Top Endurance

The human body is able to gain or lose heat in four ways: conduction, convection, radiation, and evaporation. Conduction is heat transfer from contact between two surfaces. For example, when a person sits on a cold metal bench, heat is conducted from the body to the cooler bench.

Convection is a specific type of conduction that occurs via the transfer of heat between a person and air or fluid. When an athlete is immersed in water (e.g., a triathlete or distance swimmer), the conductive and convective heat exchange that occurs between the athlete and the water is much greater than heat exchange created by other conditions. This is the main reason that wetsuits are allowed for many triathlons. The wetsuit traps a layer of water between the skin and the suit, and it reduces the rate of convective heat loss. Another example of convection in the triathlon occurs during the bike and run. Because of the greater speed of the bike compared to the run, the convective heat exchange is much greater. In fact, during a race, triathletes may not be aware of extreme heat conditions until the start of the run.

Radiation is heat gain or heat loss by way of electromagnetic waves. An extreme example of radiative heat exchange for endurance athletes is the environmental stress seen by Ironman athletes competing on the lava fields of Kona. The athletes absorb heat from the environment, both from the sun and from the heated lava fields.

Evaporation is the conversion of water to water vapor. Athletes lose a lot of heat through the evaporation of sweat. The body loses 580 kilocalories (kcal) of heat for each liter of evaporated sweat. However, if the sweat does not evaporate and soaks clothing or drops to the ground, then no evaporative cooling can occur. The environmental humidity has the greatest effect on the rate of sweat evaporation. The more humid the environment, the lower the level of sweat evaporation. The cooling effect of sweat only occurs when the sweat evaporates. Higher air humidity reduces sweat evaporation.

When exercising in a cold environment, athletes need to be concerned about maintaining core temperature above 95 degrees Fahrenheit in order to avoid developing hypothermia. In cooler environments, heat loss from the body often occurs in windy conditions or when the athlete’s clothing becomes wet, such as when a novice competitor wears clothing that does not wick sweat (e.g., a cotton T-shirt). Minimizing skin exposure to the wind and protecting the extremities from the cold can go a long way toward ensuring safe exercise. By taking these precautions, athletes can minimize the occurrence of frostnip (cold damage to the epidermis of the skin) and the more severe frostbite (fluid freezing in and between the skin cells). Many of the effects of cold exposure can be controlled by making well-planned clothing choices. Dressing in layers, using clothing that wicks moisture, and wearing an outer layer that protects against the wind help to minimize the development of cold-related problems.

Open-water swimmers and triathletes need to be aware that heat transfer in the water occurs at least twice as fast as it does on land. This means that these athletes have an increased risk for hypothermia, especially during the longer-distance swims. To help avoid this concern, many triathlons have specific rules that allow wetsuits when water temperatures are below certain levels. However, many open-water swim events do not allow wetsuits. Figure 3.4 on page 58 shows an estimate of survival times with water immersion. The time increases when a wetsuit is worn. Most triathlon swims will not occur in extremely cold water that will be life threatening; however, endurance swimming events in northern parts of the world could take place in temperatures cold enough to be a concern.