COLD WEATHER EXERCISE

I have been skiing up at Stratton, Vermont, and yesterday the temperature was minus five at the base and minus sixteen at the summit, with 25-mile-an-hour gusts, which translates to a wind chill of minus 45. I skied perfectly happily all day, albeit with a couple of breaks for cocoa, and it led me to think a little bit about outdoor exercise in extreme cold. I will talk about the physiological adaptations in a moment, but the first adage is “There is no such thing as bad weather, simply bad clothing choices.” The point being that with the right clothing virtually any cold climate is not only tolerable, but enjoyable. Freezing rain, bitter cold, and howling winds only affect you if they reach your skin. If the weather stops even a millimeter away from your skin, and within that millimeter it is a balmy 70 degrees and dry, you are effectively exercising on a pleasant spring day in Florida.

The key concept here is the microclimate. Animals in Nature seek out microclimates by burrowing in around the base of trees where it is a little bit warmer, etc., but the most important adaptation is in terms of their fur, which traps air close to the skin in bubbles or air cells. Since the air in these cells doesn’t circulate, it forms an extraordinarily effective insulator (which is the same reason styrofoam works so well, and is so light). Actually, we have yet to come up with any fabric that is anywhere near the efficiency of animal fur in terms of warmth to weight ratio. You can come pretty close, however, with a few layers.

The most important feature of the outer layer, if you are going to be more than 30 minutes from shelter is that it be waterproof. NOT water-resistant, but truly waterproof. The easiest way to actually die in cold weather is to get wet, and it is hard to get younger when you are dead ! If you are going to be near enough to shelter that you don’t have to worry about this, you can settle for wind proof , and this includes your head. There are now any number of hats made out of wind blocking fabrics on the market, and when you are skiing you will find that a helmet, in addition to adding a margin of safety, is far warmer than a hat. The ability of fabrics to breathe is a little overrated, in my opinion. Breathing works very well on cold dry days, when vapor is driven from your body outward across the Gortex shell by a combination of the heat gradient and the moisture gradient. But any time you actually need it to work (let’s say it’s in the 40s and raining) there is really no moisture gradient across the fabric, and so you will retain just as much water inside as sweat as you would if you wore a plastic garbage bag.

This is a bit of an exaggeration, and the new fabrics certainly are more comfortable than the old rubberized ones, but they are not miracles, and if you are exercising at all hard on a day when it is damp out, don’t expect the Gortex to miraculously wick all the moisture away from your skin and across a zero moisture gradient, against all the laws of physics.

The next layer to consider is the very innermost one, and for this I really do think the newer fabrics have replaced wool in terms of comfort. I am becoming increasingly a fan of the very tight microfiber fabrics like Under Armor, etc. I don’t particularly think they wick any better than other synthetic fabrics, but they are more comfortable.

The middle layer(s) is where you gear up or down for cold weather as appropriate, and the key here is to remember that it is the trapped dead air that does the insulating, not the actual fabric. That’s why a few layers of light weight fabric usually do more to keep you warm than a single heavy layer does. It is also why having those layers a tiny bit on the loose side is important not only for ease of movement, and comfort, but for insulating efficiency. Of course, the other point to the layers is that you can take them on and off as appropriate for the amount of heat you are generating through the exercise.

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THE PHYSIOLOGY OF COLD WEATHER EXERCISE

When you are exposed to cold, and we will get into exactly what exposed means, your body instantly goes through a series of adaptive responses to shunt blood away from the extremities and into the core, where it can preserve your overall body warmth. This has been most extensively studied as what is called the “mammalian diving reflex”, which is how seals and walruses can dive and stay below the surface for prolonged periods of time, even in the near freezing waters off the Antarctic coast. In their case it’s as much to limit oxygen consumption than conserve heat, but it serves both purposes. Basically, what happens is that your body constricts the arteries leading to your arms, legs, and skin, dumping this extra blood into the middle of your body where it can stay warm. This is great if you are actually a walrus plunging into the ocean, but there a few reasons why it is not so good if you are out for a hike or skiing.

The first reason is that the arterial constriction required to dump all the blood back into the core actually raises the peripheral resistance your heart has to work against, and makes your circulation less efficient. One of the interesting lessons from this is that a thorough aerobic warm up is actually even more important when you are doing cold weather exercise, like skiing, than warm weather exercise – not only for injury prevention, but also to redistribute the blood flow back out to the extremities where it belongs, so you can have enough flow through the muscles to keep them active at their ideal aerobic and strength capacity.

The increase in core blood flow also forces more blood through the kidneys, which automatically increases the amount of water your kidneys excrete, in a phenomenon known as cold-induced diuresis. This, in conjunction with the cold weather dehydration I talked about in a previous article, increases fluid loss quite substantially on cold days – so don’t be fooled into thinking you don’t need to stay aggressively hydrated just because the temperature has dropped. The reverse is far more likely to be true.

The physiologic triggers for this shift in blood volume distribution are two fold: One is the actual sensation of cold, and in this respect the face is probably the most sensitive area, though the rest of the head and the extremities count for quite a lot as well. If you are in a sauna, and put your face into a bucket of ice water, leaving the rest of you exposed to the heat, the cold effect on your face alone will trigger a substantial diving response, and you will shunt blood away from the extremities and skin and into your core despite the fact that 98 percent of your body surface is superheated. This gets back to the importance not only of covering your head well enough to prevent heat loss, but covering your face well enough to keep blood flowing where it should be flowing, i.e., out into the muscles you are actually using as you exercise.

It works the other way as well, and there have been some fascinating experiments about having people wear heated vests while working in bitter cold refrigeration rooms. It turns out that if you heat your core enough, you can overcome the cold reflex, and even though your hands are naked in sub zero weather, blood flows fully out through the fingertips, and keep the hands warm as toast, allowing people to do fine manual work for hours on end, with bare hands, in Arctic environments.

So, it’s more complex than just keeping the face and extremities warm, but I find that most people wear plenty of clothes on their torso but underdress their hands, face, and sometimes their feet. Mittens are so clearly superior to gloves that it makes no sense to wear the latter unless you really need to be using your fingers for some specific activity.

I also find when downhill skiing that it makes sense to wear two layers of long underwear bottoms, and less on top. My theory being that the legs are the most active component, and keeping them nice and warm improves muscle efficiency and circulation through them. I would rather dress more lightly on top and more heavily on the bottom to accomplish this, but that’s more a personal preference than anything else.

COLD IS A STATE OF MIND:
The final and most interesting component of this is the psychological component. Your emotional reaction to the cold has a lot to do with changing blood flow patterns within your body. So if you walk outside and instantly hunch your shoulders, draw your arms in, and huddle against the cold, you will activate the diving reflex instantly.

It is interesting to think about this from a physiologic perspective. It is impossible to actually lose any significant amount of heat from your body over the course of a short time. So walking from the supermarket to the car on a bitterly cold night, your ear lobes, which are mostly cartilage, may actually cool down significantly over the course of the walk, but the rest of you will remain a warm and glowing 98.6 degrees. You could walk to your car in your underwear and not have changed your core temperature, or global heat equation in any significant way. You would, however, feel cold, and it is this sensation that would induce all the cold related changes in your physiology.

One thing to try to do is to be consciously aware of the difference between being cold and feeling cold. It is impossible to actually be cold without a substantial period of time exposed to cold when you are underdressed. Once you get used to this, it becomes a fun challenge to walk outside in your shirt sleeves for short errands on bitter cold days. You can simply refuse to feel cold, and it works, for far longer than you would expect.

This equation changes entirely if you are immersed in water, because the efficiency of heat transfer is so dramatically greater there – so you can become hypothermic quite quickly in water, but still if you look at people who drown in cold water, my bet is that it is partly because the diving reflex is redistributing blood flow away from the arms and legs, and therefore the movements become spastic and uncoordinated within about 5-10 minutes. It’s not just that they become hypothermic, but that they lose the ability to swim along the way.

In any event, next time it is a cold day and you have a very short walk to do, try doing it in your shirt sleeves and try to force yourself to feel warm along the way. Be especially careful not to change your posture in response to the cold, but to walk as if you were striding through a
95 degree day across a burning parking lot to get to your car. It doesn’t work perfectly, but it is kind of an interesting exercise to go through, and it is a way of taking partial control of your physiology. It is also a good way to approach winter sports, where really if you dress appropriately there are almost no weather conditions in which you can’t go out and have a wonderful time.

Jack London’s admonition in To Build a Fire was that when you get down to 50 below zero the old timers advise against traveling alone. That’s not the range any of us are likely to see! There are however a couple of caveats to keep you alive.

First: DO NOT experiment with this out in the woods. You can play with these concepts on the ski slopes where it is always possible to go inside and warm up, but if you get this wrong on the other side of the equation, and actually slip into a truly negative heat budget and become hypothermic, you can die. This is especially so because most people begin to lose judgment before they are aware of having slipped over the line from simply feeling cold into truly being cold.

Second: STAY DRY. You can die from hypothermia in the woods on a 60 degree day if you are wet, and once wet, there is pretty much nothing you can do

So if you are intending to head out into the woods, which I strongly recommend, worry far more about your total heat budget and ability to stay truly warm, than about any of the refined concepts of regional blood flow distribution via the diving reflex as outlined in this article!

I am finished writing this, and my daughter and I are going back out on the slopes at Stratton, where it has gone up from the minus range to somewhere in the single digits, perhaps 5 or 8 degrees. We are going to have a great time, and while we may break for cocoa here and there, we will be out all day, while most people are huddled inside their ski condos watching soap operas on TV.