On a per capita basis, Australian athletes win more Olympic medals than any other country. The Australians’ success is due in part to their systematic application of sport science. One example of their attention to detail is the development of special vests to cool off their athletes before competition in the heat. The technique is called pre-cooling and the theory behind it is that by cooling off prior to hot weather exercise, the body has more capacity to store heat, and loses less fluid, during the competition. This is a relatively new area, but several studies have already shown that pre-cooling before exercise can improve running performance in the heat.
How does pre-cooling improve hot weather running performance?
When you run on a hot day, your body must deal with both the heat of your surroundings and the heat produced by your muscles. In fact, over two thirds of the energy produced by the muscles is lost as heat. During hot weather running, more of your blood is sent to your skin for cooling so less blood flows to your working muscles. This means that your heart must beat faster to run at a given pace. Pre-cooling is simply reducing your body temperature slightly before exercise, and can be accomplished in a variety of ways. After pre-cooling, less blood is sent to the skin, so more oxygen-rich blood goes to the muscles. This results in a lower heart rate running at a given pace, which allows you to maintain a faster pace in the heat.
The scientific evidence
Although only a handful of scientific studies have been conducted on pre-cooling for athletes, the results indicate a real benefit for hot weather endurance performance. In a 1995 study published in the Journal of Applied Physiology, 14 runners ran to exhaustion in 75 degree heat under normal conditions and after pre-cooling in a chamber for 30 minutes at 41 degrees. Pre-cooling reduced the runners’ rectal temperatures by 0.6 degrees. The investigators found that pre-cooling led to a significant increase in running endurance, and hypothesized that the improvement was due to reduced stress on the runners’ metabolic and cardiovascular systems.
During an earlier study published in the Canadian Journal of Physiology and Pharmacology, subjects cycled as hard as possible for 1 hour at 65 degrees under normal conditions and after pre-cooling. After pre-cooling, the athletes increased their work rate by 7%. In another phase of this study, athletes cycled as long as possible at 80% of VO2 max. Time to exhaustion increased by 12% after pre-cooling, and both sweat rate and heart rate were lower after pre-cooling. This study suggests that pre-cooling may improve endurance performance even in relatively moderate temperatures.
A 1997 study published in Medicine and Science in Sports and Exercise investigated the effects of cold water immersion on running performance in hot and humid conditions. Subjects ran as far as possible in 30 minutes in 90 degree heat and 60% humidity. Before one trial, subjects reclined in a cold water bath up to the neck for one hour or until they started to shiver continuously. The water temperature was initially 83 degrees but then was lowered to 73 degrees (that may not sound cold, but after a few minutes in water this temperature, you would be anxious to get out). Subjects started the 30 minute performance test 3 minutes after getting out of the bath.
After pre-cooling, the runners’ rectal temperatures were lower by 1.2 degrees Fahrenheit. Subjects averaged 7,252 meters during the control trial and 7,556 meters after pre-cooling, for an increase of 304 meters (just over 4%). This improvement is equivalent to over 1 minute for a 5 mile race.
How to pre-cool:
The safest and most convenient way to cool yourself off before hot weather competition is to wear an ice vest. Ice vests are effective in lowering skin temperature, but do not have as large an effect as sitting in ice water. But, then again, who would willingly sit in a tub of ice water?
According to Dr. Gordon Sleivert of the Human Performance Center at the University of Otago in New Zealand, the reduction in skin temperature from wearing an ice vest is likely sufficient to improve performance by allowing more blood to be pumped to your working muscles. Several companies manufacture sleek, relatively inexpensive cooling vests for athletes.
Dr. Sleivert recommends the following cooling vest protocol before hot weather racing, “The runner should put on an ice vest about an hour before the race, and wear it for the duration of his or her warm-up. This will reduce sweating during the warm-up and will preserve precious body fluids for the race. Some of the athletes I have worked with are a little uncomfortable during the first few minutes of their warmup, but they feel better once they get going.”
For the greatest effect, take the ice vest off as close to the start of the race as possible. In addition, since the heat your muscles produce contributes to your body’s heat load, you should limit your warm-up on a hot day. You are actually more likely to suffer from a heat injury during a 10K than during a marathon because of the higher rate of muscle heat production during the faster paced race.