Hot weather exercise

What’s the key to hot-weather exercise? Produce ” weaker” sweat.

Athletes who can exercise at moderately high intensities for 90 minutes under mild weather conditions can usually only last 60-80 minutes as the air temperature and/or humidity increase. Part of the problem is that skin temperature stays at about 35 degrees Centigrade during exercise. As air temperature moves above 25 degrees and advances toward 30-32 degrees, the difference between skin and air temperatures is so slight that it’s impossible for the skin to ‘radiate’ much body heat into the atmosphere

That leaves athletes totally dependent on evaporation for cooling, but as humidity increases, the evaporative cooling process can’t work, either. Sweat simply pools on the skin, and athletes are unable to control their body temperatures. To put it in the vernacular, they begin to ‘cook’

Of course, athletes should avoid exercising when temperatures and humidity are extreme, but it is possible to adapt to 25-32 degree temperatures, provided the humidity is not too high. One key adaptation is that athletes’ sweat becomes more dilute (there are fewer electrolytes, including sodium, dissolved in it). At first glance, this seems like a bad thing (it sounds as though too-copious amounts of watery sweat would be pouring on to the skin’s surface), but the increased dilution of sweat is actually good

After all, the ‘weaker’ sweat means that more sodium is being conserved inside an athlete’s body. This preserved sodium actually ‘pulls’ more water into the blood, keeping blood volume high even in the face of fairly heavy sweating. A key benefit is that the higher blood volume helps to keep heart rate fairly low during hot-weather exertion, making exercise feel less demanding and troublesome

The extra blood volume also lessens the intense conflict which is usually set up between the muscles and skin during hot weather. The muscles selfishly want more blood because of the oxygen it contains, while the skin demands blood for cooling. The increased blood volume allows both demands to be met fairly successfully.

Elite Kenyan runners say that they can tell they are ready for hot-weather racing when their sweat begins to taste weak and unsalty. As usual, the Kenyans are right. The lack of saltiness in their sweat is a sign that their bodies are clinging to sodium and are therefore ready for running under steamy conditions

While your sweat glands are being stingy with sodium, your kidneys also help you adapt to the heat by holding on to water more tightly (lowering your urine output), which also boosts blood volume. You become more resistant to both overheating and steep, hot-weather-related declines in performance

What’s the best way to stimulate your sweat glands and kidneys to ‘get with the programme’ and make you more heat-resistant? Simply exercise under warm conditions for about 30 minutes per day for seven days, with your heart rate at around 80 per cent of maximum. Don’t exercise when weather conditions are too severe, however, and don’t believe that your seven-day heat indoctrination provides foolproof protection against the effects of heat. Runners’ body temperatures have reached an extremely dangerous 105 degrees Fahrenheit after races as short as 5K, so be very cautious when heat and humidity are present!

(Ethan Nadel, Ph.D., Presentation made at the 42nd Meeting of the American College of Sports Medicine, Minneapolis, Minnesota, May 31-June 3, 1995)

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