Exercising and Competing in the Heat--Safe Summer Outdoor Workouts

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A combination of two or more of the above factors can increase your risk of heat injury. The most formidable combination is simultaneously elevated heat and humidity. An air temperature of 60 degrees plus 95% humidity is more dangerous than a “dry” 85 degrees. This combination places an extra burden on the cardiovascular and thermoregulatory systems. It’s no coincidence that triathletes, runners and outdoor fitness enthusiasts with undetected cardiac problems “choose” hot races to collapse in.

The blood to the skin carries heat from the body core, where evaporating sweat cools the blood before its return to the body’s core. But when your skin absorbs heat faster than evaporation can cool it, you run into problems. The hypothalamus—the body’s thermostat—detects this discrepancy and responds by dilating the blood vessels in the skin to be cooled. It also makes the heart pump faster to shunt more blood to the surface, causing your sweat glands to produce more sweat.

Soon a vicious competition for blood ensues between the brain (which needs 25% of heart output to function) and the working muscles, which need more blood but are getting less and less. It’s here the inexperienced or foolhardy athlete makes a mistake. Instead of slowing down, they keep pushing themselves. By continuing to push, they worsens their current state of dehydration. And with this increased sweat loss, the plasma becomes thicker and more viscous, causing the heart to pump harder.

Continuing sweating without taking in adequate fluids amplifies these demands on the circulatory system, which are by now becoming intolerable. At this stage you are a prime candidate for heat exhaustion, and if you ignore the signs, heat stroke. Usually sports performance declines by this stage, pressuring the over competitive athlete or runner to pick up their pace. Then the competition for blood becomes unbearable and the circulatory system and hypothalamus shut down. Blood pressure drops. Unconsciousness. Possible death.