It happened one Saturday
You know that you tire more quickly when it's 110 degrees rather than 70 degrees, even if you're just taking a leisurely stroll. And you know the harder you work, the more tired heat makes you. But why is that? And what can you do about that?
When you run in the heat, your body sends more blood to your skin to cool you via evaporation. As a result, less blood, which carries oxygen, goes to your leg muscles, and they have to work harder just to maintain your usual pace. Also, the warmer it is, the more you sweat. This is good, because sweating helps to cool you as your perspiration evaporates. But it's also bad, because your blood volume decreases. With less blood returning to your heart, your heart has to pump more often to keep the same amount of blood circulating throughout your body. And when your heart rate increases, you're working harder.
Running is even tougher on humid days. You generate all that sweat, and it's suppoded to cool you down, but the air is so moist that it can't absorb much more water. So your sweat stays on your skin rather than evaporating, and your body just keeps producing more and more sweat, and you just keep getting more and more tired. That's why it's a lot tougher to run on a 90-degree day in the Northeast part of the country than here in Phoenix. Here, it's hot, but dry, and your sweat can cool you like it's supposed to.
The upshot of all this dripping sweat is a dropping pace. One study found that for each one percent of body weight that you lose because of dehydration, your running performance falls by three percent. Bear in mind that one percent of body weight isn't much - if you weigh 150 pounds, that's only 1.5 pounds, or 24 ounces. It's not unusual to love three of four pounds of water in an hour of running on a hot day. This loss causes your performance to sag by more than five percent, which can be as much as a minute slower per mile. Are you starting to see why it's so tough to feel comfortable while running in the heat?
You also need to consider cumulative dehydration. Cumulative dehydration is what happens when it's hot for several days in a row, and you don't replace all the fluid you lost after each run. Suppose that during a hot week, a 150-pound runner doesn't rehydrate himself completely each day. By the end of the week, he has a cumulative loss of two pounds of water - that's more than one percent of his body weight. As a result, his performance is off by more than three percent by week's end, but he may not make the connection to dehydration. A lot of runners go through the hottest parts of the year chronically dehydrated. As a result, their running suffers.
