Many individuals try to exercise regularly and many have neither the opportunity nor inclination to exercise indoors. Therefore, they must deal with changes in climatic conditions that can range from very hot to very cold. To safely tolerate these extremes in temperature, it is important to understand the physiological adjustments that occur with temperature.
Humans are classified as Homeotherms, or organisms that are able to maintain a constant core (internal) temperature despite wide fluctuations in ambient (environmental) temperatures. Humans maintain a core temperature at approximately 37 degrees Celsius. At rest, each cell in the body produces metabolic heat that totals approximately one Calorie/min. In essence we can think of the body as a “furnace” that is constantly producing heat.
Physiological methods of heat dissipation
Exposure to cold or heat stress initiates thermoregulatory mechanisms in the body. In very cold ambient temperatures, the body attempts to prevent excessive heat loss. In very warm ambient temperatures, the body must work at dissipating the heat to the environment, using the following methods:
Radiation – At rest, radiation is the primary method of heat loss. Essentially this involves loss of heat (by electromagnetic waves) to the cooler objects in the environment, such as buildings, walls, trees, etc. The amount of radiative heat loss is influenced by a person’s size, mass and body composition (amount of body fat). People who have a high body surface area to mass ratio, such as children and those who are tall and slender, dissipate more heat by radiation.
Conduction – Conduction involves the heat transfer directly from one object to another. Normally this method of heat loss is not significant, unless a person is exercising in cold water (less than 15.5 degrees Celsius). Water conducts heat from the body approximately 25 times greater than air. At the same temperature, a person in water will lose heat from the body two to four times faster than in air.
Convection – This method involves cooling the body by movement of molecules such as air or water currents. As air moves, heat loss can occur as convective currents carry the heat away. Air currents at four miles per hour are about twice as effective for cooling as air currents at one mph. This is the basis of wind chill, where an ambient temperature of minus 12 degrees Celsius feels like minus 34 degrees Celsius with the addition of a 40 km/h (24 mph) wind.
Evaporation – Evaporation of sweat from the body is the major method of heat dissipation, particularly during exercise. Heat is transferred continually to the environment as sweat evaporates from the skin surfaces, producing a cooling effect.
When humans are exposed to a cold environment at rest, the body attempts to prevent heat loss as well as to increase heat production. The following physiological adjustments occur:
Decrease in peripheral circulation – the body reduces blood flow to the extremities as well as the skin surface. This is an attempt to keep heat “insulated” inside the deep body tissues. Subcutaneous fat also aids in this attempt, since fat is a very good insulator.
Non-shivering thermogenesis – this is an increase in the metabolic rate caused by the release of thyroxine and catecholamines (epinephrine and norepinephrine). A greater metabolic rate generates heat.
Shivering – the rapid involuntary cycle of contraction and relaxation of skeletal muscles can actually increase metabolic rate four to five times above resting levels.
Exercise considerations in the cold
Studies have shown that when people exercise in the cold they generally exercise at an intensity that will maintain sufficient metabolic heat to offset heat loss. Therefore under most conditions, cold temperatures should not preclude outdoor exercise. Some exceptions exist, so it is important to consider the following before exercising outdoors:
There is no evidence that cold temperatures adversely affect health. Even the notion that breathing cold air will cause our lungs to “freeze” is not true. Research shows that for exercisers breathing at moderate exercise intensities – and inhaling through the nose – air temperature is almost completely warmed by the time it reaches the lungs.
However, at very high pulmonary volumes, which occur during high intensity exercise, assuming mouth inhalation, research shows that extremely cold air can cause irritation to the mouth, pharynx, trachea and even bronchi. This can be alleviated by wearing a scarf over the nose and mouth to trap water in the exhaled air; this subsequently warms and moistens the next breath.
Although most people are able to exercise at an intensity level to maintain heat production, if fatigue sets in during a long-duration exercise session the intensity level may drop, thereby reducing the ability to produce heat to offset heat loss. If a person is not dressed appropriately and this occurs, body temperature can drop and hypothermia (low body temperature) may result.
Some people are more cold-tolerant, such as those who are more muscular, short or those having more body fat.
Before heading outdoors, consider the wind chill and the effective temperature. Combining the ambient temperature and wind speed, equivalent temperatures of minus 30 Celsius or lower begin to pose danger to exercisers. If it is too cold, try to adapt your workout indoors to be safe.
Clothing serves as the major barrier between skin surface and the environment. During exercise, people often perspire and this water must be allowed to evaporate to the ambient air. If this does not occur, clothing may become saturated and accelerate heat loss by both conduction and evaporation causing a person to become chilled.
Layered clothing is the best choice because it allows a person to add or take off items as necessary. Layers close to the body should be made of fibres such as polypropylene that can transport moisture away from the body’s surface to the next clothing layer for evaporation. This second layer should be an insulating layer. On the surface, wear a jacket that acts as a windbreaker and is water repellent. Because blood flow to the extremities is decreased in very cold temperatures, it is important to wear gloves, a scarf and a hat. Thirty to forty percent of body heat can be lost through the head if it is not covered by clothing such as a hat, hood, etc.
Over exposure can lead to frostbite and early warning signs of cold injury include a tingling and numbness in the fingers and toes or a burning sensation of the nose and ears. If you feel these symptoms, move into a warmer environment immediately.
By taking the proper precautions, exercise outdoors in cold temperatures will be perfectly safe and enjoyable.
Source: Robert Girandola, Ph.D.
Robert Girandola is a professor in the Department of Kinesiology at the University of Southern California in Los Angeles, California.
To learn more about training for outdoor pursuits read:
The Outdoor Athlete
Price: £10.99 (14.30 Euros)