile distance. This number only increases further as factors, such as the weather conditions, become more challenging. Analysis by irunfar found that most DNFs happen just after mile 60 of the hundred-mile race, the point at which you are tired but still have significant miles to cover. Many DNFs happen for good reasons: injury, gastric issues, hypothermia, heat exposure, or getting lost. But sometimes, runners just quit-even when their bodies are physically capable of moving forward. Why does that happen?

I have to confess quite a while back, I was guilty of such a DNF. I had no physical reason to quit. The runners passing me were in the exact same state as I was-or worse. But I had decided I couldn’t go on and just stopped. I wasn’t proud of it for a long time, but I have since been back to the race, put it right and learned a massive amount from the experience. Failure can be a great teacher.

The reality of ultrarunning is that we sometimes operate on a bit of a knife’s edge. A wrong tactical decision here, some missed nutrition there, and things can unravel quickly. You may find yourself sitting in an aid station at mile 75 completely free of genuine physical excuses but mentally defeated by the final 25 miles. The next day, you look back with a clear mind and know your body could have gone on. So why didn’t you?

Fatigue isn’t just physical

It used to be thought that fatigue was purely physical. Modern endurance science tells a different story, one that recognises that your brain isn’t just there to watch your body run out of fuel. Your brain is the ultimate governor of effort, constantly scanning your body for signs of ‘danger.’ There are two main schools of thought:

1. The Central Governor Model (Tim Noakes): Your brain acts as a built-in safety system. It’s constantly getting feedback about fuel levels, muscle damage, core temperature, hydration, and anything else you can think of. It uses this to calculate how you are doing against what you have left to do and adjusts accordingly. If it feels you are going too hard it will recruit less muscle fibres, make things feel harder and slow you down. It is trying to protect you.

2. The Psychobiological Model (Samuele Marcora): As your muscles tire, your brain has to work harder to stimulate them. This extra brain effort increases your RPE and once that level outweighs your motivation you make a conscious decision and stop. It’s not your body failing-it’s your brain hitting its own kind of ‘wall.’

And then there’s adenosine-a substance that builds in your brain up as you burn through energy. This stuff makes it harder for dopamine to bind to its receptors by changing their shape. The result? Every step feels heavier and you become less motivated to carry on. The harder your brain works (or the more exhausted you get), the more adenosine accumulates. That’s why, after a long boring day at work, the run home can feel like a slog even though your running muscles are well rested. At the same time glucose is being redirected away from areas in the brain responsible for executive control toward motor command regions. This makes it increasingly difficult to focus, to make decisions and it reduces your emotional control, making you more vulnerable to catastrophising. So overall not only do you feel the effects of exhaustion a long time before you are actually exhausted but you also become less emotionally resilient to them.

All this is very interesting (I hope, or maybe you are really bored and experiencing first hand what high levels of adenosine feels like). Either way, how is it going to help when you are shivering on a plastic chair in the dead of night at a checkpoint with your crew trying to shove a flapjack in your mouth, while you are trying to tell them that you are going to quit?

Outsmarting Your Brain

Your body is tougher than you give it credit for and your overprotective brain is not being entirely truthful with you. So how do you outsmart your brain when it’s screaming at you to quit?

It starts with preparation—not just of your body, but of your mind.

Find Your ‘Why’-And Make Sure It Means Something!

Motivation is so important, the more motivated you are the more discomfort you will tolerate. Just wanting to finish is ok but why do you want to finish? 1984 Olympic marathon bronze medallist Charlie Spedding used to ask himself three things, what do I want? Why do I want it? And how badly do I want it? (give his book ‘From Last to First’ a read). It’s a great framework – what do I want? Define your goal, you can’t hit a target you can’t see so define it clearly. Make it specific, measurable and as much as possible controllable, for example ‘I want to finish in a certain position’ is not controllable because you can’t control who turns up. Once you have set your goal ask yourself, Why do I want it? The ‘why’ is the emotional / psychological fuel that will keep you going so make it good! Something emotive - happy, proud or even angry will work. How badly do I want it? Are you all in? Is this non-negotiable? Or do you not really care that much. If it’s the latter you need a different why. If it’s more towards the former .. great! See you at the finish line.

Fuelling

Modern gels are great at delivering fuel, they are one of the reasons we are seeing old records fall. The most effective strategy is going to depend on your size and individual tolerance but some general advice would be to aim for 60g carbs per hour and work up from there. Once you exceed 90g/hour the performance benefits diminish and the potential for gastrointestinal issues increases so there is no need to go totally insane.

Train, you gut! It doesn’t take long, practice with the amount of fuel you want to use a few times per week for a couple of weeks before the race and you will get better at processing it, making gastrointestinal issues less likely on race day. Flavour fatigue can be a big issue so having a few different fuelling options is good practice. Make sure you can tolerate them on a run, its no good switching to something that you haven’t tested. If you get really sick of eating just swishing glucose in your mouth can trick your brain into thinking fuel is on the way and give you a boast.

Caffeine!

Caffeine can really help to reduce the effects of adenosine build up in the brain

Caffeine has a few beneficial effects when it comes to endurance performance but the most relevant to ultra-endurance is probably that is occupies the same receptors as adenosine without altering their shape. With adenosine blocked dopamine is allowed to function more freely making you more awake, happier, and more motivated. It can be a double-edged sword though, too much can leave you jittery, elevate your heart rate and later cause a crash so it is best to wait until later on when you are actually tired and use it sparingly. Again, it’s going to come down to personal preference and tolerance. I don’t drink caffeine at all, so I’d never use it in a race because although I’d run really fast it would only be to the next toilet where I’d have to stop with a pounding heart and stomach issues!

Be Happy, or at Least Pretend.

A positive mood will keep your brain happier. If you are miserable your brain will look for every possible reason to quit. Stay present, break the race into chunks and don’t think too far ahead. Surround yourself with positivity, a well-prepared crew, briefed to remain positive and humorous at all times can work wonders (and avoid negativity, if you find yourself running with someone really negative either try to cheer them up or make excuses and get away!). Forcing a smile sounds silly but we all know it can release feel good substances so give it a go.

Brace yourself!

It is going to get tough so brace yourself! Remember everyone is in the same boat. Night sections in particular can be littered with DNFs. Bring good lights! Nothing ruins your motivation like tripping on a tree root and faceplanting in the mud. Have the right gear! There are required kit lists for good reason so use your kit, stay warm, or cool. When it inevitably gets tough embrace it! This is what you trained for; this is the moment you get to prove yourself this is the moment your ‘why’ really takes centre stage.

So, it’s not all about being the toughest, it’s about preparing well, setting the right, meaningful goals and having a solid reason as to why you are out there, fuelling right and staying positive. If you do happen to DNF at some point, don’t beat yourself up too much, isn’t a total failure, it’s more a lesson. Analyse what happened, find out what went wrong, put it right and go again.

If you are interested and want to read more.....

Mallory Richard 2024 Diving into Ultramarathon DNF Data

A statistical analysis of DNFs (did not finishes) in ultramarathons and their various causes. irunfar.com

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