In my speedier days I figured that hot weather slowed my 10K times by about a minute. Not very scientific, but semi-accurate. Recently, I saw a New York Times article by Gina Kolata discussing the subject of heat and running in greater detail. Her article noted that our blood supply is one of the body’s cooling mechanisms, circulating to the skin to remove body heat. But on hot days, that means less blood reaches the muscles since it’s needed for cooling duty. Your legs feel like lead as muscle enzymes increase, burning glycogen (muscle fuel) more rapidly, and your running performance suffers.
The evaporation of perspiration is our other cooling mechanism. Interestingly, when runners pour water over their heads during road races it may feel good, but “is a useless ploy,” according to the article, since sweat must evaporate to provide body cooling. And high humidity is a major factor since perspiration won’t evaporate as quickly on a humid day (but I doubt that science will deter runners from sloshing water on themselves or running through spectators’ lawn sprinklers).
Ms. Kolata goes on to say, “An elite runner capable of finishing [a marathon] in less than two and a half hours on a cool day (41 to 50 degrees) would be 2.5 percent slower in warmer climate (68 to 77 degrees). A three hour marathoner on a cool day would be slowed by 12 percent in the heat,” the researchers reported. But she doesn’t explain why elites are slowed by only 2.5 percent while the rest of us are affected more severely. Maybe it’s physical size-we normal-to-heavier people are affected more by the heat than the featherweight elite runners with practically zero body fat.
But our bodies adapt-in about a week-according to scientists cited in the article. Annually we suffer through those first hot days of spring, but after a week or two it’s not so bad anymore as our bodies get used to the higher temperatures, producing higher blood volume and increased sweating. Anybody who has lived in a hotter climate (like Southeast Asia in the 1960s, for me) can attest to the fact that you adapt, whether or not you’re a runner. It’s still hot and uncomfortable, but you manage to get along.
Since it’s August already, this is stating the obvious, but we all remember how uncomfortable it felt during that first hot spell. If we were just training, we could slow down, take more breaks or cut the distance. But there are those times in the spring when you’ve registered for a particular race after training in nice, cool weather. Then a sudden hot spell rolls in, and race day dawned 75 degrees and humid. By the start of the race it’s over 80 with blazing sun. We’ve all been there and it’s miserable.
But as summer stretches on, we’re running in similar conditions without suffering so much. Not that running in the heat is much fun, but we adapt. And if we pay our summer “dues,” our race times will be better in the fall when it’s cooler.
Paul Oppenheim is a member of the Oak Park Runners Club.