How the body reacts to loss of heat during exercise

AL Gondi

In Sports, effects of heat on exercise is a topic frequently discussed. But there are also effects of cold on exercise.

Activities during which cold injuries tend to occur include those involving water immersion like swimming, those performed in very cold climates like skiing and those that require prolonged activity in cold weather such as marathon and ultra-endurance events.

This effect of cold on athletes is called hypothermia. It occurs when temperatures are below 35 degrees celcius and when the body loses more heat than it generates.

At air temperatures of above 28 degrees celcius, basic body metabolism maintains the core body temperature of about 37 degrees celcius. In states where air temperatures are below 28 degrees celcius, the body must produce additional heat to remain thermo neutral.

Extra Heat

The extra heat can be produced by greater physical activity and spontaneous shivering, which is physiologically accepted of healthy individuals in a reasonable environment.

Shivering involves involuntary muscle contractions in response to cold and uses reserved energy stores quickly.

The ability or capacity to shiver reduces as the stored energy (glycogen) is depleted. The intensity of shivering is usually related to the rate of change of air temperature. Shivering reduces muscular co-ordination, and, therefore limits sporting performance.

Non-shivering heat production occurs in small children due to metabolism of brown fat. However, this mechanism is not available to adults who have little stores of brown fat.

In human beings, heat is lost mainly through the skin and is subsequently regulated by circulation, the degree of insulation, and perspiration.

Heat loss occurs through conduction, convection, radiation and evaporation. Conduction occurs by direct contact with a cold object or air.

Heat transfer depends on the area of contact and the relative conductance of the objects.

Conduction is best experienced in water immersion, as the conductivity of water is approximately 23 times than that of air. Conduction can also occur when lying on cold wet ground.

Movement Of Air

Convection is due to the movement of air close to the body. It occurs during windy conditions when the temperature is below 20 degrees celcius. This is usually prevented by wearing clothes.

It is common in sports such as cycling and running. Radiation is the emission of heat energy to objects nearby. This occurs in uncovered skin and is the greatest source of heat loss under normal conditions.

However, in cold conditions, heat loss through radiation is less as the skin temperatures is usually almost equal to that of the environment.

Evaporation is the loss of heat as sweat on the external skin or on clothes. Heat, in this case, is converted from liquid to gas by evaporation.

Evaporation is part of the normal water losses from the body continuously.

Evaporation is greater during dry windy conditions. Heat is also lost through faeces, urine, and respiration.