Photographs by Filip Kwiatkowski for The New York Times
By GINA KOLATA, New York Times
Published: July 3, 2008
YOU already know that if you exercise outside on hot and humid days, you should drink plenty of water. And you are probably well aware of the risk of heat stroke given the countless reports about the warning signs.
But if you’re going to be out exercising anyway, you may have different questions: How long does it take to acclimate to the heat and humidity, and what is the best way to do it? How much does your performance time slow when it is sweltering and humid, and why? Does it help to douse your head with water?
Should you go out in the morning, when it is cooler but the relative humidity is higher, or at night, when it tends to be hotter but less humid?
The answers, some exercise physiologists say, are not always what you might expect.
There is no question that heat can take a toll on performance. Look, for example, at results from races on the second weekend in June, when a heat wave gripped the Northeast.
On June 7, over 4,000 women ran the New York Mini 10-K race in Central Park. When the race began at 9 a.m., it was 71 degrees and the humidity was 78 percent. The winning time, 32 minutes 43 seconds, by Hilda Kibet, was the slowest in a decade.
“From the beginning, my legs were not really moving,” Ms. Kibet told The New York Times.
That same day in similar weather and humidity, in Cambridge, Md., nearly 1,400 athletes raced in the Eagle Man Half Ironman — a 1.2-mile swim, a 56-mile bike ride, and a 13.1-mile run. Among them was Amy Roth, 32, the director of corporate partnerships at the Whitney Museum in Manhattan. She had trained hard, but the run, in particular, was difficult in the intense heat.
“I felt like I was dragging along but I couldn’t move any faster,” Ms. Roth said.
Still, she ran at a mile pace of 8:07.
“There were very fast people, very good athletes, who were walking, who just couldn’t do it,” she said.
Afterward, some posted comments, agonizing over their sluggish times, on Slowtwitch.com. “You could see the neuroses: ‘Oh, my God, am I getting slower? What does this mean?’ ” Ms. Roth said.
The next day, 190 professional cyclists started the Philadelphia International Championship, a 156-mile race. It was 79 degrees at 9 a.m. start, and 94 degrees when the last cyclist finished in mid-afternoon. About half of the competitors dropped out. The winning time, 6:14:47, by Matti Breschel of Team CSC, based in the Netherlands, was nearly a half hour slower than last year’s time, when it was cooler and drier.
One reason performance declines on sultry, humid days is that working muscles have to compete with the skin for blood. Directing more blood to the skin removes body heat and helps keep your body’s temperature from rising to dangerous levels. But that can mean less blood reaches muscles. At the same time, when your body becomes hotter, muscle enzymes speed up, burning glycogen more rapidly, depleting stores of the sugar that the muscles use for fuel.
Until now, most studies of the effects of heat on performance used treadmills or stationary bikes. If the subjects simulated a 5-kilometer road race lasting 15 to 20 minutes, their times would be 10 percent slower at 100 degrees than at 70 degrees. The longer the subjects ran, the more the performance declined.
One concern is that studies with treadmills may not accurately reflect what happens outside on a scorching day. With no wind indoors, for example, sweat will not evaporate as effectively.
Scott Montain and Matthew R. Ely, researchers at the United States Army Research Institute of Environmental Medicine in Natick, Mass., analyzed real-world data from seven major marathons, comparing performances over years when temperatures and humidity varied but the race course remained the same. Heat affected slower runners more, probably because they were on the course longer and ran in packs. Warm bodies close together make it harder for one’s body heat to dissipate.
An elite runner capable of finishing in less than two and a half hours on a cool day (41 to 50 degrees) would be 2.5 percent slower in warmer climes (68 to 77 degrees.) A three-hour marathoner on a cool day would be slowed by 12 percent in the heat, the researchers reported.
It may seem like a brilliant idea, then, to pour water over your head to cool down. That is what Floyd Landis did during a grueling ride on a hot day in the Alps during the 2006 Tour de France.
And last month, on that balmy Saturday, amateur runners used the same trick, dousing their heads, in an 8-kilometer race in Moorestown, N.J. Town residents also squirted runners with their garden hoses.
It is a useless ploy, said Samuel N. Cheuvront, another researcher at the Army institute. “Sweat must evaporate to provide cooling,” he said. “Dripping does not help.”
In fact, he added, if you get too wet you risk hidromeiosis, when sweat pores become blocked, which makes you even hotter.
AT least most races are held in the morning, when it is usually cooler and more humid, than later in the day, when it is hotter and drier.
Cold and humidity stresses the body less; you heat up less when it is cooler. Relative humidity may be greater on cool mornings, but what really matters for sweat evaporation is water vapor pressure. And water vapor pressure is lower when the air is cooler, meaning sweat evaporates faster.
Dr. Cheuvront said that if you have to choose between exercising in the morning when it is 60 degrees and 80 percent humidity, or in the evening when it is 90 degrees and 50 percent humidity, choose the morning.
Yet as challenging as heat and humidity are, people can acclimate. Blood volume expands, which reduces the strain on the heart from the increased demand for blood flow to the skin and muscles. And sweating increases — people who are heat adapted sweat sooner and more profusely, allowing their bodies to cool more efficiently.
For example, if you are not acclimated and run for an hour in 98-degree heat, your core temperature may go up to 103 degrees, bordering on the danger zone, said Craig Crandall, who studies heat acclimation at the University of Texas Southwestern Medical Center in Dallas. But if you are acclimated, your temperature might be 101 degrees after an hourlong run, which is well within the safety zone. Acclimation takes at least five days, Dr. Cheuvront found. He first asked participants to walk on a treadmill for 100 minutes in a room that was kept at 100 to 120 degrees.
On Day 1, Dr. Cheuvront said, they usually last 30 to 45 minutes. Then, he added, they will either request to get off the treadmill; collapse; or reach the safety-limit core temperature of 104 degrees, at which point they are stopped. By Day 5, just about everyone lasts 100 minutes.
It is possible to adapt even more. Dr. Cheuvront’s subjects continued to improve when they walked on the treadmill in that hot room for five more days.
Some people naturally adapt to heat much more than others. But Dr. Cheuvront said he had never come across a person who did not adapt at all.
The key to acclimation, he said, is to exercise in the heat daily and to be sure you are sweating profusely — wearing extra layers of clothing can help if you are exercising indoors or in cooler weather. Given a choice between spending more time in the heat but exercising less intensely, or less time and exercising more intensely, it is safer to choose to go longer and work less intensely, he said.
MS. ROTH’S impression that running was much harder than cycling in the heat was correct, physiologists say. And it is not just because there is more cooling wind when you ride. It is also because you don’t cycle upright, so your heart has less of a fight against gravity to pump blood to skin. That is especially true in the heat, when blood vessels in the legs are distended and blood tends to pool in the feet, making the flow of blood up to the head even more difficult.
But no matter how much you train in the heat, it will never be easy, athletes and researchers say. So perhaps the best strategy is to just accept discomfort and slowness.
“Heat is the X factor,” Ms. Roth said. “Sometimes you have to just forget it and move on.”