Q: With colder temperatures outdoors, should I shift my exercise indoors?
A: Although there are many appealing options for exercising indoors, outdoor exercise does not automatically need to be abandoned as the temperature shifts downward. As stated in the current edition of ACSM’s Guidelines for Exercise Testing and Prescription, “Although unpleasant at times, cold temperatures are not necessarily a barrier to performing [physical activity]” (1). To maximize safety, individuals need to be aware of personal aspects in addition to environmental conditions (2). In a survey of mountaineers who had developed frostbite, the top reason given as the cause of frostbite injury was inappropriate clothing followed by lack or incorrect use of equipment, lack of knowledge, and environment (severe cold weather) (3). Both preparation and appropriate response to changing weather conditions are key to safely exercising in a cold environment (4).
PHYSIOLOGY OF THE COLD
To maintain a core body temperature near 37°C (98.6°F), the human body will respond to cold exposure by increasing heat production and decreasing heat loss. To decrease heat loss, vessels to the periphery constrict to shunt blood away from the skin and subcutaneous tissue so that heat transfer to the environment is decreased (5). Although this helps to maintain core body temperature, skin and muscle temperatures decline; this may contribute to cold injuries in the periphery, especially the hands and fingers (2).
Box 1. Impact of Exercise
Exercise intensity and mode along with environmental conditions interact to determine the effect of exercise on thermoregulation (2,6,7). With increasing levels of physical activity, heat production via metabolism is increased (thermogenesis). This is helpful in maintaining core body temperature when faced with cold environmental conditions. However, when muscles are active, there is an increase in blood flow to those muscles as well as the skin. This results in heat loss due to heat being transferred from the core to the periphery. These responses occur concurrently; the ability to maintain thermal balance depends on the thermogenesis due to activity being sufficient to meet or exceed heat loss due to the increase in peripheral blood flow. An additional consideration is limb movement. Heat transfer is furthered by limb movement (i.e., increase in convective heat loss from the surface of the body). In particular, arm exercise presents a greater challenge to maintaining a thermal balance due to a greater surface area–to-mass ratio for arms compared with legs plus a thinner subcutaneous fat layer. The convective heat transfer also is impacted by the environment — whether exercising in air or water. Water presents a much greater challenge because of its higher thermal capacity (convective heat transfer coefficient is 70 times greater for water compared with air). Thus, thermal balance is more difficult when swimming or exercising in rainy weather.
Heat production can be increased with voluntary physical activity as well as involuntary muscular activity (5). The heat production with physical activity will vary depending on many factors, including intensity of the activity (see Box 1 for additional insights on the impact of exercise on thermoregulation). For example, higher intensity exercise (>60% V˙O2max) allowed for core body temperature to be maintained when the temperature was 5°C (41°F), clothes were wet, and the wind was 5 m/sec, but heat losses exceeded heat production when the exercise was light (<30% V˙O2max), resulting in a decrease in core temperature (2). Shivering involves involuntary muscle contractions that occur in a repeated, rhythmic pattern (2). Shivering can increase metabolic rate more than 4 times that found at rest (8), or even higher (2).
When the balance between heat production and heat loss is not maintained, cold injury risk increases, including frostbite and hypothermia. Background information on frostbite and hypothermia are found in Box 2; management and treatment information sources are found in the resource list at the end of this article.
Box 2. Cold Injuries
Cold environments can result in a number of medical conditions, including the following (1,2,9):
- Frostbite is a concern when the temperature of the body tissue drops below 0°C (32°F). Common areas for frostbite include exposed skin (e.g., nose, ears, cheeks, wrists) as well as hands and feet due to the vasoconstriction of blood vessels to the periphery. Numbness is the most common initial symptom.
- Hypothermia involves a core body temperature below 35°C (<95°F). Heat losses exceed heat production, resulting in the decline in core temperature. Symptoms vary between individuals at the same core temperature; preventative measures should be taken with early symptoms such as feeling cold, shivering, apathy, and social withdrawal. As core body temperature drops, confusion, sleepiness, and slurred speech can occur; cardiac rhythm changes may occur if temperature drops below 31°C (87.8°F).
With ongoing or repeated exposure to the cold, does the body “adapt”? Unlike acclimatizing to the heat, cold acclimatization results in small, slow-to-develop adjustments, providing “…little practical advantage for defending body temperature and preventing environmental injury” (9). Three patterns of thermoregulatory adjustments have been proposed in cold acclimatization with chronic or repeated cold stress (2,6):
- Habituation: response to the cold becomes less (e.g., blunting of shivering or cold-induced vasoconstriction)
- Metabolic acclimatization: body heat loss results in an exaggerated shivering response to counter loss of heat
- Insulative acclimatization: enhanced heat conservation (e.g., enhanced vasoconstriction, improved muscle blood flow) when metabolism does not offset heat loss
Thus, depending on the situation (i.e., maintenance or loss of body heat, adequate or insufficient heat production via metabolism to maintain body temperature), there may be some changes in the typical body responses of cutaneous vasoconstriction and shivering thermogenesis (6).
Other factors also can decrease shivering response and heat production, including hypoglycemia, some endocrine abnormalities, and low caloric intake (4). Calorie requirements may increase 10% to 40% as a result of shivering, weight of clothing, equipment, and increased work output of walking in snow (5). Hypoglycemia seems to impair shivering and increase the risk for hypothermia (2). Maintaining a thermal balance in the cold may be impaired by exertional fatigue and sleep restriction (9). Heat loss will be greater with some skin conditions and burns (4). Various medications impact the nervous and vascular function, both important in regulating body temperature, and thus may increase cold-injury risk (4). With an awareness of the potential influence of factors such as these, an individualized plan to decrease risk of cold injury can be developed.
Factors that should be considered include air temperature, wind speed, and wetness (1,2). Wind promotes convective heat loss, and clothing that becomes wet increases evaporative heat loss (10). The Wind Chill Temperature Index (see Figure) combines the temperature of the air with the speed of the wind to estimate cooling danger for exposed skin (11). If moisture is a factor (i.e., wet skin), cooling will occur faster, and thus, the temperature used within the Figure should be 10°C lower than the actual measured temperature (1). The impact of wind is not on changing the temperature, but rather, in causing a more rapid cooling toward the ambient temperature (2). When accounting for wind speeds, the impact of the individual’s movement (e.g., running, skiing) also must be considered because of the air movement across the body compared with standing still (9).
Exercise in water and rain increases the risk for developing hypothermia; heat loss by convection and conduction is increased because of the increased thermal gradient between the individual and the environment (2). When considering immersion in cold water, many factors come into play, including the water temperature (decreases in water temperature will increase the gradient), amount of surface area immersed (greater immersion will increase the area for heat exchange), and the type and intensity of exercise (complex interaction given the increase in circulation to exercising muscle that increases heat loss coupled with the heat production from metabolism, which varies with intensity) (2).
Many aspects interact to determine one’s response to cold-weather conditions. A challenge in preparing for cold conditions is differing risk for cold injury among individuals when faced with the same weather conditions (4). Awareness of various factors that can impact the body’s response, along with knowledgeable clothing selection, can help to avoid cold-related concerns on an individual basis.
The body seeks to maintain core temperature, and this is impacted by body characteristics. Core temperature is better maintained for those with higher combined body fat, subcutaneous fat thickness, and muscle mass (2). Differences in core body temperature between men and women seem to be due to anthropometric differences; women (compared with men of similar age and weight) tend to have greater body fat content, thicker subcutaneous fat layer, less muscle mass, and higher surface area-to-mass ratio (2).
Another consideration is fitness level and training; fitness and training alone do not seem to impact one’s response to the cold (2). However, physically fit individuals may be able to maintain core body temperature longer because of their ability to sustain physical activity at a higher metabolic rate for a longer period of time (2). In the International Olympic Committee consensus statement on thermoregulatory and altitude challenges for high-level athletes, an example is given related to winter Olympic Nordic skiing and biathlon events (10). These athletes compete at high levels (13 to 18 METS) for long durations, and such high rates of metabolism have prevented athletes from developing hypothermia even in very cold conditions by offsetting heat loss (10). Thus, higher fitness levels may contribute to the ability to maintain core body temperature (2).
Although vasoconstriction and shivering reflect the body’s physiological responses, personal choices also are of importance when preparing for cold environmental conditions — in particular, clothing choices. General guidelines include the following (2,4):
- Inner layer contacting the skin (e.g., polypropylene, polyester) should wick moisture away from the skin’s surface to provide a layer of air next to the skin, with the water transferred to outer clothing layers.
- Middle layer(s) (e.g., polyester fleece, wool) should provide insulation. Layering allows for flexibility in adjusting the amount of insulation.
- Outer layer should allow moisture transfer and ventilation along with protection from wind and rain. Because sweating may exceed the ability of the material to transfer moisture, the outer layer can cause moisture to accumulate inside; this is not desired. Often, an outer layer is worn during rest periods or only when rainy or very windy.
Proper layering of clothing will need to be individualized to meet the dual goals of staying warm and limiting body sweat (4). The latter increases cold risk due to wetness. Layers can be added or removed as needed.
Covering for the hands, feet, and head also should follow the general clothes recommendation to layer and keep dry with some additional considerations (2). Mittens provide for greater protection from cold injuries compared with gloves but hinder one’s ability to manipulate items. Liner gloves may help with the clumsiness associated with mittens and also add another layer of insulation. To decrease heat loss from the head, knit caps and balaclavas can be used. Headbands are an option to allow heat loss from the head while covering the ears. In addition, consider the fit of clothing items, taking care to avoid constriction, which may impair peripheral circulation and increase frostbite risk (e.g., if an extra pair of socks is needed, shifting up a shoe size may be warranted).
Insulation from clothing is in addition to insulation by body fat and other tissues, a factor that differs among people (9). Thus, individual adjustments will need to be made to address the balance between heat loss due to the environment and the heat production, or thermogenesis, during exercise. Where there are group or team uniforms, policies should allow for individual adjustments given the differences among individuals (9). When it comes to cold-weather activity, clothing must be personalized and adaptable.
Exercise intensity impacts the insulation needed from clothing. Although metabolic heat has a beneficial aspect in defending against the cold, being overdressed can result in excessive sweating, resulting in wetness. When clothing is wet (due to sweat or rain), insulation may be compromised and evaporation of heat increased (9). This must be considered during activity, with less insulation required with higher intensity. However, clothing required before and after activity may likely be greater because intensity is less during warm-up and cool-down (9). Thus, assuring availability of needed clothing and adding/removing layers as needed should be part of a risk reduction plan for exercise in the cold.
Box 3. Risk Factors for Cold Injury
Cold-injury risk considerations (12):
- Risk of hypothermia is increased when exercising in water or rain
- Core temperature maintenance will be more difficult for those with lower combined subcutaneous fat thickness, percent body fat, and muscle mass
- Risk of hypothermia is increased in older individuals because of blunted responses to the cold
- Risk of hypothermia is greater for children compared with adults because of body composition and anthropometric differences
- Risk of hypothermia is increased with hypoglycemia due to impaired shivering
- Risk for those with coronary artery disease may be increased because of hemodynamic changes that occur with exposure to the cold
- Incidence of exercise-induced bronchospasm is higher in winter athletes than the general population
Prevention of cold injury is the goal. Characteristics associated with increased risk of cold injury are found in Box 3. Understanding physiologic responses to the cold, along with evaluation of environmental and personal aspects, can help to address overall risk of cold injury. A risk management plan is intended to identify hazards in advance and seeks to address those hazards. To promote safety when exercising in cold weather, the ACSM position stand recommends a risk management strategy be followed, guided by the series of questions, as follows (2):
- How cold is it?
- What clothing protection is available?
- Who is at risk for a cold-weather injury?
- What is the health condition of the exerciser?
- What effective strategies do I have available to mitigate the cold stress and injury risk?
- Is there a contingency plan in place to deal with changing conditions?
To be effective, plans must be dynamic and responsive to changing conditions.
Regulation of body temperature includes physiologic and behavioral factors (6). Responses by the body at an unconscious level (e.g., shivering) are coupled with voluntary actions (e.g., activity, clothing selection). Prevention of cold injury requires preparation and a readiness to respond to changing conditions. With adequate risk management plans in place, individuals can maximize safety when exercising in cold environments.
ACSM full-color, downloadable brochure: https://www.acsm.org/docs/brochures/selecting-and-effectively-using-clothing-for-inclement-weather.pdf
Wind chill charts and online calculator: http://www.nws.noaa.gov/om/cold/wind_chill.shtml
Background information on extreme cold: http://www.nws.noaa.gov/om/cold/index.shtml
Information on treatment of cold injuries:
1. American College of Sports Medicine. ACSM’s Guidelines for Exercise Testing and Prescription
. 10th ed. Philadelphia (PA): Lippincott Williams & Wilkins; 2017. 472 p.
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3. Harirchi I, Arvin A, Vash JH, Zafarmand V. Frostbite: incidence and predisposing factors in mountaineers. Br J Sports Med
4. Fudge J. Exercise in the cold: preventing and managing hypothermia and frostbite injury. Sports Health
5. Fudge JR, Bennett BL, Simanis JP, Roberts WO. Medical evaluation for exposure extremes: cold. Clin J Sports Med
6. Swaka MN, Castellani JW, Cheuvront SN, Young AJ. Physiologic systems and their responses to conditions of heat and cold. In: Farrell PA, Joyner MJ, Caiozzo VJ, editors. ACSM’s Advanced Exercise Physiology
, 2nd ed. Philadelphia (PA): Lippincott Williams & Wilkins; 2012. p. 567–602.
7. Young AJ, Sawka MN, Pandolf KB. Physiology of cold exposure. In: Nutritional Needs in Cold and High-Altitude Environments: Applications for Military Personnel in Field Operations
. Washington (DC): National Academies Press; 1996. p. 127–47.
8. Nimmo M. Exercise in the cold. J Sports Sci
9. Castellani JW, Young AJ. Health and performance challenges during sports training and competition in cold weather. Br J Sports Med
10. Bergeron MF, Bahr R, Bartsch P, et al. International Olympic Committee consensus statement on thermoregulatory and altitude challenge for high-level athletes. Br J Sports Med
11. National Weather Service Web site [Internet]. Wind Chill Chart
. Silver Spring (MD): National Weather Service; [cited 2017 August 23]. Available from: http://www.nws.noaa.gov/om/cold/wind_chill.shtml
12. American College of Sports Medicine. ACSM’s Resource Manual for Guidelines for Exercise Testing and Prescription
. 7th ed. Philadelphia (PA): Lippincott Williams & Wilkins; 2014. .