As ultrarunners, most of the time we are worried about hot weather rather than extremely cold weather – and for good reason. Thermoregulation of body temperature in warmer weather is problematic. While running, about 75-80% of the energy that we use goes towards heat production rather than mechanical energy (i.e. movement). As our core temperature heats up, performance declines. Throw in the need for added hydration and heat makes ultras even harder. So, cooler is better, but are there temperatures that are too cold? In mountaineering and Nordic skiing, 20% of all injuries are due to hypothermia and frostbite.1 So yes, too cold is a thing and in this winter edition of UltraRunning Magazine, we will explore the science of being out in the cold.
One of the simplest adaptations is increasing the amount and type of clothing that you wear. Researchers in one study compared the effects of wearing impermeable or semi-permeable clothing during 8 hours of exercise where they alternated 60 minutes of rest with every 60 minutes of activity. Subjects wearing the semipermeable clothes maintained body heat just as well as those wearing impermeable clothing, and had an even faster rewarming of the body during exercise. During rest, subjects in the semipermeable clothing had a 30% reduction in VO2 as well,2 suggesting that rather than a rubber suit, an outfit with a little bit of breathability can work just as well when exercising in the cold.
If you find yourself shivering uncontrollably you may have mild hypothermia and should remove damp clothing, cover your head, apply heat to the trunk (and not extremities), drink warm carbohydrate-rich liquids and use friction massage to restore core temperature.
In addition to technology, we have physiological mechanisms to help us stay warm. When skin temperature falls below 95 degrees, blood is shunted away from the skin and remains in the core. At a skin temperature of 89 degrees, this shunting reaches its maximum amount.4 However, in cold conditions, blood flow may intermittently return to the extremities to rewarm tissues, prevent frostbite and re-establish nervous system activity, but reduce core temperature. Another physiological strategy is to generate heat, or the process of thermogenesis, through either shivering or non-shivering mechanisms. Non-shivering mechanisms is just a fancy way of saying increasing the amount of exercise or physical activity can increase your core temperature. For instance, when exercise intensity is light, cold and windy conditions can lead to a lowered core temperature, but if you are able to maintain an exercise intensity above 60% VO2 max, then body temperature may be maintained.5 However, the cessation of exercise immediately stops heat production and results in an “afterdrop” and lowering of body temperature. Conversely, shivering is an involuntary response to cold temperatures in which repeated rhythmic muscle contractions occur to liberate heat, but accomplish little external work (shivering does not bring you closer to the finish line). Shivering responds to changes in core temperature rather than skin temperature, reaching a maximal level at 93-95 degrees and ceasing at a core temperature of 93 degrees (which is a problem). If you are shivering, that is a sign that your core temperature is lower than it should be and can generate an impressive 200-250W of work in cold air. If you find yourself shivering uncontrollably, you may have mild hypothermia and should remove damp clothing, cover your head, apply heat to the trunk (and not extremities), drink warm carbohydrate-rich liquids, and use friction massage to restore core temperature if you cannot increase exercise intensity.
Although it’s not as robust as acclimatization to the heat, we can also acclimatize to the cold. Repeated short bouts of cold exposure lead to modest improvements in our physiological response to the cold. We can get improvements in shunting of blood to the core rather than the limbs, and increase our ability to create heat by shivering. These adaptations are much slower and more variable than the adaptations to heat and certainly don’t replace having extra insulating layers.6
Tremendous variability exists between people’s cold tolerance.1 Primarily, differences are due to different body shape and body composition of the individuals. Bigger individuals have larger surface areas and lose more heat than smaller individuals, and those with the highest levels of subcutaneous fat have the most insulation and are more cold tolerant. Differences between men and women also exist, with women two times more at risk to the cold. Differences between race exists with African Americans being 2-4 times more at risk. Age matters as well. Those older than 50 tend to have less ability to vasoconstrict and move blood to core than younger individuals, and they also have a lower sensitivity to the cold, creating a situation in which older individuals may be at a greater risk of hypothermia. If you have had a cold injury previously, you are at least twice as likely to succumb to a subsequent cold injury. Lastly, if you are physically exhausted, then you are much more likely to get hypothermia due to the inability to maintain both shivering and non-shivering thermogenesis, especially when caloric/carbohydrate intake has been low.
Environmental factors such as wind and rain make the cold substantially worse. Water transfers heat 70 times more efficiently than air. That’s great for ice baths, but not great for maintaining warmth. In dry conditions, the combination of the air temperature and wind, or wind chill, dictates how many layers we need to wear and our cold injury risk. If the windchill is below minus 18 degrees, frostbite can occur within 30 minutes, whereas below minus 50 degrees, frostbite can occur within five minutes. Use insulating layers wisely in these extremely cold conditions, but be aware that exhaustive exercise in modest temperatures can also result in mild hypothermia. For instance, 1.3% of 1983 Boston Marathon runners were mildly hypothermic despite the ambient temperature of an almost warm 76 degrees.3
Cold weather running shares at least one similar challenge which occurs in warm or neutral conditions. For instance, in a 161k ultramarathon taking place in extreme cold, nearly half of the 16 athletes that were followed ended up with hyponatremia.2 The hyponatremic athletes drank more fluid, lost less weight, and consumed less sodium than those without hyponatremia, similar to what is seen in warm or hot conditions. Not many other studies have been completed on ultrarunners competing in cold conditions.
In summary, most of the time we are able to maintain body temperature due to normal heat loss with exercise, but if we are out on a long run and the weather gets extreme or we are no longer able to exercise, then cold-induced injuries can become serious. So, that extra layer you were thinking about taking may not be such a bad idea during the cold weather months.
- Cappaert TA, Stone JA, Castellani JW, Krause BA, Smith D, Stephens BA, National Athletic Trainers’ Association. National Athletic Trainers’ Association position statement: environmental cold injuries. J Athl Train 43: 640–58, 2008.
- RISSANEN S, RINTAMÄKI H. Thermal responses and physiological strain in men wearing impermeable and semipermeable protective clothing in the cold. Ergonomics 40: 141–150, 1997.
- SALLIS R, CHASSAY CM. Recognizing and treating common cold-induced injury in outdoor sports. Med Sci Sport Exerc 31: 1367, 1999.
- Savage M V., Brengelmann GL. Control of skin blood flow in the neutral zone of human body temperature regulation. J Appl Physiol 80: 1249–57, 1996.
- Weller AS, Millard CE, Stroud MA, Greenhaff PL, Macdonald IA. Physiological responses to cold stress during prolonged intermittent low- and high-intensity walking. Am J Physiol 272: R2025-33, 1997.
- Young AJ. Homeostatic Responses to Prolonged Cold Exposure: Human Cold Acclimatization. In: Comprehensive Physiology. John Wiley & Sons, Inc., p. 419–438.