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Exercise in the Cold

Morrison, Grant M.D.

doi: 10.1249/FIT.0b013e3181fb693f

Exercise in the Cold.

Grant Morrison, M.D., practices and teaches at the University of Minnesota Department of Family Medicine and the University of Minnesota Athletics Department, and is associate medical director of the Twin Cities Marathon.

"I always gain weight in the winter; it's too cold to go out and exercise."

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"My mother always used to say I would catch pneumonia if I went outside without a jacket."

"It's not good to exercise in the winter, it will burn your lungs."

These and other sayings you may encounter are common. Many people are afraid to exercise in the winter because of folklore like these. With proper precautions and armed with knowledge, exercise in the cold can be performed safely.

Cold weather presents unique challenges to the human body. There are physiological changes that occur in response to the cold. These occur as the body attempts to maintain its core temperature, or that temperature that is optimal for internal organ metabolism. The most significant change is shunting of the blood from the extremities via vasoconstriction of the outer blood vessels near the skin and other superficial skin layers. This reduces the amount of blood flow in the extremities, reducing the amount of heat loss, and turning them into another insulating layer, as if the body were donning another jacket on its own.

Vasoconstriction, although defending the core temperature, can contribute to cold-induced injuries such as frostbite (discussed later) but certain protective mechanisms also are at play. Shivering is one of those protective reflexes as it generates a significant amount of heat, up to a maximum of six times that produced at rest.

Another self-protective, although seemingly paradoxical, reflex occurs in specific extremities vulnerable to the cold. Cold-induced vasodilatation is a short-lived repeated increase in blood flow. The trigger for this effect is unknown, and the duration can vary. Its benefit is limited, in that once the core temperature drops after prolonged cold exposure, cold-induced vasodilatation shuts down.

Can you get used to the cold? Yes, but only to a very limited extent, and not nearly as well as the body can adapt to heat. People who are repeatedly exposed to the cold seem to be more resistant to shivering and are able to limit the onset of vasoconstriction. This ability comes from exposures of less than 1 hour a few times per week. Another unique adaptation in some individuals, with cold exposures of several hours per day for a couple of weeks, is their ability to drop their core temperature less quickly than the rest of us.

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Regrettably, being physically fit by itself does not seem to provide any advantages in being protected from the cold. For example, elite swimmers may be able to swim a longer distance in cold water, but they will get just as cold just as quickly (i.e., drop their core temperature) as a nonelite swimmer, according to the 11th International Conference on Environmental Ergonomics. Instead, body composition changes that occur with fitness are protective. Muscle, simply by its tissue thickness, can be an insulating layer in the extremities, like putting on a jacket, but only when the body is at rest and the muscle is relatively metabolically inactive. This insulating effect is lost during exercise, as increased blood flow to the working muscles in the extremities allows greater heat loss because there is more diffusion of heat to the extremities and externally as the limbs are narrower, and heat transfer occurs more easily than in the increased diameter of the core tissues. Body fat also insulates the core, and those with a higher level of body fat are better able to protect their core temperature than lean people.

Many different factors are in play when it comes to trying to determine the effect of body composition on protection from the cold. Sex differences illustrate this well. Although women generally have more body fat content, their lower overall muscle mass (compared with men of similar weight and even subcutaneous fat thickness) actually creates a greater heat loss because of a larger surface area and less muscle insulation, the latter being thermally active and therefore capable of producing more heat, so less tissue generates less heat. The effects of hormonal influences are debatable, although amenorrheic women fail to defend their core temperature as well as women with normal cycles even at the same body composition.

Cold-induced injuries include frostbite, where skin actually freezes, and other nonfreezing injuries. In frostbite, after the pain of cold exposures in continued cold exposure, numbness develops and the extremities take on a "woody" sensation. Dexterity, if not already compromised by the numbness, becomes nearly nonexistent. This sensation develops at about 10°C, before skin tissue actually freezes. Rewarming creates excruciating pain and makes the frozen tissue extremely sensitive to further damage with even minimal trauma or repeated freezing. For this reason, those in the wilderness or at altitude extremes will often choose to avoid rewarming frozen tissue until definitive treatment can be performed and refreezing can be assured not to occur. Other nonfreezing cold injuries can cause superficial skin injuries, which may be initially alarming but are not necessarily permanent. Many other factors play a role in the development of frostbite, as seen in the Table.

Exercising in the cold successfully requires appropriate strategies to prevent temperature loss and injuries. Clothing recommendations typically consist of the use of three layers. An inner layer of non-water-absorbing clothing such as polypropylene wicks away moisture from the skin where it can evaporate. A middle layer of fleece or wool insulates best, and an outer layer lets the moisture escape while repelling environmental conditions such as snow or rain. Unfortunately, exertion can cause sweating, often overcoming the clothing layer's ability to wick it from the skin, setting up moisture near the skin that can enhance heat loss. A factor problematic to many exercising outside in the winter is that they dress for the cold temperatures when they are at rest, but quickly get overheated with movement. Being able to discard layers during exertion and don them again at rest is a necessity.

Approximately 50% of heat loss can come from the head (measured at rest and while wearing winter clothing, in subfreezing temperatures), and vasoconstriction does not occur in the scalp, so particular attention must be given to minimizing heat loss by some kind of head covering, preferably one that is water and wind resistant. Because of the vasoconstriction in the extremities, wind and waterproof gloves are advisable as well. Disposable hand-warming packets, available commercially at sporting goods and outdoor stores, can be very handy, but special attention must be paid to the duration of action. They can last anywhere from 30 minutes to several hours depending on the brand, so take this duration into account when planning an outdoor excursion in the cold.

Cold weather climates, especially those at altitude in the mountains, are notorious for changing rapidly. An awareness of these changing conditions and the ability to respond to such rapid fluctuations become crucial. This can include having ready access to another layer of dry clothing, cutting short a long run in the cold when it rains or the wind picks up, or just knowing when to quit whether or not the goal or summit has been reached.

The best attempts at awareness and prevention can be cut short, however, when good decision-making ability is blunted. Cold environments, such as altitude, can adversely affect our ability to make the best decisions. Altitude, with its associated colder temperatures and hypoxia, may cause similar impairments. Alcohol intake, although well established to impair judgment in general, also can be an independent risk factor when it comes to developing hypothermia and cold injuries.

Exercise in the cold can be safely performed when you are adequately prepared. Fitness gains made in the more congenial weather months need not be lost, and the benefits of exercise, in general, warrant encouragement to assist those who may be misguided by fear.

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Recommended Readings

Auerbach PS, editor. Wilderness Medicine. 5th ed. Philadelphia (PA): Mosby-Elsevier; 2007.
Castellani JW, Young AJ, Ducharme MB, Giesbrecht GG, Glickman E, Sallis RE. American College of Sports Medicine position stand: prevention of cold injuries during exercise. Med Sci Sports Exerc. 2006;38(11):2012-29.
Lounsbury DS, Ducharme MB. Self-rescue strategies during accidental cold water immersions: performance and thermal considerations. In: Proceedings of the 11th International Conference on Environmental Ergonomics: Ystad (Sweden). Lund University; 2005. p. 553-6.
Young AJ. Homeostatic responses to prolonged cold exposure: human cold acclimatization. In: Fregly MJ, Blatteis CM, editors. Handbook of Physiology: Environmental Physiology. Bethesda (MD): American Physiological Society; 1996. p. 419-38.
© 2010 American College of Sports Medicine.