The Science Behind the Winter DNF

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Disclaimer: this article will not tell you what to wear this weekend, but it might be helpful for life.

It’s race week and not just any old race week – (not) Tough Guy. I challenge you to use a word other than a synonym of ‘cold’ or ‘brutal’ to describe the original obstacle course race-based over the unforgiving landscape in Wolverhampton in Winter. As we have come to learn more and more over the last 31 years, around this time of year the online experts come one and all with their wisdom of how best to tackle this event and survive the impending cold. But this year I’ve decided to bring the big-guns – science. The aim of this piece is not to give recommendations on clothing, training or nutrition but to inform the racer, completer or DNF-er of what is really going on with your body this winter. In fact, this stuff is useful for life so stick with my tentative links to OCR. Here goes.

Physiology – or how your body functions – is complicated because everything, including your temperature, is tightly controlled and integrated. Let’s start with something that I hope you are familiar with if you are considering taking part in an OCR… exercise.


Sweaty man exercising in the heat chamber (40°C), with several different physiological monitoring systems attached.

When we exercise our muscles do work. We are not as efficient as machines so some of our energy ends up as heat, rather than useful exercise. The harder we work, or the more muscle we have capable of working, the more heat we produce; if you run faster you get hotter, quicker. Heat is effectively carried in your blood and in order to lose heat to the outside through radiation (providing it is cooler outside than you are) blood needs to be close to the skin-surface. To achieve this, your veins respond by getting wider (vasodilation) and you may notice this e.g. on your forearms. Next, you start to sweat. Now sweating is only useful for heat loss if it evaporates. If you are hot, do not wipe it off or let it drip, this will only continue to dehydrate you without the benefit of cooling! Sometimes you might not think you are as sweaty/wet as other times, but this can depend on the environment around you. If it is easy for the sweat to evaporate (warm and not very humid air) you might not think you’re as sweaty because it has evaporated so quickly, whereas humid places you might notice it more – think desert vs. jungle. Winter is quite dry and you are probably still sweating if running. You can check your sweat rate by weighing yourself before and after exercise (but remember to include anything going in or out of your body in the calculations). I can help you with this if you’re interested. Also, people have different sweat compositions (estimate yours with precision hydration), and the electrolytes are there to help the sweat evaporate. It might seem like I’m talking about heat a bit too much for a winter blog, but this concept is important because if you are sweaty, or wet from some freezing cold TG lollipop-dunks then you return to a chilly winter breeze, this water is probably gonna evaporate from your body and take that precious heat with it (depending on clothing: cue discussion). Plus, I don’t want any heat casualties later on in the year (or when you’re running the first few miles of TG in a wetsuit?!). Still with me? Here comes the chill…

Cold air

You know you’re cold when… shivers – an actual electromyography trace I took of someone shivering in cold water, taken from the muscle down the side of the neck.

I’m going to start by making it clear that cold air and cold water can mean different things to how your body responds. It’s to do with the density and specific heat capacity of water, which are greater than that of air, which ultimately means heat will be dragged out our your body (conduction) faster; this is why we see more hypothermic casualties in wet races than dry races in the winter. Usually, if you are running a dry winter race in the UK you are able to produce enough heat through the exercise to offset any cooling from the cold air. If not, you need to either work harder to produce heat or have more insulation. Wind is an issue though – this will bring you to lose more heat through convection as it messes up the nice boundary layer of warmer air forming around your body (this layer is destroyed with movement too). If it rains, this will cool you down a bit like the sweat we talked about earlier, although at a different rate due to its temperature and lack of electrolytes, and a bit like being immersed in water…

Cold water

Hello TG and Winter Nuts. This part could go on a bit, so I’ll give you the basics and if you want to know more, the best person to read is Professor Mike Tipton (MBE), a world leader on cold water immersion. You might have heard about cold shock before, particularly with last year’s RNLI float to live campaign, based on ~40 years of Prof Tipton’s research. To recap (oh and FYI, immersion = up to the neck, submersion = head-under the water):

Cold shock. When you enter cold water the sudden lowering of skin temperature will cause an involuntary response that is short-lasting but threatens survival. The blood vessels in the skin constrict (leading to an increase in blood pressure), you can’t help but ‘gasp’ (large enough to drown you if you breathe in water instead of air!), and your heart rate will increase rapidly. All in all, this can make you panic and occasionally it all gets a bit complicated for the heart, even in fit young healthy people close to safety, and a heart attack ensues. Wearing a life-jacket, to help keep your head out of the water if you fall/jump in cold water will reduce the risk – it is not there to help you swim faster in the race. On this note, RDs, lifejackets are better than bouncy-aids, if you can please! Float to live. Practicing calming yourself down in a lifeguarded swimming pool wouldn’t go amiss. But if you really want to help yourself, you can habituate slightly to this shock response. Short repeated exposures to cold water will minimise the reaction, but it will still be there to some extent. Where possible, enter cold water gradually, even when racing, and keep your head out.

Swim-failure. So now you’ve overcome this shock and you’ve gotta make it to the other side of the lake in the race. In this time your muscles and nerves get gradually colder, especially in the arms and legs. This can reduce your manual dexterity, grip strength and the speed at which you can move. Long enough in this situation and swim failure (and subsequently drowning) will occur – you can notice this by someone going from a more horizontal position to a more vertical position in the water, and they probably won’t be able to put their arms in the air and wave for help either – it’s the quiet ones you’ve got to look out for. Again, wear a flotation device.

Hypothermia. Contrary to popular belief in the OCR world, this occurs a bit later in the stages of cold water immersion. Of course, you are going to feel cold and a bit useless before this because of your low skin temperature and the effects described above, but clinically to be hypothermic your core temperature needs to be below 35°C (normally you’re are around 37°C) and eventually leads to loss of consciousness. The rate at which this occurs will depend on a few things: the water temperature; whether you are floating or moving; your body fat; your body surface area; and of course clothing. I will go into a bit more about your body shape etc. in the next section. It is difficult to practically measure accurate core body temperature out in the field and sometimes the numbers can’t be taken too literally so accompanying symptoms should also be taken into account – see this video of an experiment I did once). One day I hope to give you guys some core temperature pills and measure how cold you really get, but until then, I’ll continue to race the 1 lap event at Winter Nuts (too much knowledge for my own good here/excuse for being a bit of a wimp)!

Rest vs. exercise.  In cold water, moving can sometimes make things worse. If you are floating in still water, you can create a cosy boundary layer around your body to help keep you warm. Plus because you are not using your muscles, they can insulate you too. When you exercise, blood gets sent out to your muscles and skin close to the surface where it is colder and returns to the heart cooler, which can speed up your cooling rate. Your arms are particularly at risk as they have a high surface area for heat loss and lower muscle mass for heat production than your legs – try and keep your arms out of the water if possible… but often we don’t get a choice in OCR.

Post-immersion collapse. Something that is not often considered is what happens when you leave the cold water. When you are in water (goodbye gravity), particularly cold water, there are fluid shifts inside you and you might realise you need a wee. This affects your blood pressure so when you leave the chilly but buoyant environment (hello gravity), the blood sinks to your feet, your head and heart aren’t too happy and you might feel dizzy/collapse. If you’re racing keep that muscle-pump in your legs going to get the blood back up your body. If you are in a rescue situation, try and keep the person horizontal when they are removed from the water and tell them to keep fighting for their life, not to relax.

Afterdrop. This is a thing where you continue to cool down after you stop being in a cold environment. I can explain it with a melon experiment if you want (or here it is in some humans). Just like it takes time for the cold to set in, it also takes time to re-warm. This can also depend on what you’re doing as well as your body’s characteristics. For example, if you exercise, blood will go to the muscles and cold skin surface, cool down and return to the heart cooler than it was before. Or, if you sit in a warm bath after a cold training run, your protective vasoconstriction will turn to vasodilation, but you will warm up quicker than doing nothing or exercising (again, here’s me in the bath vs. some poor people in a survival bag). As long as you’ve done things to warm you up you will warm up eventually – patience is a virtue.

Body shapes and sizes

OK, this is where it gets a bit more individualised. As I mentioned earlier, if you have more muscle and work harder, you can generate more heat to keep you warm. The other half of the equation is body fat. Fat is an insulator so helps prevent heat loss, but doesn’t help generate it, unlike exercise or shivering. Remember shivering is a good thing and helps keep you warm, so don’t be afraid, although it is using up energy too – eat more. It follows that the more fat you have, the slower you will cool down, so embrace that winter body! Plus, fat is distributed differently between men and women (females also tend to have more fat than males). Another complication is body surface area. If you are tall and slim you will lose more heat because you have more blood vessels close to the surface – this is why your hands and feet get a lot colder, they have a high surface area-to-mass ratio. Just a note on Raynaud’s here… Raynaud’s is a phenomenon where the blood vessels in your extremities constrict more than normal (amongst other things). It is more common in women than men and puts you at risk of other cold injuries, so don’t try and train your fingers out of it! Now, remember that boundary layer we spoke about? That’s what your wetsuit is doing – creating a nice warmer layer of water/neoprene around you. Everybody loves layers.

There are other factors which can influence temperature or your perception of it e.g. alcohol, paracetamol and menthol, but I won’t go on any longer. Plus there’s debate on whether cold water swimming can boost immunity. But for now, find me at a race this year and talk to me about other environmental stressors (e.g. warm-weather training, altitude etc.) and exercise performance physiology (e.g. V̇O2max, lactate threshold etc.).




We would like males and females of all shapes and sizes to participate. We are looking in particular for male volunteers aged 18-30 years.  They will get £20 for participating. 

The study involves attending 1 session at the RNLI in Poole.  The volunteers will be asked to wear summer clothing and fall into the water in their pool and stay there for two minutes, the water will be a bit cold at 15oC, they will then repeat that in winter style clothing. 

We will be running two study dates on the 21st and 22nd Feb. It will take a couple of hours, one session in the morning and one in the afternoon, volunteers only need to be available for one of the sessions.


With thanks to The University of Portsmouth, King’s College London (Dory Video) and Bournemouth University.

By, Dr Becky Rendell (PhD) – Lecturer in Exercise Physiology and Member of Nuclear Phoenix OCR team.

Follow her @therunnerbeanuk on Instagram for Sport or @BeckyNeal7 on Twitter for Science.

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