Cold weather environments present a challenge for any outdoor adventurer. We tend to think of being outdoors in the cold as an uncomfortable experience. Given this, we dress abundantly to create a microclimate of warmth between our body’s outer shell and clothes to help maintain body core temperature and a degree of comfort. However, this “front-country” dressing strategy does not work well during times of backcountry exertion and may, in fact, be detrimental to one’s health. In this post, we explore the physiology of cold weather hiking in the Winter. What happens to the body in the cold, both at rest and exercise. How do we manage our behavior in the backwoods to be out there with relative comfort and maximum safety? I will not address submersion in water.
What happens to your body in the cold?
Our body works to maintains a core temperature of 37℃ (98.6℉) (Tipton et al., 2006) with a range from 36.1℃ to 37.8℃ (97℉ to 100℉) depending on factors such as sleep, emotions, feeding, activity, and external environment (Porcari et al., 2015). Any significant deviation from 37℃ approaching 35℃ and 41℃ results in impaired exercise performance (Tipton et al., 2006, McArdle et al., 2015). Further variations outside of 35℃ and 41℃ body core temperature may be lethal (McArdle et al., 2015). Humans may briefly endure core temperatures as low as 26℃ or as high as 41℃ (80.5℉ – 105.8℉) after which, cells begin to die as intracellular proteins begin to unravel (Porcari et al., 2015). To maintain “thermal homeostasis,” our body must contend with environmental conditions such as air or water temperature, air humidity, air or water motion, solar sky, and ground radiation (Tipton et al., 2006).
It may be noted here that windchill may be of notable concern. Wind Chills of -20°F on exposed skin can lead to frostbite in 30-minutes. However, windchill below -10°F may pose a relatively low risk for frostbite to the adequately dressed hiker after which above -20°F to about -60℉ requires total protection and above -60℉ exercise should be done indoors (Robergs & Roberts, 1997).
This said, the hiker with inadequate protection from the wind, especially with moist clothing, is at risk of cold injury (hypothermia or frostbite) due to the effect from windchill (Robergs & Roberts, 1997). Many people are surprised to learn that most hypothermia cases occur between the temperatures of 30℉ and 50℉ (National Oceanic and Atmospheric Administration – National Weather Service, 2015), especially when in breezy conditions.
Metabolic rate and clothing are factors that impact adaptation and resistance to cold. It must be noted that humans have a minimal potential for long-term adaptation to cold exposure compared to our ability to adapt to hot environments (McArdle et al., 2015). During exposure to cold, the human body responds by vasoconstriction of the peripheral circulation as an initial attempt to retard heat loss and defend the core (Tipton et al., 2006). This reduction in peripheral blood flow, if not addressed, may leave the fingers and toes at elevated risk for damage as skin temperatures have dropped.
Factors that increase or decrease body core temperature
For the ill-prepared person, once vasoconstriction of the peripheral circulation has occurred and skin temperature reaches 62℉, metabolic regulation begins (Porcari et al., 2015). The first metabolic response is nonshivering thermogenesis. As its name indicates, this does not include shivering. The body initiates metabolic changes via the preoptic-anterior hypothalamic (POAH), which is regulated by norepinephrine. This metabolic adjustment can create a small and temporary upregulation in heat production in the cells. Once this adjustment is not adequate, POAH signals skeletal muscle with a shivering neural response (Porcari et al., 2015). Shivering, involuntary twitching of skeletal muscle can increase metabolic rate and heat production by several-fold.
As indicated above, a core at 35℃ may be lethal. At this core temperature, the shivering rate would be maximal at 5 times the resting metabolic rate equivalent to ∼40% of maximal oxygen consumption (Haman, 2006). Thus it is uncompensable without other warming or heat-producing methods (Haman & Blondin, 2017). It must also be noted that shivering at high levels interferes with voluntary muscle action, so finding ways to reduce shivering to a minimum can maintain locomotion.
Shivering can be sustained for hours and produce significant heat; thus, it is essential in survival situations (Haman & Blondin, 2017). However, shivering is not unexhaustible. The body is limited in its ability to shiver by its blood glucose stores and subsequent fatigue (Procter et al., 2018). In addition to shivering, as indicated above, the peripheral response to cold involves vasoconstriction of the cutaneous and skeletal muscle blood flow to the core, thus reducing heat loss (Robergs & Roberts, 1997). In early circulatory adjustments, no vasoconstriction occurs in the brain, so the head is a potent location for heat loss, up to 25-40% of the total (McArdle et al., 2015, Robergs & Roberts, 1997).
There may be a degree of acclimatization to the cold by humans, but it is probably modest at best, especially compared to our ability to adapt to hot environments. The three main adjustments include (1) habituation, (2) metabolic acclimatization, and (3) insulative acclimatization. Habitation primarily occurs when a person is exposed to cold, but heat loss is minimal (e.g., insulation is given by clothes or warmth provided by an external heat source). Both metabolic acclimatization and insulative acclimatization are acclimatization adjustments that occur when the body is exposed to the cold for extended periods of time with significant heat loss. In reality, these can be considered a continuum of adjustments rather than distinct changes of their own.
Habituation is a state in which the individual demonstrates less pronounced physiological responses to cold than before acclimatization. Specifically, blunted shivering and blunted cutaneous vasoconstriction. Some individuals also demonstrate lower core temperatures, this is termed hypothermic habituation. As cold habituation increases body heat loss is lessened.
The metabolic acclimatization probably takes chronic exposure to cold environments over many weeks, months, or years (Tipton et al., 2006). This acclimatization pattern is hallmarked by an increased thermogenic response such as enhanced shivering or possible non-shivering thermogenesis by increasing brown adipose tissue thermogenic capacity and activity (Gordon et al., 2019). It may be noted that some data suggests that when non-shivering thermogenesis occurs in metabolic acclimatization, shivering may reduce by >20% without decreases in heat production (Gordon et al., 2019).
Finally, insulative acclimatization is characterized by improved heat retention. The primary adaptations are identified by more rapid declines in skin temperature and lower thermal conduction at the skin with more significant vasoconstriction in response to the cold (Tipton et al., 2006). Additionally, the body’s enhanced ability to redistribute muscle blood flow to the core (Tipton et al., 2006).
During prolonged exposure to the cold, exercise performance can be impacted if the body’s core temperature falls. In temperate environments, the body demonstrates an intricate balance of using both carbohydrates and fats as energy sources depending on exercise intensity (Spano, 2011). With or without a fall in body core temperature, fatty acid production is lessened in cold environments due to the blood vessels’ peripheral vasoconstriction supplying the skin and subcutaneous fatty tissues (Porcari et al., 2015). This increases the body’s dependence on glucose stores in the blood, liver, and skeletal muscle for energy. This is why the hiker must engage in a planned nutritional strategy that includes adequate carbohydrate, fat, and protein on the trail.
Humans can generate enough heat from physical activity to maintain body core temperature in air as cold as -22℉ without the need for heavy insulation (McArdle et al., 2015). If undue fatigue sets in and exercise capacity is impaired, body core temperature may fall, and shivering may begin. When cold stress induces shivering during physical activity, O2 consumption values are increased compared to similar metabolic workloads in temperate environments (McArdle et al., 2015. Tipton et al., 2006). If core or muscle temperatures drop, impairments in aerobic capacity and strength and power will occur (Tipton et al., 2006). However, if exercise is sufficient to maintain core and muscle temperatures, O2 consumption values are similar to that of temperate environments (Tipton et al., 2006).
The cardiovascular complications associated with reduced body core temperature are seen as cardiac output increases under cold stress well over that at the same metabolic workload in temperate environments (Tipton et al., 2006). Thus, shivering during exercise may induce cardiac strain. As skeletal muscle core temperature cools, blood lactate levels may be higher at any given workload if shivering is present along with disruption in energy utilization (Tipton et al., 2006).
Fluid balance is critical during exercise. In cold weather settings, thirst drive may be blunted. Additionally, cold weather stress induces cold-diuresis, which may increase fluid loss. Furthermore, breathing into dry cold winter air may speed fluid loss; however, this is probably negligible (Tipton et al., 2006).
Dressing and heat management
As many people enter cold environments, they dress to reduce the perceived discomfort of the cold. To an extent, this is determined by how conformed a person is in cold climates and/or because of less heat loss due to how much subcutaneous body fat a person possesses (Robergs & Roberts, 1997). Greater amounts of subcutaneous body fat reduce the temperature which requires increasing metabolic rate, but this is more true in water than air (Robergs & Roberts, 1997).
Overdressing to blunt the sensation of cold
Dressing to reduce cold also reduces heat loss by creating a microclimate under the clothes near the body. If the heat loss from evaporation, radiation, and/or convection is excessive due to “overdressing” heat injury can occur even in cold environments. Read more about dressing for the cold.
Always start your hike with a slight chill and adjust your clothing as needed
Humans can sustain long-duration exercise in extreme environments, including cold. Metabolic heat production begins immediately at the start of an activity, followed in a few minutes by thermoregulatory sweating. This increase in body core temperature causes heat storage and a difference between the body’s set-point temperature (which is not affected by exercise) and now elevated core temperature (Tipton et al., 2006). This difference leads to a load error, and signals are sent to begin circulatory adjustments to dissipate excess stored heat, usually via sweating. This is why it is important to wear clothing that wicks sweat away from the body and does not lose insulative value. Cotton cools but kills in cool weather and winter. Using clothing made from polypro or wool is critical. In the winter, we rely mainly on convective and conductive heat loss. Exercise can increase core temperature up to 20-25 times that of rest. With the correct amount of insulation during vigorous exercise, our heat production may maintain core temperature to an ambient air temperature of -20℉ (Porcari et al., 2015). With that said, we do rely on a delicate balance of exercise intensity and clothing layers to help maintain body core temperature.
When we exercise in cold environments and have inadequate insulation, we either need to increase exercise intensity and/or add more insulation to balance heat conservation, or our body will increase heat production from shivering (Robergs & Roberts, 1997). The problem with continually increasing exercise intensity is the increased risk of fatigue that may lead to “fatigue induced shivering.” There is a relationship between exercise intensity and the amount of insulation (clo units) we need to use. As we enter cold environments, we need to increase insulation to manage heat loss, but with increasing exercise intensity, our need for insulation decreases. As we exercise in the cold, we need to be aware of sensory feedback of how chilled or warm we are so we can add or subtract clothing as needed.
What to do?
You have just completed a tough uphill section while breaking trail in deep snow and you have removed all upper body layers but your base. You reach the top of the section and the trail is now level for the next half a mile. Remember, you are breaking deep snow.
What do you do?
Can you quicken your pace to avoid getting chilled? But since you are breaking trail, you may fatigue. Do you add another lightweight layer to keep you warm while on the easier level stretch? Do you simply do nothing as there is another uphill coming in a half a mile?
As your exercise intensity has decreased and you have removed layers on the last uphill you’ll probably do well to add back another layer on the level section.
The behavioral response to how we manage cold will alter the need for physiological adjustments. In other words, how we manage our layering, pace, rest time, feeding, and hydration is key to complete thermal management.
However, the behavior methods and responses to managing cold are often insufficient as the hiker does not make the correct adjustments in the field. The reasons for this may include; a desire to obtain a goal, not recognizing overheating or being cold, ignoring the body and/or clothes wetness, being underprepared, being underfed or dehydrated, no breaks or breaks that are too long, etc.
Physiology and Behavior
As one may see the physiology of cold weather hiking is directly related to the “psychology” of cold weather hiking.
Fitness effect on cold-weather tolerance
Neither aerobic fitness nor improving muscular strength or power has been shown to improve tolerance to cold significantly. With that said, improving one’s fitness will allow an individual to engage in voluntary physical activity at higher intensities without premature fatigue. This could mean that people who are more fit can generate heat via physical activity for more extended periods of time than people with lower fitness (Tipton et al., 2006). Individuals, fit or not, are at risk for “Hiker’s Hypothermia,” a condition in which fatigue due to overexertion, sleep deprivation, and/or underfeeding occurs and resulting in a hypothermic sequela. When fatigue is a factor, the exercise intensity and heat production rate decline, thus leading to core cooling (Tipton et al., 2006). Further, when blood glucose levels fall (hypoglycemia) due to underfeeding, an activity can not be sustained to maintain core temperature (Tipton et al., 2006).
Additionally, people with greater amounts of slow and fast-twitch muscle mass may generate more heat via shivering mechanisms for more extended periods of time. There is evidence that during prolonged shivering, the body will alter shivering efficiency by modifying muscle fiber recruitment, increasing or decreasing the recruitment of type II fibers (Haman, 2006). By “adjusting” fiber type recruitment, various macronutrients (carbohydrates and fats) are used in different proportions (Haman, 2006). As improved fitness (both cardiorespiratory and muscular) influence the body’s ability to extract and use energy more efficiently, one could theorize that the ability to use varied muscle fiber types during shivering would be positively affected by fitness.
Gordon, K., Blondin, D. P., Friesen, B. J., Tingelstad, H. C., Kenny, G. P., & Haman, F. (2019, June 6). Seven days of cold acclimation substantially reduces shivering intensity and increases nonshivering thermogenesis in adult humans. Journal of Applied Physiology, 6(126), 1598-1606. 10.1152/japplphysiol.01133.2018
Haman, F. (2006, May 1). Shivering in the cold: from mechanisms of fuel selection to survival. Journal of Applied Physiology, 100(5), 1702-1708. doi.org/10.1152/japplphysiol.01088.2005
Haman, F., & Blondin, D. P. (2017, July 11). Shivering thermogenesis in humans: Origin, contribution and metabolic requirement. Temperature, 4(3), 217-226. 10.1080/23328940.2017.1328999
McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Exercise Physiology Nutrition, Energy, and Human Performace. (8th ed.). Wolters Kluwer Health.
National Oceanic and Atmospheric Administration – National Weather Service. (2015, March 4). Wind Chill Safety. National Weather Service. Retrieved January 10, 2021, from https://www.weather.gov/bou/windchill
Porcari, J. P., Bryant, C. X., & Comana, F. (2015). Exercise Physiology. F.A. Davis Company.
Procter, E, Brugger, H, Burtscher, M. Accidental hypothermia in recreational activities in the mountains: A narrative review. Scand J Med Sci Sports. 2018; 28: 2464– 2472. https://doi.org/10.1111/sms.13294
Robergs, R. A., & Roberts, S. O. (1997). Exercise Physiology: Exercise, Performance, and Clinical Applications. Mosby.
Spano, M. A. (2011). NSCA’s Guide to Sport and Exercise Nutrition (B. I. Campbell, Ed.). Human Kinetics.
Tipton, C. M., Sawka, M. N., Tate, C. A., & Terjung, R. L. (Eds.). (2006). ACSM’s Advanced Exercise Phyiology. Lippincott, Williams & Wilkins.
Backwoods wanderer with a passion for backpacking, hiking, and exploring the wilds of the Catskills and Adirondacks in New York. A Catskill 3500 Club Member and Adirondack Forty-Sixer. Climbed Mount Rainier. Professionally an Exercise Physiologist.