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Tuesday, January 29, 2008

Exercise in the Cold: Part II

A physiological trip through cold water exposure

While we are all still thinking about the incredible video of Kayoko Fukushi's marathon finish, we thought we would carry on with our series on exercise in the cold. Most of us are still firmly in the depths of the northern-hemisphere winter, and so we hope you found Part I of this series relevant---it was meant to introduce some basics of heat loss and temperature regulation, and also look at some of extreme stories such as the runner who "won" an ultra-marathon by being the only one to complete the race in below-freezing temps. The price of victory? Losing two toes!

Water vs. air exposure

In tackling this series on exercise in the cold, we differentiate between cold AIR and cold WATER exposure - the key difference is that the heat LOSSES in water are so much greater than on land, that you're far less likely to develop any kind of hypothermia (low body temperature) on land. In fact, water conducts heat about 25 times better than air. So when water moves across your skin you will lose heat more quickly compared to air simply because of its physical properties.

As as we said in Part I, the key to managing exercise on land is simply through appropriate clothing. But the introduction of some water to the mix ('Just add water') makes the situation far more complex and challenging and that's what we'll tackle today!

Too much heat to cool down - why hypothermia is the least of your worries

Perhaps the first, and maybe the most surprising fact about cold water physiology, is that your body has too much heat to become hypothermic within about 30 minutes, no matter how cold the water is! In other words, it is not possible to get so cold that you're in danger unless you are in the water for more than about 30 minutes. The graph below shows this:

So from this graph, you can see that even at water temperatures of 0 degrees celsius, 30 minutes falls within the marginal zone, not the lethal zone. Many would probably survive for close to an hour - this is demonstrated by shipwreck victims, who have survived freezing water for this long.

The implication of this is that if someone is immersed in cold water, and dies, the cause of death is unlikely to be hypothermia unless that person has been exposed for a long time! Hypothermia is often wrongly blamed for death in people exposed to cold water - getting too cold is actually the least of your worries! We'll take a look at the main challenges a little later, but first, a key discussion about body composition and its effect on your ability in the cold.

Lessons from English Channel swimming

Perhaps the best forum in which to examine the physiology of cold-water swimming and immersion is English Channel swimming. The great British Exercise Physiologist Lawrence Griffiths Pugh performed a series studies in this area on the channel swimmer Jason Zirganos. Zirganos died from exposure to cold water while attempting to cross the Irish Channel, but his legacy was to leave behind a solid understanding of cold-water physiology.

The importance of body fat - 'fatter = warmer'!

The first important point about cold-water exposure is that body composition has a profound effect on core temperature during immersion. Pugh demonstrated this when he compared himself---the scrawny scientist type---to Zirganos, the chubby cold-water swimmer type. When just sitting in 16 C water the rectal temperature of both men fell, but after approximately 80 min Zirganos was sitting a full one degree higher than Pugh. The more dramatic difference was when swimming in the same water, though. Zirganos was able to maintain his temperature at around 38 C for nearly two hours while Pugh's temperature began to plummet after just 30 min until he exited the water after about 70 min, when his temperature was less than 34 C.

This early research from the 1950's pioneered this area of physiology, and today we have a substantial body of evidence that demonstrates both the effects of cold-water immersion and how we adapt to this stress.

The cold-shock response - the biggest challenge to survival in the cold

One of the first things you experience when submerging yourself in cold water is something called the "cold-shock response." This is characterized by an uncontrollable gasp for air, followed by a prolonged period of hyperventilation - more rapid breathing. In fact, this response is one of the most likely causes of death in most cold-water immersions such as when one falls out of a boat into icy water. It's not difficult to see that if the timing of that "gasp" is slightly wrong, you'll take in a huge lungful of air, and one or two gasps while underwater is all it takes to drown.

The other big 'killer' is a heart attack, which can result when the temperature of the blood returning to the heart is suddenly cooled - this can affect the electrical conduction within the heart, causing fibrillation. So it is these two possibilities - drowning and cardiac arrest that are most likely the cause of death. However, as we said, most times, people blame hypothermia for death, when in fact the body temperature does not need to fall for an unlucky 'swimmer' to perish in the cold.

Swimming in the cold - a problem of breathing and muscle weakness

Once you've overcome that problem, however, the next thing to worry about is swimming. And again, the hyperventilation that happens in the cold has a profound effect on the ability to swim in an efficient manner. The graph below, from a paper published by Eglin and Tipton in 2005 (EJAP) shows the breathing response of a swimmer exposed to cold water. It shows the BREATHING RATE in breaths per minute against time in a person who stands in a cold shower at 10 C.

So the rate of breathing goes up from about 16 breaths per minute to 75 breaths per minute, within the first 20 seconds. It then stays up at 40 breaths per minute for the next few minutes. It is not difficult to see how that would affect your ability to swim, because your stroke rate would have to change substantially to allow you just to breathe!

Next problem - the "DiCaprio" problem - a cold muscle, and cold skin, equal a weak muscle

The next problem is equally significant - when the muscle and the skin are cooled, the muscle becomes weaker! So the cold water on the skin will make a powerful swimmer incapable of swimming, simply because his skin is cooled. There is evidence from studies that shows that the ability of the muscle to produce force is as much as 25% lower immediately after exposure to water at 10 degrees celsius - this would only drop in even colder water. Then we add to that the fact that as you get cold, your body's natural response is to shiver. But when you shiver, your co-ordination is affected, making it even more difficult to swim!

This obviously has profound implications on ability to swim. And for all those who watched in despair as the character played by Leonardo DiCaprio could not swim to safety in the movie Titanic, you now have a physiological explanation - he simply could not swim, because his skin and muscles were too cold to contract normally! (Far be it for us to suggest that Hollywood portrayed that accurately!). The principle remains, however - a good swimmer in warm water will be an average swimmer in the cold. And a weak swimmer in the warm...well, that's a recipe for trouble.

The good news - adapting the cold shock response

So that is the bad news. . .but the good news is that humans are adaptable organisms, and just like we make adaptations to things like marathon training, we also make adaptations to stressors such as cold-water immersion. The data show that exposures to cold water as short as three minutes in a 10 C shower will attenuate the cold-shock response by as much as 20-30%. In the graph below, you can see the same data we showed above (Eglin and Tipton, 2005, EJAP), but this time, we've added in a comparison with the breathing rate AFTER six 3-minute long exposures to the cold water.

So you can see that only six exposures is enough to reduce the cold shock response by 20%. If you have even longer exposures, you can bring it down by 50%. That is obviously a significant reduction, and the implication is that swimming will be far easier if you are simply adapted to the cold.

The second important adaptation has to do with blood flow and heat loss. When at rest your muscle tissue actually acts as in insulator. This changes when you exercise because now you are pumping lots of blood to the working muscles, and it is the blood that transports heat around the body. Therefore when you start to swim in cold water you send more blood to the muscles, and all this does is increase your heat losses as now the blood----and the heat it contains----it close to the surface of the body and the cold-water. Since water conducts heat very well, the heat from your body readily moves to the water. . .and the consequence of this is a decrease in core temperature even though you are producing some heat with your muscle contractions.

Decreases in sh-sh-sh-shivering

Another big change that occurs with repeated cold-water exposures is that we lower our "shivering threshold," or the temperature at which we begin to shiver. The bonus of with shivering is that we produce heat as our muscles are contracting, although involuntarily. The bad news is that when trying to perform a complex movement such as swimming (or any dynamic activity), shivering can really foul things up. So we adapt by lowering the temperature at which we begin to shiver, and the result is that you can swim for longer before being hampered by shaking limbs and uncontrolled movements.

Evidence for non-shivering thermogenesis?

Finally, there is evidence that humans actually increase their core temperature either acutely or chronically in response to repeated cold-water exposures. The net effect of this response is that they can then remain in a cold environment for much longer before suffering any detrimental effects of the exposure, such as decreased nerve conduction velocity and then shivering (and a loss of coordination as a result of that shivering). Simply put, they have more heat in their bodies, and together with the other adaptations we mentioned above it means they reach a critically low temperature much later than someone who is not adapted to the cold.

So the take-home message here is that cold-water exposure is just like any other "stressor" or training stimulus. Our physiological response to these stimuli is to make adaptations that allow us to cope better with the , which in this case is cold-water immersion.

That wraps up Part II of this series, but stay tuned for Part III, when we will examine how the cold actually affects exercise performance!


Anonymous said...

GREAT POST! At a triathlon in Kenosha, WI last spring I developed hypothermia after being in the water for over 33 minutes. [.9 mile swim USUALLY only takes me 24 minutes but due to the cold, I could feel my lats and traps tighten half way through the swim and breathing became EXTREMELY difficult.] This only reasserts some of the comments you've made here.
But immediately exiting the water, I started to take off my thin, surfer-style wetsuit. Soon following this pocketed release of body heat, I collapsed and didn't know where I was. Luckily medics were close by to get me into a warming tent.
Any suggestions for low % body fat individuals who need to practice swimming more in open water (usually without a wetsuit)?

Ross Tucker and Jonathan Dugas said...


Thanks for the post - interesting story and it does certainly complement the post!

The phenomenon you observed in your triathlon is a very typical one - the body temperature falls AFTER you get out of the water. Physiologically, what is happening is that as you 'warm up' (once out of the water, that is), your body sends blood back to the skin, having previously shifted it all to the core to help prevent heat loss. So once this blood is sent back to your very cold skin, it's a matter of time before it recirculates and carries that cold to the rest of the body!

This "afterdrop" is almost always seen and I think it happened in your case! Good thing they had the right treatment for you.

As for what you can do - training in the cold is one, as we mentioned, because it helps you defend body temperature far better than when in an unadapted state. But failing that, the key to "survival" in cold water is that you must be a strong enough swimmer that you can resist the peripheral effects of the cold. Once again, your example showed that - had you swum a couple minutes faster, you may well have avoided the big drop at the end. So it really is key that you learn to swim in the cold, and also develop your swimming to the point where the impact the cold has on performance is not that great.

But of course, the best thing of all is to wear a wetsuit when you know it will be cold!

We'll be discussing a few more tips and strategies for cold weather exercise later in this series!


Andrew said...

Once again, amazing info! Two questions for you guys:

1) Many folks die from falling in cold water for less than 30 mins - you suggest drowning and heart attack as the culprits, but what about hypothermia due to "afterdrop"?

2) How do you explain a guy like Wim Hof? Is he just very well trained and adapted?

Ross Tucker and Jonathan Dugas said...

Hi Andrew

Unlikely that the Afterdrop is the cause of death - remember, the afterdrop only happens once the person is rewarmed or removed from the cold environment. By definition, it happens because the blood that was initially sent AWAY from the skin to conserve heat now gets sent back to the skin and then cools the body once it starts circulating. So no, the predominant cause of death is drowning or heart failure, and only if the person is submerged for longer than 30 minutes (at a minimum - remember, 30 min is the limit in VERY cold water - in anything above about 2 or 3degrees, survival time is much longer - see top graph).

For obvious reasons, though, it's difficult to pinpoint cause of death after the fact, because often, the body is found days later. There is obviously evidence for aspiration of water, and hypothermia - telling the two apart is very difficult. And an electrical failure of the heart is impossible to detect post-mortem. So the confusion exists - but in theory, and based on physiology, it can't be hypothermia within the first 30 minutes, at least.

Then secondly, thanks for the link to Wim Hof. At the risk of sounding blase about it, yes, it's just adapted physiology. Not dissimilar in fact, to an athlete who starts out being able to run a 4 hour marathon with zero training and ends up running 3 hours after months of training. And maybe even 2h30 with some more effort.

Of course, it's still impressive physiology, but too often it's seen as "freakish" when in fact it's not. I'm not 100% sure of his physiology, but I'd guess he has quite a high body fat percentage, has reduced that "cold shock" response right down, and likely displays hormonal adaptations that allow him to increase his core temperature, reduce shivering and distribute blood really effectively. And then of course, there's mental willpower and training - another analogy would be the guys who walk over hot coals, but have trained themselves not to feel it.

So it's interesting physiology but certainly not "freakish".


Anonymous said...

Hi Guys

Fascinating series I have to say. I have intimate experience of exercise in the cold under lab conditions and armed forces selection courses.

Maybe you know Mike Tipton of Portsmouth University, UK? He carries out a vast amount of thermal physiology research, some of which i have had the pleasure of participating in.

I seem to remember that the amount of fat around the triceps is a good indicator of the time to failure of swimming technique/ability in cold water.

The Sports Scientists said...

Hi Daniel, and thanks for participating in the discussion here on The Science of Sport.

We do indeed know of Dr. Tipton, and read much of his work when we worked with a cold-water swimmer names Lewis Pugh. Dr. Tipton has really published substantially in that area and consequently is a bit of an authority in that area of physiology.

Interesting that the triceps skinfold is a good predictor of swimming ability in cold water, and thanks for sharing that with us.

If we can ask, in which experimental trials did you participate? It would be really interesting to hear about them here.

Thanks again for visiting!

Kind Regards,

Anonymous said...

Firstly, it's a pleasure to be involved in the commentary of this blog. The posts are top notch and i admire the time and effort that you put in.

Secondly, Professor Tipton was a member of the academic staff during the time of my undergraduate degree. An associate lecturer,also a member of the thermal physiology department, conducted lab sessions in which we were immersed into a cold water tank and instructed to swim until it was no longer possible or we were deemed unable.

On another occasion I was a subject for a cooling methods experiment. 3 subjects wore Royal Navy issued fireproof boiler suits and performed a stepping exercise. Each subject was then assigned to either hand immersion in warm water, an air conditioning fan or nothing as a means of control.

These were 3 years ago so my memory of them is sketchy but i have notes in the attic somewhere.

I didn't take part in any published work but certainly learned a great deal about thermal physiology.

I took part in a piece of Ph.D. research as part of my Masters degree last year also. Ice baths and heart rate patterns. Also, treadmill running in 39 C heat and high humidity drinking cups of tea versus sports drink. The list goes on so i'll stop here.

Keep up the good work lads. Rest assured i'll be back for my 2 pence worth in the future.

Ross Tucker and Jonathan Dugas said...

HI Daniel

Thanks for the kind words. I know Jonathan replied last time, so this is my response to that email and this latest one!

I met Mike at a conference in France in 2004. It turned out that one of our graduates from UCT went and worked with him for a while - Paula Robson (later Ansley, perhaps you know her too?).

In any event, two of the graphs in this post were from Mike's work (I have actually gone back and added the reference) - the ones showing the breathing rate and the adaptation of the cold shock response were from studies by his group.

He is the guru on cold water research for sure, and his work will come up in the next couple of posts in this series as well. I'm sure you'll recognise them!

Anyway, thanks so much for the postive comments and encouragement!


Unknown said...


Just found a link to your blog from a site on winter open-water swimming.

In the second main paragraph of your text, you state that "water conducts heat about 25 times better than air".

This is correct, but of course conduction is not the only route of heat loss from the body. As a result, this comment in isolation is slightly misleading.

In air, the human body looses heat through conduction (very little), convection, radiation, and evaporation. In water; evaporation & radiation are reduced to virtually nil (only from the head (presuming the head is out of the water), convection is virtually nil (due to the effeciency of the water at removing heat), and conduction is maximised.

As a result, heat loss from a semi-nude human body is only four to five times greater in water than in air of the same temperature.

This is an important element of Tipton's work, and is (I feel) overlooked in his book "Essentials of Sea Survival". There are many myths about how much heat is lost from the human body into water, and I thought I would take the time to clarify your point.

Congratulations on a very interesting blogpost, I'm now going to spend some time reading the rest of them.

Nancy said...

Hello! I know you posted this a long time ago, but I was wondering if this explains my issue every time I do an ocean water competition. I always blamed it on bad circulation because I'm of average build - and body fat, I think. I'm usually the only one that gets so cold my feet and hands get numb, and as a result, my performance is not very good compared to my peers - in ocean water, times are more dependent on conditions in the water, etc. I'm about to give up on open water swims, or just do them in Hawaii. I live in Southern California by the way, where the temp is usually in the 60's in the summertime. If you have any advice on how I can combat this (more swimming?), I'd much appreciate it. :-)

Unknown said...

I have been swimming for about 20 mins each day in an unheated pool wearing a shortie wet suit in winter. The water temperature dropped to around 5 Deg a few days ago and I noticed quite severe pain in my hands and (less) in my feet when I showered after swimming. I was concerned this might result in some tissue damage if I carried on at still lower temperatures. Over the last couple of days the temperature has fallen further (about 3 deg) and so I have felt it safer to stop for the moment. I really enjoy the swimming and would prefer to carry on, but am I at risk of damaging toes / fingers? I am 53 and healthy.
Thanks, Robin