Carlson, Mark J. M.D.
EXERCISING IN THE COLD
Mark Carlson, M.D...Image Tools
Exercising in the cold, for some, provokes thoughts of cold hands and feet and the inconvenience of wearing heavy layers of clothing. Others find the cold a challenge that enhances their exercise. Indeed, winter sport athletes cannot even compete without cold. For those of us in northern climates, cold weather dominates much of our calendar year, and avoiding the cold dramatically reduces exercise options. Fortunately, with planning and preparation, exercising in the cold is safe, comfortable, and even fun.
The key to comfort and safety during cold weather activity is balancing the body’s heat production and loss to prevent both overheating and overcooling. Losing heat energy faster than the body can produce it will eventually lead to hypothermia, a potentially lethal condition that occurs when the core temperature drops below 35°C (95°F) (2). Conversely, producing heat too fast leads to increased sweat. Sweat moistens skin and clothing, which in turn increases the rate of cooling. A rate of heat loss from wet skin and clothing that is too fast could tip the balance back toward too much heat loss and possibly hypothermia. For this reason, balancing heat production and loss is critical for exercise in cold weather.
Heat is created in the body by several processes. The most important for our consideration are as follows: basal energy expenditure and muscular activity — exercise or shivering.
Basal energy expenditure is the heat energy created by the body as a result of resting metabolic processes. The heart must pump blood to the tissues, and the chest muscles and diaphragm must contract for us to breathe, releasing heat energy. Cells metabolize energy in the course of their normal activities, and heat is one of the byproducts.
Muscular activity is one of the most important sources of heat production in the body during exercise. It is such a good source of heat that the body will initiate involuntary muscle activity, in the form of shivering, to increase core temperature if it must. Although, in late stages of hypothermia, shivering may actually diminish or cease.
Most of the heat loss exercisers must manage depends on three things: outdoor temperature, wind speed, and moisture. Controlling exposure to these three factors is usually adequate to manage the heat production/loss balance. Colder temperature, higher wind speed, and wetness will all increase the body’s rate of heat loss. Because water increases the efficiency of transfer of energy compared with air, exercising in cold and wet conditions (e.g., swimming or exposure to cold rain) may be more dangerous for hypothermia than in freezing but dry conditions (2).
Exercise clothing should be assembled not just for performance but also to manage heat balance, sweat, and protection from wind and wet. In cold weather, there are three important clothing layers: a wicking layer, an insulating layer, and a shell layer.
The first layer next to the skin should be a lightweight wicking layer. The purpose of this layer is to allow the inevitable sweat to be drawn off of the skin. When the moisture is trapped in contact with the skin, heat loss is accelerated because of evaporation and increased conduction. A wicking layer pulls moisture away from the skin and reduces these effects. Synthetics, such as polypropylene, are good at this. Silk is a natural fiber option.
The insulating layer goes over the wicking layer, and its main purpose is to trap air. Motionless air is a great insulator. This is why bulkier fabrics often are warmer. This layer must maintain its insulation even when damp because some sweat during exercise is inevitable. Cotton is a poor insulator when wet, so cotton sweatshirts are not recommended. Synthetic pile or fleece fabrics are excellent. Natural wool maintains good insulation when damp but is less comfortable for some. Increasing the number or thickness of the insulating layers increases the amount of trapped air and thereby decreases heat loss.
The shell or protection layer must protect the body from wind and external moisture while allowing some internal moisture to escape. The ideal shell would allow all sweat out and no wind or moisture in. Most shell fabrics available to us today provide a balanced compromise of these traits. Because exercise usually generates sweat faster than shell fabrics allow it to transfer out, a shell layer may be more useful during rest periods or in wet or windy conditions (2).
An exposed head and neck represents a large contributor to total body heat loss, especially when overlooked and left unprotected (1). Hats are very useful as they are easily removed and replaced to manage heat loss and minimize sweat. Headwear that covers the ears is critical when wind chill is a factor, as ears are very susceptible to frostbite.
Some cold weather sports dictate a certain type of footwear (e.g., hockey or skiing). For walkers or runners, however, there is some opportunity to select footwear that may provide more comfort and safety. When possible, layered socks are helpful by using a thin wicking-type layer against the skin with a thicker insulating layer over it. However, wearing more than a single sock layer in well-fitted shoes may require a larger shoe size. Footwear that is too small increases heat loss and the risk of frostbite. In snow or slush deeper than a couple inches, gaiters can be useful to protect feet from moisture and conserve heat.
Keeping hands warm is easier when adequate core temperature is maintained. The peripheral blood vessels constrict with lower core temperature, and without an external source of heat or an increase in the core temperature to allow dilation of the blood vessels, it can be difficult to rewarm them. It is important to note that frostbite can occur even with a normal core temperature, so hands must be protected. Again, keeping the skin dry is critical. Changing gloves during exercise and even cycling them on and off in milder temps to keep hands warm, but not perspiring, can help. For extreme cold, use a layered wicking-insulating-shell approach to hand wear.
Snow, ice, and irregular surfaces are common in cold weather and can make running quite dangerous. Traction devices that slip over the shoe sole are effective at walking speeds but may be difficult to keep on when running. When traction is poor, a slower pace may be necessary. Low visibility also is common in cold conditions. Daylight hours are reduced in the winter. Snow accumulates in piles along streets and intersections obscuring drivers’ views. Cloudy gray skies over a white landscape create a low contrast light, and falling or blowing snow can even reduce visibility to mere feet. Runners and other pedestrians or road users should wear bright reflective clothing if exercising in these conditions.
Even in the cold, hydration remains important for both performance and safety. Fluid is lost through sweat in any temperature, but fluid loss from respiration and urine production can increase in the cold. Additionally, thirst can be blunted by the cold, which complicates hydration management. For activities of 1 to 2 hours or less, a starting point would be consuming similar amounts of fluid as would be required for that activity in 10°C (50°F) weather and then adjusting based on actual fluid weight lost. In very cold weather, warm fluid also can be an external source of heat.
Breathing Cold Air
Cold winter air is usually very dry and is partially warmed and humidified by airway passages in the mouth, nose, and throat. Dryness, and the cold air itself, can cause irritation of the tissues lining the airways and trigger bronchospasm or asthma exacerbation. Cold conditions can sometimes cause enough irritation to provoke a cough, even in people without the airway hyperreactivity of asthma. However, if wheezing or coughing during exercise is significant enough to affect performance, further medical evaluation is warranted. Breathing through a scarf or mask can warm the air and improve humidification to reduce irritation from the cold dry air.
DANGERS OF COLD
Cold must be respected as it can cause serious injury or even death. Two of the more common serious cold dangers are hypothermia and frostbite.
Hypothermia is a reduction in the body’s core temperature below 35°C (95°F) (2). Hypothermia can occur at even mild temperatures, but it is more likely at colder temperatures because of the increased rate of heat loss. When the body’s core temperature gets low enough, metabolic processes within the body stop working, the victim suffers cardiac arrest and dies. Symptoms of hypothermia may vary between people with the same core temperature (2).
Signs of hypothermia include the following: shivering (or diminished shivering in later stages), stumbling or decreased coordination, fatigue, drowsiness, impaired judgment, and lack of self-concern.
These last signs are potentially the most dangerous because they make it difficult for the hypothermia victim to recognize the danger and take corrective action. Even experienced people with the knowledge and resources needed to rescue themselves can succumb to hypothermia if they allow it to progress to the point of impaired judgment and apathy. Respect for the danger of hypothermia shown in careful planning and preparation can help mitigate the risk of hypothermia. Still, exercising with a partner and planning for emergency contingencies such as carrying a cell phone or identifying places for emergency warming can be life saving.
The most important treatment for a victim of hypothermia is to stop further heat loss. Hypothermia victims should be moved to a warm, dry place, have all wet clothing removed immediately, be covered with warm dry blankets or coats, be given warm fluids, and monitored closely.
Warming a victim by simply removing cold, wet clothing and covering with wind and waterproof materials will allow most otherwise healthy adults to recover easily from mild hypothermia.
Do not expose the victim directly to hot liquids (>42°C or 107°F) or extreme heat sources like heat lamps or space heaters. Warm (not hot) compresses like warm water bottles or a dryer-warmed towel may be placed around the victim’s trunk.
Immersion of a victim in a warm bath or hot tub is usually unnecessary but probably does not cause any more cardiovascular stress than a low-to-moderate level of exercise (1). Do not immerse hypothermia victims in water above 40°C or 104°F, and do not expose them to heat for a prolonged period as hyperthermia is a risk at these water temperatures.
If a person suspected of hypothermia displays confusion, disorientation, stumbling, difficulty with speech, or other concerning signs, emergency medical services should be activated for immediate transport to an appropriate medical facility. Even victims with no signs of life should be treated aggressively until rewarmed and evaluated at a medical facility.
Frostbite is a freezing of the skin and superficial tissues. Small, peripheral, or exposed areas of the body are most susceptible. The nose, ears, cheeks, chin, fingers, and toes are commonly injured. Corneas also can be damaged with lower wind chill exposure. Corneal freezing is more common in faster sports like skiing or snowboarding, but it can occur with any activity when wind chills are low enough. Frostbite to the scrotum and penis has been reported in cold weather runners (1). Frostbite cannot occur when the ambient temperature is above freezing (0°C or 32°F) (2). Lower temperatures and longer duration of exposure are two main risks for frostbite, but lower wind chill temperature index (Figure), higher humidity, and dampness also increase the risk (1). Particular caution should be used when the wind chill index temperature is below -27°C (-18°F), as these conditions can cause frostbite to exposed skin in 30 minutes or less (2).
Figure. Wind chill t...Image Tools
Before actual frostbite injury occurs, the skin may become cold and numb or prickly. It is not considered a frostbite injury until the skin suffers actual damage. Frostbite signs include pale yellow or mottled bluish color of the skin, sense numbness (unable to sense touch at all), a feeling of the affected digit or tissue being absent or missing, or the appearance and firm texture of frozen flesh (1).
The initial treatment for actual or suspected frostbite is to immediately move the victim to a warm environment, remove wet and damp clothing, elevate the injured part to minimize swelling, warm the injured part with lukewarm water (40°C to 42°C or 104°F to 107°F), and protect the injured skin until it can be treated at an appropriate medical facility.
Treat frostbitten skin carefully as it is already damaged and is fragile when it rewarms. Be very careful removing clothing. It may be better to thaw the whole area before removing clothing than to attempt to separate frozen fabrics from frozen skin. Because of the fragility of the skin, do not rub or massage the area, especially with ice or snow, which can be abrasive. Do not apply a direct heat source, as the injured skin is more easily burned. Damaged skin between fingers and toes should be protected with clean gauze or soft cloth (3).
Once frostbitten skin is rewarmed, it is very important to avoid reexposure to cool or cold temperatures as reinjury is much worse than delayed treatment. Even recooling the thawed tissue, without refreezing, can cause reinjury from vasoconstriction. The length of time the tissue is frozen has little correlation with the amount of tissue damage. In fact, if there is a chance that the skin may refreeze, it should not be thawed until it can be kept warm indefinitely. However, progression of frostbite to the surrounding tissues also must be avoided (3).
Frostbite is prevented by protecting susceptible parts from cold, wind, and moisture. Maintaining adequate core temperature is critical to prevent frostbite, as the body’s compensatory mechanisms will sacrifice the digits and limbs to support the heat needs of the central organs.
External Heat Sources
Sometimes, the best way to get warm or stay warm is to use an external source of heat. Disposable chemical heat packs are small and lightweight and provide excellent source of warmth to cold fingers, toes, or even noses and ears if needed. They can be carried in pockets, inside gloves, or even inside shoes. However, in extreme cold, nothing is better than just going inside for a warm-up break.
Cold weather brings its own challenges to exercising safely and comfortably. The dangers and risks of exercise in the cold include hypothermia and frostbite. Fortunately, a careful balance of heat production and loss with appropriate clothing, preparation, and planning can keep one safe, warm, and comfortable.
Disclosure: The author declares no conflict of interest and does not have any financial disclosures.
CONDENSED VERSION AND BOTTOM LINE
Exercise in the cold provides challenges for managing heat production and loss for warmth and comfort while avoiding hypothermia and frostbite injuries. Cold weather exercise also brings some unique challenges to safety that must be considered. With appropriate clothing and other preparations, exercise in the cold can be safe and enjoyable.
© 2012 American College of Sports Medicine.