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!
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Tuesday, January 29, 2008
A physiological trip through cold water exposure