Prevention of Cold Injuries during Exercise

John W. Castellani, Ph.D., FACSM; Andrew J. Young, Ph.D., FACSM; Michel B. Ducharme, Ph.D.; Gordon G. Giesbrecht, Ph.D.; Ellen Glickman, Ph.D., FACSM; Robert E. Sallis, M.D., FACSM

Disclosures

March 01, 2010

In This Article

Hypothermia

Clinically, hypothermia is defined as a core temperature below 35°C (95°F), which represents ~ 2°C (3.5°F) fall from normal body temperature,[143] while physiologically, hypothermia is a core temperature below the value observed typically observed during active phases (< 36.8°C). Hypothermia develops when heat losses exceed heat production causing the body heat content to decrease. Declines in core temperature may eventually impact exercise performance.

Hypothermia is characterized as mild, moderate, or severe.[143] Table 2 lists the core temperatures and physiological changes associated with these low body core temperatures. The symptoms of hypothermia are quite variable from person to person even at the same core temperature. Early symptoms of hypothermia include feeling cold, shivering, and exhibiting signs of apathy and social withdrawal. Coaches and athletes should be aware of these early symptoms so that proper preventative measures can be taken at this time. More pronounced hypothermia manifests as confusion or sleepiness, slurred speech, and a change in behavior or appearance.[158] Severe hypothermia is associated with changes in cardiac rhythms requiring immediate treatment to rewarm and restore normal temperature. Resuscitation has been successful even with core temperatures as low as 13.7°C.[60] At these temperatures, life signs are almost impossible to discern and no one should be pronounced dead until they have been rewarmed. Hence the use of the adage, "a person is not dead until they are warm and dead."

Table 3 presents a list of factors thought to predispose individuals to hypothermia. Important risk factors commonly encountered by exercisers are considered in detail below. The majority of factors known to influence the onset of hypothermia have been identified in experiments that used whole-body cold water immersion, but the risk factors are likely to have similar influence, if less pronounced, on hypothermia onset in cold-air exposure.

Immersion, Rain and Wind

Water has a much higher thermal capacity than air, with the convective heat transfer coefficient being 70 times greater compared to air.[54] Therefore, swimmers and athletes exercising in rainy weather can experience considerable body heat loss even in relatively mild water or air temperatures.

Thermal balance during exercise-cold water immersion and cold-wet air exposure depends on a complex interaction among metabolic heat generated, exercise mode, anthropomorphic and clothing factors that insulate, and magnitude of cooling caused by water temperature, rain, and wind. Individuals vary with respect to the water temperature that can be tolerated without experiencing a dangerous decline in core temperature during exercise.[178] A decrease in water temperature increases the thermal gradient between the person and the environment and leads to significantly greater heat loss via convection and conduction. The more of a person's surface area is immersed, the greater the effective heat exchange area is between the person and the water. As surface area immersed increases, core temperature will decrease more rapidly.[109]

The maintenance of a normal core temperature also depends on the ability to generate enough heat to offset heat lost to the environment. Exercise in cold water may either increase or decrease core temperatures compared to rest in cold water,[157,178,180] depending on whether exercise was performed with the legs-only or if a combination of arms and legs were used. Arm-leg exercise (e.g., swimming) increases the circulation to the extremities where peripheral heat loss is optimized due to the small diameter of the extremities and short conductive pathway for heat transfer from the limb core to the skin surface.[19,181] However if the exercise intensity is high enough (~75% V·O2max or 2.75 L·min−1), core temperature will increase,[49] even though combined arm-leg exercise is performed. Muscle provides an important insulator at rest during cold-water immersion,[39,185] but during exercise this insulation is reduced, as blood flow increases to support metabolism.[178,185] Thus there may be some benefit from adding clothing insulation to cover active muscle areas during prolonged, active immersions as in long-distance cold water swimming.[186]

As with cold water immersion, many factors interact to determine if core temperature can be maintained during exercise in cold, wet, and windy conditions. At an air temperature of 5°C, heat loss in wet clothes may be double that in dry conditions.[97] Furthermore, wind increases convective heat loss. Exercise performed at intensities greater than 60% V·O2max can maintain core temperatures at or above 37°C[146,188,189] when the ambient temperature is 5°C, clothes are completely wet, and the wind is 5 m·s−1. However, when only light exercise is performed (< 30% V·O2max), heat losses exceed heat production leading to declines in core temperature.[20,145,146,188] Also exercise performed before cold-water immersion[163] or exercise in the rain[20] leads to more rapid declines in core temperature compared to not exercising. Evidence Statement. Exercising in water and rain significantly increases the risk for developing hypothermia. Category A

Anthropometry and % Fat

Subcutaneous body fat provides a relatively high thermal resistance[178] and persons who have a high % fat tend to maintain core temperature better than lean people,[64,78,99,124,145,146,178] although this has not always been observed.[62] There is also some evidence that individuals with fat greater than 25% have a higher threshold for vasoconstriction and this enables them to limit heat loss.[96] Unperfused muscle also provides insulation during resting cold exposure[148] and can contribute as much as 85% of the limb insulation.[39] During exercise, perfused muscle loses its insulatory potential and skin and subcutaneous fat provide most of the insulation.[179] Some studies suggest that people with large surface area-to-mass ratios have a more rapid fall in core temperature,[16,165] although in two studies where subjects were matched for body fat but differed in body mass and surface area-to-mass ratio, core temperature was the same at rest and during exercise.[63,179] Evidence Statement. Individuals with high combined values of subcutaneous fat thickness, % fat, and muscle mass can maintain core temperature better than individuals with less fat and muscle. Category B

Sex

Sex differences in thermoregulatory responses during cold water exposure are primarily attributable to the woman's generally greater body fat content, thicker subcutaneous fat layer, less muscle mass, and higher surface area-to-mass ratio than men of comparable age and weight.[176] However, in women and men of equivalent subcutaneous fat thickness, the women have a greater surface area and smaller total body mass and musculature (and lower total body heat content) than men. Thus, total heat loss is greater in the women, versus men, due to the larger surface area for heat loss and less insulation provided by muscle, and body temperature falls more rapidly during resting cold water immersion.[124] Interestingly, during exercise in cold water, men and women of equivalent percent body fat exhibit the same decline in core temperature, perhaps due to loss of insulation in perfused muscle in men and a favorable distribution of subcutaneous fat over active musculature in women.[125] Other data also suggest that men begin shivering sooner and at higher mean body temperatures than women, i.e., men are more sensitive to a change in body temperature,[69] although Glickman-Weiss et al. did not find a sex effect on thermosensitivity between men and women.[61] Cyclic changes in female reproductive hormones also may impact thermoregulatory responses to cold. Data suggest that the onset of shivering occurs sooner in the luteal phase[80] when estrogen and progesterone levels peak, although this finding has been challenged[68] and there are no data to suggest that differences in the absolute starting core during cold exposure places a women at higher risk for hypothermia in the follicular vs. luteal phase. Amenorrheic women cannot maintain their core temperature during exercise in cold air as well as their eumenorrheic counterparts, even if they have a similar body composition profile.[69] Evidence Statement. Core temperature responses to cold exposure between average men and women are primarily attributable to differences in body composition and anthropometry. Category C

Age

People who are older than 60 yr may be less cold tolerant than younger persons, due to reduced vasoconstriction and heat conservation in comparison to their younger counterparts.[48,102,166,194] Older people also experience a decline in physical fitness. If they are exercising at the same absolute metabolic rates as younger individuals, the older person will be working at a higher %V·O2max, will fatigue sooner, and must decrease their absolute heat production if they fatigue, increasing the likelihood of a reduction in core temperature. Older individuals also appear to have a blunted thermal sensitivity to cold. For example, in studies where subjects have control of setting a thermostat as the ambient temperature fluctuates, older individuals will allow the air temperature to fall to lower levels before readjusting the thermostat.[137,173]

Children, in comparison to adults, typically have a higher body surface area-to-mass ratio and lower subcutaneous fat amounts and this leads to substantial falls in core temperature when swimming in cold (20°C) water.[165] Interestingly, in 11-12-yr-old boys who had similar amounts of subcutaneous fat as men (19-34 yr), core temperature was the same at rest and during exercise in 5°C air between the groups, but the mechanism for achieving this was different with the boys exhibiting a more pronounced vasoconstrictor and metabolic response compared to the men.[167] Premenarcheal girls do not defend core temperature as well as eumenorrheic girls during exercise-cold stress, due to a diminished vasoconstrictor response.[105] Evidence Statement. Older individuals (> 60 yr) are at an increased risk of hypothermia due to blunted physiological and behavioral responses to cold. Children are at a greater risk of hypothermia than adults due to differences in body composition and anthropometry. Category B

Hypoglycemia and Fasting

Recent findings show that shivering, like low intensity exercise, relies on lipid as the predominant metabolic substrate in well-fed individuals, but that blood glucose, muscle glycogen and even some protein are also metabolized.[72,73] Underfeeding can lead to hypoglycemia, and acute hypoglycemia impairs shivering through a central nervous system mediated effect.[55,140] Also, declining peripheral carbohydrate stores probably contribute to an inability to sustain exercise thermogenesis during cold exposure.[141] Glycogen depletion, itself, has been shown, during cold-water immersion, to either impair initial shivering rates[121] or to have no effect on shivering thermogenesis.[196] Muscle glycogen depletion has been observed to be more pronounced during low intensity (e.g., below 25% maximal oxygen uptake) exercise-cold stress compared to temperate conditions, but differences between environments are not seen when bouts of higher intensity exercise are compared.[162] Shivering bursts also affect muscle glycogen levels, with more bursts leading to greater glycogen utilization.[71] Complete food restriction for 48 h, even in the absence of hypoglycemia, impairs shivering and causes core body temperatures to decline more rapidly.[116,120] Evidence Statement. Hypoglycemia impairs shivering and increases the risk for hypothermia. Category B

Physical Fitness and Training

Overall, physical training and level of fitness appear to have only minor influences on thermoregulatory responses to cold.[48] Cross-sectional comparisons of aerobically fit and less fit persons find relationships between maximal aerobic power and temperature regulation in the cold,[9,90] but in those studies, differences in thermoregulation appear more likely attributable to anthropometric differences between the aerobically fit and less fit subjects, rather than an effect of fitness state, training, or the level of maximal aerobic power, per se.[9] In a recent study comparing novice and expert swimmers,[115] the expert swimmers could swim further, but not longer, before becoming incapacitated in cold water compared to the novice swimmers. This is likely because arm fatigue due to muscle cooling was the primary cause of swim failure. Longitudinal studies have shown interval training has no measurable effects on thermoregulatory response to cold,[160] and that while endurance training appears to strengthen cutaneous vasoconstrictor responses to cold, that effect has little impact on core temperature changes experienced during cold exposure.[195] The effects of resistance training programs on thermoregulatory responses to cold have not been documented, but it seems likely that any such effects would be primarily attributable to training-related changes in body composition. The primary thermoregulatory advantage provided by the increased strength and aerobic power resulting from physical training is that the fitter individual can sustain voluntary activity at a higher intensity, and thus sustain higher rates of metabolic heat production than less fit persons during cold-exposure. Evidence Statement. Physical fitness and training, per se, do not improve thermoregulatory responses to cold. Physical fitness does allow someone to exercise for a longer period at a higher metabolic rate, and may contribute to maintenance of normal core temperatures. Category C

Prevention Strategies for Hypothermia

Risk Management

Hypothermia is best prevented by first assessing how cold it is by monitoring the temperature, wind, solar load, rain, immersion depth, and altitude.[34] Then the hazard of exercising in the cold is assessed by analyzing the exercise regimen to be performed, the clothing available, and identifying those who are at higher risk of getting hypothermia. Specific factors that can be evaluated include the exercise intensity, duration, experience of the athlete, condition of the athlete (fit and rested or fatigued), general health, and nutritional status.

Risk management is the process of identifying potential hazards before performing in cold weather and taking the steps necessary to control these hazards,[34] because hypothermia can occur during athletic events.[93,149] Figure 1 outlines a cold strain risk management process for preventing cold injuries. An important aspect of this is recognizing changes in weather conditions so that people can be alerted to potential modifications that may be necessary to reduce exposure and susceptibility to cold injuries. Therefore, the risk management process must continually be reevaluated as input changes. Planning ahead could mean bringing additional clothing, cutting short the duration of an event, changing venues, offering warming facilities, or possibly even canceling an event. The greatest occurrence of hypothermia happens when people are not prepared for it, i.e., when people are not expecting it (rainy weather in spring/summer/fall; ocean/lake swimming on a hot day in spring and early summer). As stated above, cold, wet, and windy weather poses the greatest risk for developing hypothermia. Heat loss is much greater in these conditions and if the exercise intensity is not high enough to match heat loss[123,146,189] due to fatigue or if fatigue occurs before cold exposure,[20,21] an individual may be more susceptible to hypothermia.

Figure 1.

Strength of recommendation taxonomy.

Clothing

Cold weather clothing protects against hypothermia and peripheral cold injuries by reducing heat loss through the insulation provided by the clothing and the trapped air within and between clothing layers.[7] Typical cold-weather clothing consists of three layers: an inner layer (lightweight polyester or polypropylene) which is in direct contact with the skin and does not readily absorb moisture, but wicks moisture to the outer layers where it can evaporate, a middle layer (polyester fleece or wool) which provides the primary insulation, and an outer layer, which is designed to allow moisture transfer to the air, while repelling wind and rain. Sweating can easily exceed the vapor transfer rate of the outer shell layer, causing moisture to accumulate on the inside, even if the outer layer has substantial venting (e.g., zippers in armpits) to allow moisture to escape. The outer layer should typically not be worn during exercise (unless it is rainy or very windy), but should be donned during subsequent rest periods.

Clothing insulation needs during physical activity can vary with changes in the ambient temperature and exercise intensity. Figure 2 depicts the insulation needed to maintain thermal balance at different ambient temperatures and exercise intensities.[7,66,82,88] Table 4 presents the approximate insulation of various clothing articles and ensembles.[89] As the exercise intensity increases (jogging, skiing—consult[2] for a comprehensive list of activities and metabolic requirements), the amount of clothing insulation needed to maintain body heat content and thermal balance decreases at any given air temperature. Imposing a single standard clothing ensemble for an entire group could result in overheating and sweating during exercise in some, while others would not be kept warm, therefore people should adjust clothing according to their own needs. A common problem is that people begin exercising while still wearing clothing layers appropriate for resting conditions, and thus, are "overdressed" after initiating exercise. If the combination of environmental conditions, work intensity, and available clothing suggest that body heat content cannot be maintained (e.g., low exercise intensity in rainy conditions), then supervision of the exerciser or use of the buddy system should be encouraged. All exercisers need to be aware that the risk of hypothermia increases if the weather is wet and wet-weather clothing is not available and exercise intensity is low (e.g., stop running and begin walking). Remaining dry, especially for those exercising in remote regions, is extremely important and dictates that carrying extra clothing that is waterproof and dry clothing to change into is vital.

Figure 2.

Approximate amount of clothing insulation needed at different air temperatures and physical activity levels. Wind speed is assumed to be less than 5 mph (2.2 m·s−1). One MET refers to energy expenditure at rest. One clo of insulation is the clothing necessary to allow a resting person to be comfortable when the air temperature is 21°C (70°F) (7). Refer to Table 4 for a list of typical clothing ensembles and their respective clo values.

Wet suit use is becoming more widespread, especially during triathlon competitions. They are primarily used in recreational diving and commercial fishing to maintain body core temperatures and increase survival time during immersion.[177] The international swimming association (FINA) has adopted guidelines to allow wet suit use during triathlons to aid in thermal protection (www.fina.org). This guidance is based on athletic status (elite or not), swim length, and water temperature. For example, an elite triathlete swimming a course between 1500 and 3000 m cannot use a wet suit if the water temperature is above 23°C, but must wear one if the water temperatures is below 15°C. Studies have shown that wet suits reduce drag,[182] increase buoyancy and lower oxygen consumption at any given swimming speed,[183] and thus, their use in swimming competitions is primarily as a performance enhancer. For this reason, their use has been banned in open-water swimming competitions (e.g., English Channel). Core temperature goes up slightly when swimming with a wet suit at 20°C[184] and wet suits have no negative impact on further triathlon performance (cycling, running) after swimming in 25°C water.[103] At lower water temperatures, wet suits with arm protection may provide the best thermal protection during swimming[132] since arm exercise causes a greater cooling rate than leg only or combined arm-leg exercise.[181]

Heat losses from the head have been measured up to 50% of the total resting heat production in a person sitting in −4°C (25°F) air while wearing winter clothing.[53] Knit caps and balaclavas can decrease this heat loss substantially. Headbands can be used to cover the ears, but allow for heat loss through the head. Socks should not fit tight and constrict blood flow. Shoes can be ½ to one size larger with thick socks. Feet perspire even in the cold, particularly in heavy winter boots. This necessitates changing socks at least 2 times per day, but perhaps even more if activities levels are high. Evidence Statement. Clothing insulation requirements during exercise are a function of metabolic rate and ambient temperature. Layering provides the most flexibility to adjust insulation to prevent sweating, overheating, underdressing, and remaining dry in wet conditions. Category C

Food and Fluid Intake

Athletes can expend more energy during cold weather (by 10-40%,[52]) due to a combination of heavy clothing and equipment and the increased effort required for working or walking in snow or mud.[139] The body may also expend more energy keeping warm through shivering when the weather is cold, but this depends on how well the individual has protected herself through proper clothing choices. If the core temperature remains above resting values during exercise, cold exposure does not increase oxygen uptake or caloric requirements above normal.[188,189] In most cases, people do not need to change from their normal diet to meet their caloric needs in cold weather since they are not continually exercising for days and weeks (e.g., mountaineering). If caloric requirements are indeed higher, the 10-40% extra calories needed per day can be obtained by eating a "normal" breakfast, lunch, and dinner, and then supplementing with frequent snacks throughout the day. For the majority of exercisers who do not experience a decline in core or muscle temperature, fatigue is related to carbohydrate availability rather than thermoregulatory limitations[56,141] and exercise can be sustained by ingesting carbohydrate beverages of 6-12%.[56,57] Furthermore, since carbohydrate availability appears limiting, carbohydrate loading to maximize muscle glycogen stores before exercise in the cold is beneficial.[141] Thus, as with exercise in temperate environments, the majority of people who exercise for very long durations in cold weather (cross-country skiers, marathoners) will maintain performance by eating carbohydrate-rich foods such as crackers, potatoes, cereals, bread, and pasta.

Fluid balance may be affected by cold-weather exercise. Exercise can increase sweat loss in the cold just as in temperate climates by increasing core temperature and initiating thermoregulatory sweating.[52] Sweat losses occur if activities are performed at a high intensity, while wearing heavy cold-weather clothing systems, and traversing in snow resulting in high metabolic rates.[139] In these conditions a person could become dehydrated if fluid intake is substantially lower than fluid loss. In addition, if skin temperatures fall significantly, thirst is less noticeable in cold compared to hot weather.[101] Exposure to cold air or immersion in cold water may also increase urine flow rate, the so-called cold-induced diuresis (CID). This response is likely caused by a redistribution of body fluids from the periphery to the central circulation as peripheral vasoconstriction occurs.[52] CID is self-limiting because the response diminishes as body water content falls. CID is also prevented by moderate-intensity exercise.[112]

Moderate fluid loss may not be as important for exercise performance in the cold as it is for temperate and hot environments. Recent data[23] show that if the skin temperatures are low, 4% dehydration has no effect on cycling performance in the cold. But if cold strain is minimized by clothing, thereby maintaining skin and core temperatures near that observed in temperate or even hot environments, dehydration will likely degrade performance.[52] Dehydration does not alter heat conservation, heat production, or CIVD responses[134,136] and thus does not appear to increase the likelihood of cold injuries.

Simple solutions can be instituted to ensure adequate hydration before and during exercise. Before exercise, athletes can monitor hydration status by noting the color and volume of their urine and their body weight. Dark, low volume and infrequent urination indicates that fluid consumption should be increased. Likewise, frequent and large volumes of clear urine indicate that fluid replacement is adequate. Body weight can be assessed daily. People usually drink most of their water with meals, and eating food improves fluid consumption.[87,170] During mealtime individuals can drink a variety of fluids (milk, juice, tea, sports drink, coffee), as each will be equally effective in replacing body water.[87] In addition, meals provide the salt intake necessary to retain body water. Sodium-containing beverages, compared to pure water, have been shown to aid in fluid retention (~ 1 kg more fluid retained with Na+) over several days of a cold survival scenario,[150] but little information is available on their effectiveness during short-duration exercise bouts in cold-weather. Snow, in most cases, should be avoided because it can potentially lower body temperature, contains dirt and other pollution, and provides relatively little water per volume of snow to counteract dehydration. However in a person with a normal or high body temperature, snow is not contraindicated if it is the only source of water. During exercise, frequent fluid intake can be an effective strategy for maintaining hydration. Evidence Statement. Cold environments can increase energy expenditures and may cause fluid losses; dehydration does not impair vasoconstriction or shivering, thus dehydration does not increase susceptibility to cold injuries. Category C

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