Heat Regulation

Our bodies are supremely adapted to maintaining a very stable core operating temperature of 37 degrees. Any deviation above or below this can be tolerated at the peripheries but the brain, heart, lungs and muscles will not tolerate even a small deviation of one or two degrees.

The biochemical systems that allow us to operate so efficiently compared with animals that can’t regulate temperature so well (reptiles, insects) does mean our bodies have to got to extreme efforts to maintain this exact core temperature.

For the athlete, good temperature regulation will make a big difference in the ability to perform and maintain a good power output. To do this, the body and the individual have to make choices between tolerating temperature variation and losing efficiency or expending lots of energy and fluids in an effort to maintain an exact temperature.

Compared to most all other mammals we have several important, fairly obvious differences. We are relatively hairless, very very sweaty and have a large surface area for our mass. We are clearly a heat-optimised mammal and are one of the few large mammals that can function in the tropical noonday sun. It has also been suggested that our upright stance helps reduce our sun exposure in that same midday period!

Most of our heat tolerating advantage is in our unique ability to sweat profusely, most mammals sweat very little at all. Even between trained individuals, the ability to produce sweat can vary several-fold and in some athletes the sweat literally pours of their bodies.
Sweating works largely by evaporation, which has an effective cooling surface effect. Sweat that runs off you does carry away heat but is not nearly as effective as evaporation.

Heavy sweaters can more easily become dehydrated and need to maintain a higher fluid intake that matches the salty composition of sweat. If they do match the fluid loss only with plain water they can run the risk of becoming hyponatraemic (low on sodium). This is the most common cause of collapse in endurance events and is sometimes called “water intoxication” especially when the athlete has over replaced with water and not taken electrolyte fluids or eaten salty foods during the event.

For cold events, technical clothing is the answer and allows our heat optimised bodies to operate in environments way outside their range. The big problem with strenuous exercise in these cold environments is our body’s tendency to sweat, which can overwhelm even the best technical “breathing” fabric’s ability to get rid of it. Getting wet in cold climates is not ideal especially when exercise stops and heat generation drops.
Clothing can make a difference in hot climates also; caps and light coloured clothing can deflect the sun and prevent heat build up.

The cooling effect of evaporation can also be improved by clothing that holds sweat, allows air to circulate next to the skin and increases evaporation’s cooling effects. A thin layer of poly pro may increase cooling by this effect in the same way the fine hairs we have can do.

Studies on the effects of splashing water on the face and upper body suggests this can have a useful but modest cooling effect but probably is mostly of psychological benefit, perhaps just as important.

Hot or cold food and fluid as appropriate are also of some modest benefit but again have a powerful perceived or psychological benefit.

Lots of research has been done on pre-cooling for hot weather events, so the body has a lower temperature at the start to act as a buffer against later heating. Again, the effect is fairly small and only useful in short events in very hot climates.

It does, however, make you wonder how long a “warm up” should be in hot events, not very long I suspect!

In the heat the body directs blood to the surface, especially of the extremities, to allow cooling and in the cold it restricts flow to these areas to preserve central heat. Excessive blood restriction in the cold can expose the individual to chill blains or even frost bite in severe environments.

This mechanism also varies enormously between individuals; some almost always have cold hands and feet while others are always warm and sweaty.

In hot environments cooling can still occur above the body’s own 37 degrees by the effect of evaporation. However, if temperatures go much higher, or high humidity makes evaporation inefficient then there is a real danger of over heating.

Although our bodies might only generate 300 watts cycling the inherent inefficiencies mean our bodies have lots of heat to get rid of in the order of several thousand watts of heat.

In very hot, very humid conditions heat build up can occur in athletes, leading to “heat stroke”, a very severe condition causing muscle melt down (and rhabdomyolysis), collapse and even brain damage.

Even though we are hot weather mammals we have our limitations. Keep cool!