Not only is Becky Rendell a joy to watch, as she destroys the competition out on course, she's also a highly trained and respected scientist in her field of exercise physiology. After sharing her essay on Winter DNFs with our pals at the Mudd Queens, she's asked that we pass along her wintery wisdom on the science of the DNF.

This article will not tell you what to wear or give recommendations on clothing, training or nutrition, but inform the racer, completer or DNF-er of what is really going on with your body this winter.

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.


When we exercise our muscles do work. We are not as efficient as machines so some of our energy ends up as heat. The harder we work, 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 through radiation (providing it is cooler outside than you are) blood needs to be close to the skin-surface. To achieve this, your veins get wider (vasodilation) -you may notice this, e.g. bulging out on your forearms. Next, you start to sweat. 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 onl dehydrate you without the benefit of cooling!

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

If it is easy for the sweat to evaporate (warm, dry air) you might not think you're as sweaty because it has evaporated so quickly, whereas in humid places you might notice it more - think desert vs. jungle. Winter is quite dry and you are probably still sweating if running. Also, people have different sweat compositions (estimate yours with precision hydration), and 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) and you return to a chilly winter breeze, this water is probably going to 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 (like when you’re running the first few miles of TG in a wetsuit?!). Still with me? Here comes the chill…

Cold Air

I must make it clear that cold air and cold water elicit different responses in the body. 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 in 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 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 cause heat loss 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, a bit like being immersed in water...

Cold Water

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You might have heard about cold shock before, particularly with last year’s RNLI Float to Live campaign, based on ~40 years of Prof. Mike Tipton’s research. Oh and FYI, immersion = up to the neck, submersion = head-under the water.

an actual electromyography trace I took of someone shivering in cold water, taken from the muscle down the side of the neck.

Cold shock. When you enter cold water the sudden lowering of skin temperature will cause a short lasting, but live threatening involuntary response. 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. 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. You’ve overcome the shock and have to make it to the other side of the lake. 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 subsequent 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.

It's always best to keep your arms out of the water, though that's not always possible!

Hypothermia. Contrary to popular belief, 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 to be clinically hypothermic your core temperature needs to be below 35°C (normally you’re around 37°C) and eventually this 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. It is difficult to accurately measure 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 or an 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. (That is also how your wetsuits work, by keeping a warmer layer of water and neoprene close to the skin). 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 to keep your arms out of the water if possible, but we don't always 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 that 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 to get the blood back up your body. If you are in a rescue situation, 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 up, you will warm up eventually - patience is a virtue.

Body Shapes and Sizes

This is where it gets a bit more individualised. 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 -just 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.

Measuring pre-season skin folds and body fat with OCR team Nuclear Phoenix.

Just a note on Raynaud’s here… Raynaud’s is a phenomenon where the blood vessels in your extremities constrict more than normal. 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!

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. So for now, find me at a race this year and talk to me about other environmental stressors (e.g. warm-weather training, altitude etc.), measuring your sweat rate and exercise performance physiology (e.g. V̇O2max, lactate threshold etc.).

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

If you would like to participate in important research for the RNLI:

Follow Dr Becky Rendell (PhD) – Lecturer in Exercise Physiology and Member of Nuclear Phoenix OCR team @therunnerbeanuk on Instagram for sport or @BeckyNeal7 on Twitter for science.

This article was originally posted on UKMuddQueens, and has been reposted with permission at the request of the author.