Exercising and competing in cold weather conditions poses fewer risks to health and performance than working in extreme heat – as the elite athletes attending this year’s Olympic Games in the torrid heat of the Athenian summer can attest with feeling.

However, there are a number of environmental threats to guard against and a variety of ways to safeguard your performance as well as your health in the less-than-perfect conditions.

The human temperature is tightly controlled to within about 1°C either side of 37°C (core temperature) or 33°C (skin temperature), although it is possible to survive a fall in core temperature of around 10°C and an increase of about 6°C⁽¹⁾. The balance between heat gained and heat lost is tightly regulated to maintain thermal homeostasis, or balance.

During exercise, the two most powerful challenges to this thermal balance are metabolic heat production and environmental conditions.

Metabolic heat production is most likely to vary as a result of muscular activity, including exercise and shivering. In fact, humans are only 25% efficient, with 75% of the chemical energy produced during muscular contraction being lost as heat. This is necessary because during sustained vigorous exercise heat production can exceed 20 calories per minute. If the body were prevented from losing any of the heat it produced, a fatal level of heat storage would be reached in about four hours at rest and after just 25 minutes of moderate exercise!

The primary aim of the body’s thermoregulatory system is to maintain body temperature within safe limits. This is achieved by a complex combination of mechanisms.

In air at 25-28°C, or water at 35°C, a naked, resting individual can maintain body temperature by varying the amount of heat delivered to the skin via the circulation. As air/water temperature falls or increases, the body attempts to defend its temperature via shivering or sweating. However, these responses are limited in their effectiveness and costly in metabolic terms.

Cold environmental conditions pose a significant challenge to the maintenance of core temperature. If this falls low enough to affect physiological function, hypothermia may develop, leading to a variety of complications (see table below). And if skin temperature falls dramatically, cold injury, including frostbite, may occur.

Risk factors for hypothermia⁽¹⁾

Hypothermia exists when deep body temperature falls below 35°C. Risk factors include:

• Cold air/water temperature
• Air/water movement: faster moving fluids increase convective heat loss
• Age: children cool faster than adults due to their lower levels of subcutaneous fat and higher surface area to mass ratio
• Body stature: tall thin individuals cool faster than short fat people
• Body morphology: body fat and unperfused muscle are good insulators
• Gender: females tend to have more subcutaneous fat than men
• Fitness: high fitness enables higher heat production
• Fatigue: exhaustion results in decreased heat production
• Nutritional state: hypoglycaemia reduces shivering and accentuates cooling
• Intoxication: drugs and alcohol have depressant effects on metabolism
• Lack of appropriate clothing

These risks are particularly high for older people, whose ability to maintain core temperature is reduced, and those with circulatory problems. Children also have a reduced ability to maintain core temperature because of their high body surface area to body mass ratio. They are able to safeguard their core temperature, in part, by constricting the peripheral blood vessels, but this increases their risk of cold injury, such as frostbite.

The impact of cold on an athlete varies according to whether he or she is exercising in air or water. Water is 25 times more conductive than air, and heat is lost 3-5 times faster than with air at the same temperature⁽²⁾.

Obviously, the temperature of air or water has a marked effect on the rate of heat loss, as do factors like wind speed, body composition and body size. People with a high body surface area to body mass ratio – ie tall slender people – will lose heat at a faster rate than their short, heavy counterparts. That is because body fat is the key insulator, and those with plenty of it will lose heat at the slowest rate.

It is also important to note that on dry land a wet person will lose heat at a faster rate than a dry one. This means that cold weather poses greater-than-normal risks if it is snowing or raining, or if an individual is sweating excessively due to inappropriate clothing.

The skin is the first tissue to cool on exposure to low environmental temperatures; next affected are the superficial nerves and muscles, with knock-on effects on function.

Below a muscle temperature of 27°C, the contractile force and rate of force application is reduced and fatigue occurs earlier. Maximum power output falls by 3% per °C fall in muscle temperature⁽³⁾. And in consequence, speed of movement, dexterity, strength and mechanical efficiency are all reduced with cooling.

During low-intensity exercise in air, including walking, there is a danger that heat production will not be sufficient to counteract heat loss unless precautions are taken. In this context, appropriate clothing is of key importance, although it is important to strike a balance between the need to maintain core/skin temperature and the need to avoid excessive insulation.

Over-insulation leads to a rise in core temperature and sweat production and, consequently, to wet clothing that may effect heat balance, particularly if exercise intensity falls.

Of greater concern, though, is the potential for cold injury to the extremities – hands, face and feet. In cold air or water, skin cooling is initiated in order to reduce heat loss. The extremities are most vulnerable to this skin cooling because of their high surface area to mass ratio, and the fact that their major source of heat – blood flow – is restricted to protect the vital organs.

During moderate and high intensity exercise in air, such as running or cycling, while performance may be reduced at temperatures below –10°C there is little concern for core temperature, although obviously care must be taken to avoid cold injury to the extremities. Similar findings have been observed for maximal and supramaximal exercise in the cold.

In cold water, a fall in core body temperature intensifies shivering, which raises oxygen consumption during submaximal exercise (by 9% in water at 25°C and 25.3% at 18°C). Thus, the energy cost of submaximal exercise is increased in water cooler than 26°C, and this can lead to more rapid depletion of carbohydrate and fat supplies, with earlier onset of fatigue.

VO2max and maximum performance are both reduced during cold water immersion. This reduction occurs in water temperature as high as 25°C and is linked to falls in core body temperature, with a 10-30% reduction for every 0.5-2°C fall in core temperature. At the same time, lactate appears in the blood at lower workloads and accumulates more rapidly, suggesting a decreased oxygen supply to the muscle and greater reliance on anaerobic metabolism.

A decrease in core body temperature of 0.5°- 1.5°C leads to a reduction of 10-40% in the capacity to supply oxygen to meet the increased requirements of activity. With more profound cooling, anaerobic metabolism is also reduced due to muscle cooling and direct impairment of the processes that produce anaerobic energy.

Regardless of exercise intensity, athletes must be careful to avoid rapid cooling after exercise, when exercise-induced heat production is reduced and heat is lost to the environment. The body responds in two ways to this negative thermal balance:

1. The peripheral blood vessels constrict, reducing blood flow to the skin and increasing central blood volume and central venous pressure. The reduction in blood flow varies across different body parts, leaving some areas of skin more susceptible to cold injury than others. In response to the increased venous pressure, blood pressure increases and, despite a cold-induced reduction in heart rate, cardiac output rises;
2. Involuntary metabolic heat production occurs, leading first to increased skeletal muscle tone (pre-shivering) and eventually to outright shivering.

Avoiding cold injury

Individuals vary in their response to cold exposure, with some more affected than others. But it is important for everyone to reduce heat loss following exercise in cold environments in order to avoid possible cold injury. The key protective measures are to get out of the cold environment as quickly as possible, remove wet clothing and add extra layers, taking care to cover high risk areas such as hands, feet and head.

It is worth pointing out that energy consumption, particularly of carbohydrates, increases when exercising in the cold, so it is a good idea to ingest more carbohydrate than usual under these conditions. Note, too, that the rate of dehydration in the cold can be as high or even greater than you would expect in warm conditions, for the following reasons:

• Inappropriate clothing can lead to an excessive rise in core temperature, giving rise to high sweat rates (a problem affecting not just hydration status but also post-exercise cooling rates);
• Increased blood pressure associated with constriction of the peripheral blood vessels can boost urine production;
• Because cold air tends to be dry, large volumes of fluid are lost through respiration.

These three factors combine to cause significant dehydration during prolonged exercise in the cold. It is therefore important to increase fluid intake in the cold to as much as, or more than, in warm conditions.

Pronounced constriction of the peripheral blood vessels during cold exposure can reduce skin temperature in the extremities to levels that may lead to cold injuries. Early warning signs of cold injury include tingling, numbness and/or a burning sensation in the fingers, toes, ears or nose. If protective action is not taken at this point, tissue damage may occur, giving rise to either ‘freezing’ cold injury (FCI, or frostbite) or non-freezing cold injury (NFCI).

Human tissue freezes at around –0.55°C. Nevertheless, the risk of frostbite is low above air temperatures of –7°C, irrespective of wind speed. It becomes pronounced when ambient temperature is below –25°C, even at low wind speeds.

NFCI is caused by protracted exposure to low ambient temperatures in the absence of freezing. Immobility, posture, dehydration, low fitness, inadequate nutrition, constricting footwear, fatigue, stress or anxiety, concurrent illness or injury can all raise the risk of NFCI. Its precise cause is poorly understood but appears to be related to damage to the walls of peripheral blood vessels.

Treatment depends on whether the dominant injury is FCI or NFCI. All cases of FCI should be thoroughly re-warmed by immersion of all the chilled parts in stirred water at 38-42°C. A topical anti-bacterial should also be diluted into the water bath. Re-warming should be delayed, however, if there is a chance that refreezing may occur.

Thawing out a freezing cold injury can be intensely painful, and strong painkillers should be given, as necessary. The best continuing treatment is twice-daily 30-minute immersion of the affected part in a 38-42°C whirlpool bath containing an appropriate anti-bacterial.

By contrast, people with NFCI should have their affected extremities re-warmed slowly, by exposure to warm air alone, and must not have them immersed in warm water. The early period after re-warming can be very painful in NFCI, even in the absence of obvious tissue damage. With either form of injury, once re-warmed, the affected extremities should be treated by exposure to air and early mobilisation.

It is unusual for the respiratory tract and lungs to be in danger of damage when exercising in the cold. The air is warmed and moistened rapidly during inspiration to avoid potential damage. However, this moistening/humidification of the inspired air can dry out the airways, giving rise to such complaints as dry mouth, a burning sensation in the throat and general irritation of the respiratory tract.

Cold air inhalation may aggravate symptoms of pre-existing asthma and even cause exerciseinduced asthma in otherwise healthy athletes.

The risk of all these problems can be reduced by wearing a scarf or face mask that will enhance air humidification and reduce water loss.

Rarely is it too cold to exercise if the correct precautions are taken. In general, however, when the temperature falls to below –20°C extreme caution should be taken, and exercise avoided.

Greg Whyte

References

1. Tipton, M (2004) Environmental factors. In ABC of Sports and Exercise Medicine ed Whyte, G, Harries, M and Williams, C, BMJ Books, London
2. Nimmo, M (2004) Exercise in the cold. Journal of Sports Sciences (in press)
3. Reilly, T and Waterhouse, J (2004) Exercise in the cold. pp33-49. In Sport, Exercise and Environmental Physiology, Elsevier Ltd, London