I arrived today in St George, Utah, two days before the St George Ironman. I had signed up for this event several months ago when I had aspirations of being in top shape and chasing a Kona spot. Since then training hasn’t gone so well, but I’ve decided to show up anyway because the opportunity to participate in an Ironman is pretty awesome.

After checking in for the race and settling in the hotel, I decided to go for a short run. Immediately I was blow away by how hot it was. According to wunderground.com, the max temp today was 87 degrees, with a historical average for May 5 of 67 degrees, and prior record temp of 83 degrees in 1989. Last year at the inaugural Ironman St George there were problems with cold water temperatures and high winds. There was almost no wind today, and the water temperature today was reported as 65 degrees. What is the predicted high temperature for Saturday? 92 degrees!! Apparently we won’t have to worry about a cold race.

Writing this post will not change any suffering that occurs on Saturday. I’ve decided to write on this topic because clearly we are entering the hot season again. At its worst, heat kills. Even without significant heat injury, heat drastically affects the performance of endurance athletes. Although we frequently have to deal with this natural phenomenon, I have found that how heat affects our physiology during exercise is not well understood by most athletes.

You probably know that the average normal core body temperature is approximately 98.6 F (37 C). There is some individual variation of this, but overall the hypothalamus acts to maintain a core temperature at which your body will function well at. It is normal though for our core temperature to be lower (even less than 97 F) in the early morning, or during cold weather. During hard work, strong emotions, and occasionally in some normal adults and active children, core temperature can sit higher at 99-101 F. Exercise is not very efficient, with over 80% of the energy that we expend during exercise being converted to heat rather than work, and during hard exercise our core temperatures normally will rise to 101-104 F. Although the temperature is elevated, it is not a “fever”, which is a resetting of the hypothalamic set point to a higher temperature (most commonly induced by chemical messengers released in response to an infection). So, don’t bother trying to take Tylenol to lower you body temperature when you exercise. It will not work.

Even at rest, we are generating a lot of heat, mostly from deep organs. During exercise there is much more heat being generated from skeletal muscle contractions. This heat must be transferred to the skin where it can be lost to air. The rate of heat loss depends on how rapidly heat can be transferred from the core to the skin AND how rapidly heat can be transferred from the skin to the surroundings. To increase the amount of heat transferred to the skin (as is needed in exercise) the blood flow to the skin must increase, pure and simple.

Transferring heat from the skin to the environment occurs with the four mechanisms of heat transfer (flashback to physics): radiation, conduction, convection, and evaporation. Don’t tune out if you are already feeling confused. I’ll try to take the mystery out of these.

Radiation is loss of heat in the form of infrared heat rays. This is important to understand. Every object (be it your body, the pavement, or a lava rock) will radiate heat in the form of infrared heat rays. When you look through night vision goggles this is evident because objects that are radiating more are more visible in contrast to the cooler surroundings. Although a lot of heat is lost from the body by radiation, there is heat also radiating towards the body. If the temperature of the body is greater than the temperature of the surroundings, more heat is radiated away from the body than towards it. How significant is this? It can be very significant. It is estimated that a nude person in a room that is at 70 F will lose 60% of their total heat loss by radiation.

Conduction is heat loss that occurs when the body is in direct contact with objects or air that is of lower temperature than body temperature. A nude person in a room at 70 F loses about 3% of total heat loss by conduction.

Convection is heat loss that occurs by conduction to air, which is then carried away by convection air currents. This can be a very significant portion of total heat loss, and depends on the wind velocity. The cooling effects of wind is proportional to the square root of the wind velocity, i.e. a 4 mph wind is twice as effective as cooling as a 1 mph wind.

Evaporation is heat loss that occurs when water evaporates from the skin or lungs. Evaporative heat loss can be increased by sweating and becomes essential in high temperatures. High humidity impedes evaporation, often resulting in dripping sweat which has the effect of causing dehydration without cooling.

In response to excess heat, there is direct stimulation of the anterior hypothalamus which causes nerve impulses that result in increased sweating. Sweating can increase heat loss 10 fold. The posterior hypothalamus also acts to cause nerve impulses that result in dilation of the blood vessels in the skin, increasing blood flow from nearly nothing to 30% of total blood flow! This increases the heat transfer from organs to the skin. I will leave you now with the graphical representation of this to contemplate, and pick up the rest of this topic in another part.

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