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Thursday, January 24, 2008

Exercise in the Cold: Part I

Introduction to HYPOthermia and survival in the cold

As promised (a while back, admittedly), today we kick off a series on exercise in the cold. A challenging one to write, mostly because most of you reading this will (hopefully) never be exposed to conditions that are so extreme that your physiology is challenged to the point where it can't cope. Because most of you reading this are runners or cyclists, and it's only when you get into the water that a potentially lethal drop in body temperature is a real possibility. For the most part, exercise in the cold on land is limited by sensation and perceptions, which are of course controlled by wearing appropriate clothing. The biggest dangers of exercise in the cold on LAND? Frost-bite and other medical conditions, and not actually hypothermia.

But, there are still some fascinating studies and insights into exercise in the cold, and it is one of the most amazing areas of physiology to discuss. As I mentioned yesterday, Jonathan and I both worked with the polar swimmer Lewis Pugh when he prepared to swim 1km in the Arctic Ocean in 2005. Jonathan subsequently worked on his trip to the Antarctic as well. So we have a tendency towards COLD WATER physiology, which will make up the first couple of posts. Then we'll move onto the studies of running, cycling and muscle function during cycling and running.

So bear with us as we move through the series in a few parts:

  1. How cold is cold? What are the limits to survival in the cold? Some stories of cold exposure in sport
  2. The physiological responses to cold water immersion
  3. The effects of cold temperatures on performance during swimming, cycling and running
  4. Practical recommendations and insights
To begin with, how cold is "cold"?

In terms of body temperature, let's begin with a few basics - your body temperature as you read this is somewhere between 36 and 38 degrees - most textbooks will tell you 37 degrees celsius (or 98.6 Farenheit). As with exercise in the heat, the regulation of body temperature within a fairly narrow range is critical, not only for performance, but for health and survival. When we exercise, the body temperature very quickly rises to about 39 degrees celsius - one of the most remarkable questions in physiology, in my opinion, is why the body would allow this when it has the ability to lose the heat? It can reach 40 degrees celsius with few ill-effects, as found by numerous studies by the Danish group of Nielsen and Nybo.

The body regulates this temperature in a number of different ways. When we wrote about heat, we spoke of sweating, sending blood to the skin etc. In the cold, the opposite happens - you shift blood away from the skin, you shiver and release hormones that help keep the temperature up. The body is naturally insulated by skin, muscle and fat, and it's no co-incidence that on land and in water, the lean athlete, with low body fat percentage, is likely to get colder sooner!

What determines whether hypothermia happens ON LAND?

A drop in body temperature in recreational athletes is rarely seen ON LAND, even in quite cold conditions, provided they are not forced into prolonged exposure (discussed below). Rather, the main factor predicting body temperature is metabolic rate, and so provided you are exercising, your body temperature will rise, regardless of how cold it is...up to a point.

Remember that your body temperature will only fall if you lose heat faster than you produce it. When you are exercising in cold conditions, the fact is that you're usually:
  • Dressed very warmly and therefore not likely to lose heat too rapidly - in this case, your body temperature is a function of heat production; or
  • Dressed very inadequately, with skin exposed, or not waterproof. In this case, you'll FEEL so cold within minutes of starting, that you're likely to be unable to run altogether. You'll turn home and get warm in no time!
The point is, the SENSATION of the cold, mediated through your skin, precedes any fall in body temperature. So, by extension, provided you have free-will (and choose to exercise it - see the story of Andrew Wells below!), you'll be able to get warm without any complications. If, however, you're unlucky enough to get caught out, either because you're underdressed, far from shelter, or forced to spend a good deal of time in the cold (injury on the run/ride, getting lost etc. - think Polar explorers!), then your physiology really will be challenged, and that's what we'll tackle in subsequent posts!

Interestingly, in water, the situation is quite different. Because as we'll see tomorrow, you actually lose MORE HEAT when you exercise in the water than when you don't! So it's in water that hypothermia becomes a real threat - hypothermia, incidentally, is a fall in body temperature below about 35 degrees celsius - by this time, you'd be shivering uncontrollably.

But for today, we'll look at survival in the cold, and just what the risks are.

Some stories of cold exposure

In 1964, during the Four Inns Walking Competition in Derbyshire, England, tragedy struck when 3 hikers died as a result of cold conditions combined with poor preparation and planning for the cold. In this race (which is described by Pugh and later Noakes), competitors set out in temperatures between 0 and 4 degrees celsius, with wind speeds of about 45km/hour.

Seven hours into the event, the first reports of hikers in distress were received. In total, three hikers died as a result of the extreme cold and wind. In an analysis of these events, the following three characteristics were common to all three deaths:
  • Inadequate clothing, which was not water-proof. As a result, when it began to rain, the water, combined with the cold and wind would have meant zero insulation for the hikers
  • All the hikers had low body fat percentages. This was insufficient to make up for the loss of insulation by clothing once the clothes were wet
  • The three hikers fatigued within a few hours of starting. As a result, they were walking very slowly, often collapsing, and hence unable to keep their rates of HEAT PRODUCTION high. The hikers who survived were those who finished faster.
These three points demonstrate the principles of exercise in the cold ON LAND - make sure you're dressed well, and make sure you are not forced to stop exercise or remain exposed for any longer than is necessary (unfortunately, this is not a choice we always have!)

In marathon running - evidence of hypothermia

The story above deals with hikers, exposed to snow, rain, wind and cold for days at a time. What about marathon runners? There is some evidence of hypothermia in marathon running. And perhaps surprisingly, it doesn't take temperatures below freezing to cause this. For example, in Scotland in 1982, in the Aberdeen Marathon, temperatures were about 12 degrees celsius, it was quite windy (25 to 30km/hour), but there was no rain, and yet 4 out of 59 runners finished with temperatures lower than 37 degrees! Similarly, in South Africa in 1985, during a 56km race, a temperature of 19 degrees combined with wind and rain was enough to see 8 runners taken to the medical tent with body temperatures below 37 degrees celsius!

Once again, in these cases, the likely cause is a combination of inadequate clothing, combined with low heat production as the athlete becomes fatigued. One of the body's responses to cooling is to shiver, and use more energy in an attempt to keep your temperature up. This is hardly conducive to performance and so the exercise intensity drops and body temperature tends to fall. Also, most of the time, athletes, particularly in South Africa, would expect reasonably warm conditions - they'd dress for these conditions, wearing little more than running shorts and a vest, completely inappropriate for wet, cool conditions, once the wind starts blowing. The same would apply to cycling, perhaps even more so, since the higher air speeds move heat away from the body more rapidly.

Having been in the medical tent at the last three Comrades Marathon events (90km race), I can also testify that towards the end of the day (around 4 or 5pm), when temperatures have fallen to the low teens (not cold, by any means), many runners report to the medical tent feeling very cold, shivering uncontrollably. And a big part of it is that they are fatigued, exhausted from the preceding 80km. They walk more, which causes heat production to fall. At the same time, they are underdressed, and then unable to do two jobs at one time - provide energy for exercise and for keeping the body warm. So the athlete often pays by cooling down and feeling very cold.

More recently - a high cost for winning a race

So we've seen that on land, the biggest danger is that you'll be caught underdressed and unable to keep exercising hard enough to keep your rate of heat production up. Provided you combat these two potential problems, your risk of hypothermia is actually relatively low. However, there is still a risk of frost-bite, demonstrated as recently as this past weekend by the case of one Andrew Wells. Wells won the Frozen Otter Ultra Trek, a 64-mile trek on what is called the Ice Age Trail in the Northern Kettle Moraine State Forest.

Wells won the race not because he finished the 64 miles first, but because he was the only competitor to actually reach the fifth check-point of the race. Only two competitors reached half-way! That's how severe conditions were - it was nearly 15 degrees below freezing at times!

But Wells clearly kept warm enough to avoid the potential for hypothermia. But unfortunately, he was not as lucky when it came to frostbite. This quote comes from this news story on the race:
Wells said he never noticed the frostbite set in. "My feet were obviously frozen, so I couldn't feel them," he said. "And it was too cold to take my shoes off to check my feet. On my hands, I had mittens on, and just to take them off for 30 seconds, my hands got really cold, painful. I thought my toes were OK."
It turns out that Wells competed in only running shoes, one pair of wool socks and a pair of waterproof socks! He'll now have two toes amputated! This story demonstrates that the danger of hypothermia may be lower, but frost-bite is a real possibility. Interestingly, none of the other competitors had this problem, because most, it is reported, bailed from the race when they'd hd enough. Wells, on the other hand, pushed past the initial pain until numbness set it. So it certainly is possible to voluntarily exercise into health problems, but for most, the sensation determines the safe limit.

But the record belongs to...

However, all these stories pale into insignificance when compared to the record for LOWEST SURVIVED body temperature. This report, published in Lancet in 2000, reports the remarkable story of a skiier, who holds the distinction of record cold-survivor:
“A twenty-nine year old woman fell while skiing (18h20), and was trapped and partially submerged under a waterfall. After 7 min, emergency rescue was summoned and, after 40 min struggling in the ice water, she stopped moving. The rescue team arrived (19h39), commenced cardiopulmonary resuscitation, and transported her to Hospital…Her Tc, 14.4oC at the start of rewarming, dropped to 13.7oC (afterdrop), and then returned to near normal after 3h. She recovered to normal function…”
Remarkable survival, but I'm sure you can appreciate, a one in a million story, and most are not nearly so fortunate (she did have numerous complications requring many years of rehabilitation afterwards).

So these stories and situations are obviously what we all wish to avoid! And in our next post in this series, we'll look at what happens to the body when it's suddenly exposed to a very cold environment - we'll look specifically at water immersion.

So join us then, and in the meantime, if you're in the Northern hemisphere, remember, it's all in the clothing!



Andrew said...

Fascinating information. There's a saying: "there's no such thing as bad weather, just bad clothing". I've run in temps of 0F with -20F wind chill without problems.

After my long runs - in any weather, hot or cold - about 15-30 minutes after finishing I find myself feeling very cold. Based on your article, this is because 1) my body is focused on recovery instead of temperature regulation, and 2) I'm no longer moving so I'm not generating excess heat. Correct?

Is it also true that frostbite is only a risk when temps are below freezing? Hypothermia is a risk at any temp, but I think frostbite requires sub-freezing temps.

Anonymous said...

A serious question:

Is there any long term risk from frozen popsicle? I can be perfectly comfortable in tight, briefs and running shorts, but it doesn't protect the manhood

Ross Tucker and Jonathan Dugas said...

Hi Andrew

Thanks for the questions.

To respond, after your runs in cold weather, the feeling of coldness later on is quite typical. It happens to an even greater extent after cold water immersion, and we'll cover that in subsequent posts. But basically, what happens is that when you are exposed to the cold, your body sends blood AWAY from the skin to conserve heat. Of course, your skin and extremities are getting incredibly cold, but it doesn't affect your internal temperature, as a result of this response.

When you then stop (or get out of the water, as it were), the blood can return to the skin, which in the meantime has become incredibly cold. Pretty soon, that blood does a complete "loop" and returns to the core and the temperature drops. This afterdrop is one of the most dangerous periods in reheating people who've been exposed to the cold.

When we worked with Lewis Pugh, we saw the same thing each time he got out of the water - the biggest drop happened 10 min AFTER he got out the water, not during.

I suspect that the same happens after running, though to a lesser extent.

But of course, the main reason is that you've removed half the equation by stopping. heat storage, and hence body temperature, is a function of heat production vs. heat loss. After you stop exercise, heat loss persists (in the form of vasodilation and the sweat response and so forth), but heat production obviously does not (to the same extent anyway).

So it's probably mostly related to this. Most people will have found that simply putting on a warmer jacket helps, simply by reducing heat loss. And yes, it's in part due to the fact that the body simply doesn't see it as a 'priority' - we'll cover some of the physiology behind this in the later posts.

And then also, the severity of exercise does impact, probably via hormones. Fatigue is guaranteed to compromise your ability to keep warm. And catch-22, the cold increases the chance of fatigue, which is why it's so risky being exposed. But any prolonged exercise exposure will affect your ability to maintain temperature. That will come up in subsequent posts again.

Finally, on the frostbite issue - you can develop frostbite if the wind speeds are high enough to create an EFFECTIVE air temperature below about -20 degrees. Frostbite happens at these temperatures, to exposed skin. So it's easy to appreciate that in order to develop frostbite when the air temperature is 1 degree, you have to have some serious wind to chill the skin. But it can happen, in theory. In practice, not so common...


Ross Tucker and Jonathan Dugas said...

Hi Outdoor enthusiast

Thanks for the comments. I think your reasoning is likely sane, only because if you were doing damage, you'd likely have seen it by now. So I suspect that your timing at that water temperature is about right. I confess that I don't know of any work that has ever looked at "survival" times of human tissue when exposed to cold environments. In otherwords, i doubt it's possible to draw a graph showing time to cell death against time of exposure for different temperatures.

If I'm wrong, and someone knows of this, let me know! But interestingly, a lot of that kind of knowledge was accumulated during World War 2, in the concentration camps. As I'm sure you can appreciate, doing that kind of experiment is ethically very suspect. But in WWII, it was done, and subsequently, a lot of that knowledge has been destroyed. So there's a gap in scientific knowledge (rightly, because of the method used to gather the data), which means I don't think that your question is definitively answerable!

However, in terms of what we do know about physiology, your logic is sound. When exposed to cold water, the body's number one priority is to defend the core temperature. That's why you shift all the blood away from the extremities - it doesn't even try to keep them warm. Quite when they become vulnerable to damage I don't know, but it's another case of letting your body tell you when it will become dangerous.


Chris said...

Thanks for publishing your stuff. I find it incredibly interesting and useful.

My question:

Some friends and I who use heart rate monitors for training feedback have experienced slightly reduced heart rates in really cold weather, meaning that for any given level of perceived effort or pace, our heart rates are a few percentages of HRmax lower than in warmer weather.

Is this coincidence and chance or is there a phyisiological reason this may be happening?

Ross Tucker and Jonathan Dugas said...

Hi Outdoor enthusiast

With frostbite, very basically put, the cells freeze. Remember that the cells contain a great deal of water, and because water expands as it freezes, the cells rupture, and the tissue bursts - so you're right with your theory.

I must confess that I'm not 100% sure how the condition develops. That is, I can't tell you whether cells recover - of course, there are cases where they do, but that's very mild frostbite. If it's severe enough, as with our unfortunate runner, you lose the limb - after the Everest disaster in 1998 where Rob Hall died (the book Into Thin Air), one of the climbers lost a number of toes and fingers to frostbite. But it depends on the severity, as far as I know. Best to avoid it, in that case, I guess!

As for your trip, there's no limit to how long you can spend outdoors, provided you are sufficiently clothed. If you cover your extremities, you can spend far longer than an hour outside - but it's all down to the clothing you wear. In particular, fingers, toes and ears, and if it's really cold, a face mask is a good idea. If you have those, you won't be in any danger at all.


Ross Tucker and Jonathan Dugas said...

Hi Chris

Thanks for the compliments, great to have you along!

You're quite right, there's no co-incidence here - it's all physiology. In the cold, the demand on the body to send blood to the skin for cooling is reduced, and so the blood distribution is different - you have an increased "central blood volume", which is a physiological concept for the blood in the core of the body. To simplify, this means that the volume pumped by the heart PER contraction (called Stroke Volume) is higher in the cold. Hence, it needs to pump less often. In the heat, because more blood is sent to the skin, the stroke volume is relatively lower, and the heart rate is higher.

A couple of other things add to this - one is your body temperature, which may be slightly lower (depending on how cold we're talking). There's evidence that a colder heart pumps less often - a direct temperature effect. And finally, you sweat less, and so your plasma volume doesn't decrease as much. This means you can pump less often, because you are not compensating for a decrease over time.

Hope that helps!

Anonymous said...

Hi, Ross.

I'm in Alberta, Canada right now so this post was suddenly interesting (I am from the Philippines).

I suppose putting on a jacket helps the afterdrop for most people. In severe cases, I imagine that both factors need to be addressed, reduce heat loss as well as have a gradual decline in heat production that is matched by a gradual increase in temperature--a kind of "cool down" whose goal is to keep you warm.

Douglas Kretzmann said...

"Similarly, in South Africa in 1985, during a 56km race, a temperature of 19 degrees combined with wind and rain was enough to see 8 runners taken to the medical tent with body temperatures below 37 degrees celsius."

I remember that race well. I'd just failed to break 2:40 at the Sun City marathon that year, because the temps were in the 80's (F) instead of in the 60's as in the past 3 years, and they'd changed the course to finish uphill. I'd planned to take that fitness and get a top-100 finish at Two Oceans. In the event I was running in the usual mesh vest and shorts with 7% body fat.. got so cold I had to run too fast early just to stay warm, and fell apart at 44km. Finished 108th, c'est la vie.
Since moving to Colorado, I've found out one or more things about running in the cold.. mostly that I don't like it.

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