Exercise in the cold

Humans can be thought of as tropical animals. Most of our adjustments to heat stress are physiological, whereas many adjustments to cold environments involve behaviour, like putting on more clothing. Increased year-round participation in sport has sparked new interest in and concerns about exercise in the cold. In addition, certain occupations and military endeavors require people to work in cold conditions – conditions that often limit their performance. For these reasons, the physiological responses and health risks associated with cold stress are important issues in exercise science. We define cold stress here as any environmental condition that causes a loss of body heat that threatens homeostasis. In the following discussion we focus on the two major cold stressors: air and water.

The hypothalamus has a temperature set point of about 37°C(98.6°F), but daily fluctuations in the body temperature can be as much as 1°C. A decrease in either skin of blood temperature provides feedback to the thermoregulatory center(POAH) to activate the mechanisms that conserve body heat and increase heat production. The primary means by which our bodies avoid excessive heat loss(in the order in which they are invoked) are peripheral vasoconstriction, nonshivering thermogenesis, and shivering. Because these mechanisms or effectors of heat production and conservation are often inadequate, we also must rely on behavioral responses such as huddling(decreasing exposed body surface area) and putting on more clothing to help insulate our deep body tissues from the environment.

Peripheral vasoconstriction occurs as a result of sympathetic stimulation to the smooth muscle surrounding the arterioles in the skin. This stimulation causes the smooth muscles to contract, which constricts the arterioles, reduces the blood flow to the shell of the body, and minimizes heat loss. A continuous adjustment of skin vascular tone occurs at all times to offset small heat imbalances in the body. When changing skin blood flow alone is not adequate to prevent heat loss, nonshivering thermogenesis – stimulation of metabolism by the SNS – is increased. Increasing the metabolic rate increases internal heat production. The next line of defense of body temperature during cold stress is shivering, a rapid, involuntary cycle of contraction and relaxation of skeletal muscles, which can cause a four-to fivefold increase in the body’s rate of heat production. The overall adjustments in blood flow and metabolism tat serve to maintain body core temperature are shown in the picture below.

Factors affecting body heat loss

As in heat stress, the body’s ability to meet the demands of thermoregulation is limited during exposure to extreme cold, and too much heat loss can occur. The mechanisms of conduction, convection, radiation and evaporation, which usually perform effectively in dissipating metabolically produced heat during exercise in warm conditions, can dissipate heat faster than the body produces it in extremely cold environments.

Pinpointing the exact conditions that permit excessive body heat loss the eventual hypothermia(low body core temperature) is difficult. Thermal balance depends on a wide variety of factors that affect the balance between body heat production and heat loss. Generally speaking, the larger the difference between the temperature of the skin and the cold environment, the greater the heat loss. However, a number of anatomical and environmental factors can influence the rate of heat loss.

Body size and composition

Insulating the body against the cold is the most obvious protection against hypothermia. Insulation is defined as resistance to dry heat exchange through radiation, convection, and conduction. Both peripheral muscle mass and subcutaneous fat are excellent insulators. Skinfold measurements of subcutaneous fat thickness are a good indicator of an individual’s tolerance for cold exposure. The thermal conductivity of fat(its capacity for transferring heat) is relatively low, so fat impedes heat transfer from the deep tissues to the body surface. People who have more fat mass conserve heat more efficiently than others in the cold. The rate of heat loss also is affected by the ratio of body surface area to body mass. Tall, heavy individuals have a small surface area to body mass ratio, which makes them less susceptible to hypothermia. As shown in the table below, small children have a large surface area to mass ratio compared with adults, leading to a proportionately greater heat loss. This makes it more difficult for them to maintain normal body temperature in the cold.

Body weight, height, surface area, and surface area/mass ratios for an average-sized adult and child
Person	Weight(kg)	Height(cm)	Surface area(cm2)	Area/mass ratio
Adult	85	183	207	2.47
Child	25	100	79	3.16

                        

Women tend to have more body fat then men, but true sex differences in cold tolerance are minimal. Some studies have shown that the added subcutaneous fat in women might give them an advantage during cold-water immersion, but when men and women of similar body fat mass and size are compared, little difference is noted in body temperature regulation with exposure to the cold. As people age, they often tend to lose overall muscle mass, making them more susceptible to hypothermia.

Windchill

As with heat, the air temperature alone does not provide a valid index of the amount of thermal stress from cold experienced by the individual. Wind increases convective heat loss and therefore increases the rate of cooling. Windchill is an index based on the cooling effect of wind and is an often misunderstood and misused concept. Windchill is typically presented in charts of windchill equivalent temperatures showing various combinations of air temperature and wind speed that result in the same cooling power as that seen with no wind. It is important to remember that windchill is not the temperature of the wind of the air(windchill does not change air temperature). True windchill refers to the cooling power of the environment. As windchill increases, so does the risk of freezing of tissues.

Heat loss in cold water

More research has been conducted on cold exposure in water than in air. Whereas radiation and sweat evaporation are the primary mechanisms for heat loss in air, convection allows the greatest heat transfer during immersion in water(convection involves heat loss to moving liquids or gases). Water has a thermal conductivity about 26 times greater than air. This means that heat loss by convection is 26 times faster in cold water than in cold air. When all heat-transfer mechanisms are considered(radiation, conduction, convection, and evaporation), the body generally loses heat four times faster in water than it does in air of the same temperature.

Humans generally maintain a constant internal temperature when they remain inactive in water at temperatures down to about 32°C(89.6°F). But when the water temperature decreases further, they may become hypothermic. Because of the large loss of heat from a body immersed in cold water, prolonged exposure or unusually cold conditions can lead to extreme hypothermia and death. Individuals immersed in water at 15°C(59°F) experience a decrease in rectal temperature of about 2.1°C(3.8°F) per hour. In 1995, four U.S. Army rangers died of hypothermia after exposure to 11°C(52°F) swamp water in Florida, tragically publicizing the fact that hypothermia can occur when water temperature is well above freezing.

If the water temperature were lowered to 4°C(39.2°F), rectal temperature would decrease at a rate of 3.2°C(5.8°F) per hour. The rate of heat loss is further accelerated if the cold water is moving around the individual because heat loss by convection increases. As a result, survival time in cold water under these circumstances is quite brief. Victims can become weak and lose consciousness within minutes.

If the metabolic rate is low, as when the person is at rest, then even moderately cool water can cause hypothermia. But exercise in water increases the metabolic rate and offsets some of the heat loss. For example, although heat loss increases when one is swimming at high speeds(because of convection), the swimmer’s accelerated rate of metabolic heat production more than compensates for the greater heat transfer. For competition and training, water temperatures between 23.9 and 27.8°C(75-82°F) seem appropriate.