WASHINGTON (AP) -- An ice water bath may help distance runners prepare for a race, and researchers say it works best with a rectal thermometer.
Athletes must monitor their temperatures carefully or risk sinking into dangerous hypothermia, researchers say.
The technique, known as precooling, pushes down core temperature, the internal temperature created largely as a product of metabolism, which is the conversion of energy into heat and work.
By driving down core temperature, precooling gives an athlete a bit more time to work out or compete in aerobic sports before overheating. ''You now have a capacity to store this heat that you didn't have before,'' said researcher Emily Haymes of Florida State University. ''That allows you to go longer.''
Precooling is a way to delay the shunting of blood away from muscle as a way to lower core temperature. Normally, as the body burns calories to create energy, excess heat is transferred to the blood, which is sent to the skin, where the heat can be radiated away with the help of cooling from the evaporation of sweat.
But blood sent to the skin is not carrying oxygen as well as nutrients to hard-working muscles. So muscle performance can be impaired.
''Precooling studies confirm that increasing body heat is a limiting factor during exercise,'' said researcher Frank E. Marino of Charles Stuart University in Australia.
Marino, whose report in the April issue of the British Journal of Sports Medicine reviewed 13 previously published studies, concluded that ''precooling before an endurance event may be worthwhile if the facilities are available.''
Studies have found that precooling aids aerobic activities of 30 to 40 minutes in warm weather, Marino said. In most cases, precooled athletes were able to work harder than athletes who did not have the benefit of precooling, his article said.
''What we saw, and what most of the other studies have found, is this is most beneficial in exercise that is high-intensity,'' said Haymes, whose work was cited in Marino's article.
The study found precooled athletes averaged about 26 minutes on a treadmill -- about four minutes longer than those who started at normal body temperature.
''The issue is where do you start and where do you finish,'' said researcher Lawrence Armstrong of the University of Connecticut. Performance suffers at temperatures above 101 degrees, and precooling can delay the cooling-performance tradeoff by starting at a below-normal temperature, he said.
There are risks involved -- the chief being hypothermia, in which the central nervous and respiratory systems are depressed, and the heartbeat can become dangerously irregular. It can occur when internal temperature falls to 95 degrees or lower.
The risk of hypothermia is fairly low, said scientist John W. Castellani of the U.S. Army Research Institute of Environmental Medicine. But the risk varies from person to person, he said.
A person can cool to 97 degrees or a bit lower without becoming hypothermic, but temperatures have to be monitored closely, Lawrence said. That's where the rectal thermometer or some similar way of monitoring core temperature is needed. The experts think an under-the-tongue or on-the-ear thermometer is not accurate enough.
This may limit the popularity of precooling. ''Not many people are going to use the rectal thermometer,'' said researcher Allen Parcell of Brigham Young University.
There are other limits to precooling, such as equipment needed to do it. ''You'd have to back an ice cream truck up to the starting line,'' Parcell joked. Cold water actually is far more effective than cold air at leaching away body heat, and cold water is what he used in his research.
One cooling technique, used now against heat stroke after hot-weather races, needs only a child's wading pool with six to eight inches of ice water in it, Armstrong said. A person can sit in the pool with arms and legs dangling outside, thereby cooling the core without also chilling muscles that should be warm and pliant for competition, he said.
''I would recommend they do this, if they record core body temperature while they do this,'' Armstrong said.
On the Net:
British Journal of Sports Medicine article: http://bjsm.bmjjournals.com/cgi/content/full/36/2/89
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