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Swimrunning in cold conditions – Utö swimrun coming up

Up next week is Utö swimrun, which this year seem to become a cold race.

Expected weather forecast

In the links below you will find a good indicator of what the weather will be like at Utö. Remember that Stockholm archipelago is well-known for its occurring sea breeze, which can affect the wind direction.

SMHI (click on Idag = Today or Måndag = Monday)

Expected wind


Expected water temperature


This is the best and closest average measurement available. However measurements are taken 1 metre below the surface and doesn’t take wind, waves and currents into account. With these factors the water will be colder on the surface.


The basics of getting cold

When swimrun racing in cold water there are some important things to keep in mind, especially if you are not used to it. We have covered this topic on several occasions, pointing to out the risks and dangers of getting cold.

Before hitting the cold water it’s important to understand the basics behind why you get cold. As long as your body temperature fall short of 37 degrees Celsius it’s in a constant state of negative heat transfer, losing moisture through either sweating, insensible perspiration or respiration. If maintained over time this can lead to dehydration and make the body more susceptible to hypothermia and other cold injuries. Water conducts heat away from the body up to 25 times faster than air, and adding the wind-chill factor to this, your heat loss will become even greater. When wearing a wetsuit you are protected from the wind-chill and the high loss of heat in the water, but you sweat more which could lead to higher heat loss when swimming if your wetsuit is not a 100 % fit allowing fresh cold water in.

To sum up: You basically lose body heat from low outside temperature, rain, sweat, water and wind.

What happens when the body loses heat?

The body shuts down systematically starting with fingers and toes, undergoing a physiological amputation of the extremities to protect vital organs. There are 4 zones on the body which are better at releasing heat than the rest, the crotch, armpits, neck and head.

So, what should a swimrunner wear?

Most swimrunners prefer not to add any extra insulation, but for some races it could be smart to do so. If you’re a thin person with low body fat %, have an effective metabolism, a low-level of fitness or fuel store capacity, you might want to add an extra layer or two, especially if you’re doing a longer race meaning low energy levels towards the end. It happens from time to time that people quit races because of being too cold in the final stages. Having a neoprene cap and gloves available in your equipment could be a good idea, or lots of Vaseline to protect the exposed areas around the head and neck.

But shouldn’t you wear anything underneath and if so, what?

This is a tricky question. I would say it completely depends on the type of race you are doing, and when in the season. But to start off. If you are going to keep the wetsuit on for the entire race, it doesn’t really matter what type of material you wear underneath, just as long as it feels comfortable and doesn’t give you any chafing. You will get sweaty and most likely wet when swimming, and since the wetsuit doesn’t allow for air to circulate and release heat, having a material that “breathes” away the redundant heat won’t help you at all. Now if you on the other hand plan to cab-down during the race, then it’s another matter. During these moments you will have the possibility to release redundant heat why you should choose a material that does this well. Both polyester and wool will do this for you where the difference between the two is that wool also keeps you a bit warmer than polyester. A regular t-shirt layer and boxers, or perhaps knee-long boxers will do fine.

So, the combination of a wetsuit and functional clothing is a bit tricky?

Following the logic of how the body protects itself and where it disposes of heat, the current available wetsuits would have to be re-constructed to allow for maximal “during-the-race” adaptation. As it is now, the swimrunner don’t have that many options to stay warm, apart from having a 100 % fit wetsuit and protective clothes underneath, hoping for the best during the race. Because when you pass a certain point of losing body heat, you won’t be able to make up for it unless you quit the race completely. On the other hand, if you need to release heat, you basically have the option to cab-down or take the suit off, and swim with it slightly open to allow water in and cool off this way. Both very basic and rough options and not the fine-tuning we are used to with e.g. general running or hiking clothes. To be most effective in reducing excessive heat a wetsuit should be possible to ventilate around the neck, around the armpits and the crotch or that you have the ability to adjust the water intake during the swim, just enough to keep you levelled. Adding zippers to the heat zones mentioned above, like with traditional outdoor clothing, would be optional, but that would most likely also impede on the flexibility of the wetsuit.

To sum-up, having an extra layer can be good for longer colder races if you’re a person with a low % of body fat who usually get cold easily?

Well that’s the basic idea and the best way as always is to get out there and make a trial swimrun to get to know your body and gear. Pay attention that not all wetsuits are equally thick making them keep or lose heat differently in the water and that they really close shut around the neck, other than this just try your way through and also with the eventual clothing you want to have underneath. If you are uncertain, buying a full suit is always the better option since you can cut and modify it at your own convenience. Staying fat is another great option as a study has shown previously (below).

/The WoS Team

Swimming in ice cold water
B. Knechtle, N. Christinger, G. Kohler, P. Knechtle, T. Rosemann

Introduction: We investigated two athletes swimming in 4 C for 23 min (1.3 km, swimmer 1) and 42 min (2.2 km, swimmer 2), respectively. Materials and methods: Pre-swim, percent body fat was determined; post swim, core temperature was measured. Results: The core temperature of swimmer 2 was: 37.0 C immediately before the start, 32 C 20 min after getting out of the water, and 35.5 C 80 min after finishing the swim. Conclusion: We assume that the higher skin-fold thickness and body fat of swimmer 2 enabled him to perform longer. In addition to this, mental power and experience in cold water swimming must be considered. In any athlete aiming at swimming in water of less than 5 C, body core temperature and heart rate should be continuously monitored in order to detect a body core temperature below 32 C and arrhythmia to pull the athlete out of the water before life-threatening circumstances occur.

Read the full study here

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