Relentless Forward Progress_ A Guide to Running Ultramarathons - Bryon Powell [71]
The Heat Is On
Hot weather is by far the most common extreme condition ultra-marathoners experience. Sometimes ultrarunners face the smothering reality of “hazy, hot, and humid.” Other times that heat presents itself as a moisture-sucking dry heat that feels like you opened an oven door in your face. While these types of heat have different effects, both present performance and health issues.
Performance and Acclimation
Whether we’re looking to finish or to win the race, most of us go into an ultra with performance goals in mind. Heat can directly limit performance by diverting blood flow from muscles to the skin for cooling, and indirectly by causing dehydration, the performance-limiting effects of which were explained in chapter 8. I’ll leave it to Dr. William Henderson to explain the effects of heat on running and how to acclimate in case you’ll be racing in extreme heat.
Stream and river crossings offer a welcome reprieve on a hot day. (Photo by Glenn Tachiyama)
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Heat Performance and Acclimation
William Henderson, MD, FRCPC
There is little doubt that exercise performance is impaired in hot environments. While the effect of heat on performance varies with the sport (for example, it has less effect on cycling than running), there is a great deal of empirical data showing a link between ambient temperature and performance. Various authors have suggested performance impairments of 1.6 to 3 percent in marathon times for every 10 degrees above 55 degrees Fahrenheit. The effect seems to be less dramatic for faster runners.
How Heat Affects Running Performance
Why are we slower in hot conditions? There are a variety of proposed mechanisms, but the one that is most widely accepted is based on cardiac output limitations.
When we exercise, we produce a great deal of heat. One of the principal ways that we get rid of this excess heat is through sweating (evaporative heat loss), as well as conduction and radiation of heat from our skin. To achieve this, our bodies have to send a considerable amount of blood to the skin. This blood is therefore not available to perfuse working muscles and deliver oxygen to them. So a portion of our blood volume is essentially no longer able to participate in oxygen delivery and energy formation in our exercising muscles. The greater the amount of heat that we need to dissipate, the greater the proportion of blood that is diverted to the skin (up to a point—this can’t increase forever).
What is necessary for cooling isn’t the hemoglobin (the red blood cells in blood) but the plasma, which is essentially water with a number of different proteins and electrolytes in it. However, your body can’t separate the red cells (which are the oxygen carriers) from the plasma—they all go along for the ride to the skin.
How We Acclimate to Heat
If it’s plasma that is the essential cooling component, is it possible to improve this problem by increasing our total plasma volume? Yes, and that is exactly what happens as we adapt to heat over time. Whether you acclimate naturally to higher temperatures over the course of a season, or in a heat chamber, the most significant change that occurs is an increase in plasma volume. Other things occur as well (such as changes in sweat sodium concentration, resting core temperature, and heart rate), but plasma volume expansion is the key. After extensive acclimatization, plasma volume can have expanded by as much as 2 liters!
This may explain why the fittest athletes adapt to heat stress more quickly than the less fit. One of the by-products of endurance training (especially at high intensities) is an increase