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Saturday, July 14, 2012
Myths In The Open Water: Heat Escape From Head vs. Body
One issue that is commonly reported in the press is that "Most body heat escapes through your head."
This oft-used quote is an indication how important it is to wear a swim cap - rubber, silicone or neoprene.
But is it true?
Most open water swimmers are believe that more heat escapes through their head relative to other parts of their body. This statement is taken as one of the standards in the open water swimming world where the entire body is exposed to the elements while, generally, the swimmers wears a swim cap.
Note this article is based on swimming traditionally (i.e., with one porous swimsuit and one rubber or silicone swim cap) as opposed to swimming with neoprene (i.e., with a wetsuit or neoprene cap).
The belief that more heat is lost through the head than any other part of their body comes from a military study in the 1950s according to scientist and pediatrician Dr. Rachel Vreeman and Professor Aaron Carrol, MD MS. In the US military study, volunteer subjects were measured for heat loss while wearing arctic survival suits and no hats in extremely cold temperatures. Because their heads were the only part of their bodies that remained exposed to the cold, it stands to reason that most heat loss was through their heads.
A subsequent American survival manual in 1970 advised army personnel to cover their heads when it is cold since "40-45% of body heat" is lost from the head.
If the original military experiment was conducted with the subjects only wearing swimsuits, would they have lost 40% of their body heat through their heads? Recent studies have shown that any uncovered part of the body loses heat and will reduce the core body temperature proportionally.
"The more accurate statement is that swimmers lose a disproportionate amount of heat through their head when comparing to other surface areas," says Dr. Jim Miller, a member of the FINA Sports Medicine Committee. "The scalp is a richly vascular area with minimal fat deposits so it will readily exchange heat. As we all know, double caps make a tremendous difference in the athlete's comfort in cold water, and the reverse is true in warm water."
Dr. Otto Thaning, a heart surgeon from South Africa, explains, "Heat loss from the surface of the human body is a function of the area of the body's surface that is exposed to the elements and the differential temperatures of the relevant body area involved. In swimming, most heat loss is the result of conduction which is the direct transfer of heat from the warmer skin to the colder water whereas radiation and convection play a much lesser role in swimmers."
"Heat loss in general is related to the skin surface to mass ratio," says Dr. Miller. "The best example of this is children. Children have a lot of surface area per unit of weight and therefore are much more susceptible to extremes of heat or cold. Adults have an increase in surface area with growth; however, there is a greater addition of inner mass compared to the surface area so the ratio of surface area to mass actually declines as we reach adulthood."
Dr. Thaning, the fastest person over the age of 50 to swim across the English Channel, continues. "The rate of heat loss in conduction also depends on the temperature differential between the skin and the water. The temperature of the skin is one of the auto-regulation mechanisms that we can train by regular cold exposure. In other words, the blood perfusion rate of the skin - if minimal - will allow the surface temperature of the skin to approximate that of the water. By this mechanism, the heat loss will be reduced."
Dr. Peter Attia who has done both warm-water channel swims (in Hawaii) and cool-water swims (in Catalina) also has first-hand experience. "The head receives a disproportionate amount of blood flow. It has more to with actual blood flow, than outright surface area, per se. The head does not have the vasoconstrictive compensatory mechanisms of your arms for example. That is, you can't shunt blood away from your head as easily as you can from your arms or legs."
"In an adult, the surface area of the head is approximately 9% of the total body area. In a child, it is 18%," explains Dr. Thaning. "Heat loss there is thus not the main area of concern especially if a cap is worn and the swimmer has a good mop of hair."
Hair or not, cap or not, your head does not have as much insulation as in your torso. But does it matter?
"Most of us carry at least 50%, if not much more, of our total body fat around our torso," explains Dr. Attia. "There is variation among people and gender - males more on waist, females more on hips. This body fat does a great deal to insulate us from the large surface exposure of our torso where the bulk of the blood flow is. The kidney, for example, probably on a per mass basis receives more blood than another other organ in the body -- but fortunately for us it's so well insulated in the retroperitoneum."
Dr. Thaning agrees, "It helps to have a somewhat increased body mass index that will insulate the deeper tissues and thus also help to conserve heat." He describes what happens in the body. "The comparative volume of blood that perfuses the head is important. The brain takes about 25% of the total cardiac output per minute, the kidneys take another 25% and the rest of the body the remaining 50%. In exercise, these numbers vary significantly, but the real question is how much blood is flowing through the surface areas of the skin. In a cold-adapted swimmer, this amount is probably very low; hence the ability to drop the skin temperature to that of the water. That means the blood flow from the scalp and skin of the head and face is not equivalent to that needed by the brain.
In essence, I believe that heat loss from the head is not the main source of the problem. It is an area of [potential heat] loss that can be further limited by wearing a cap."
But what is practically true for open water swimmers is how sensitive their head is when swimming in cold water. The ice cream headache is well-known by all who enter cold water. And few are as experienced with cold water swimming as Ram Barkai, founder of the International Ice Swimming Association. "I started swimming without a swim cap as part of my training for Antarctica. Quite a few of us [in South Africa] do that now. We train at 11°C (51.8°F) without a swim cap. It makes the ice swims easier. Swimming without a cap take me up to 10 minutes (at 9°C or 48.2°F) to be able to swim comfortably. The pain your head experiences is unbearable. It feels like a multiple ice cream headaches, but not in the sinuses. [It feels] like someone put your head in a vice and squeezes. However, after 10 minutes I get used to it and [my head] feels no difference unless there is strong wind."
From practical experience, one of the world's most prolific extreme swimmers says, "I don't think that heat loss from head is different. But the nerves and blood flow around the skull is different and makes it more sensitive. In ice swims, I don't think I could do them without a cap."
That being said, most swimmers do not swim in sub-5°C (41°F) like Ram or others in the extreme swimmer community. Most swimmers are dealing with cold more likely in sub-16°C (60.8°F) water.
Long-time pool and open water swimmer Dr. Larry Weisenthal explains that when swimmers kick, their limbs require blood perfusion and make the percent of blood flow to the head less than in the case of an immobile person. "There is a big difference between leg-dragging swimmers and those who have a vigorous kick. [Those who do not kick much] shunt lots of blood away from their legs. A swim cap would make a bigger difference in these swimmers than in vigorous kickers. The vigorous kicker will also generate more heat from muscle contraction than a leg dragger. This helps those with low percentage body fat to stay warm swimming in cold water.
Kicking versus non-kicking doesn't have any effect on absolute blood flow to the head. So the absolute amount of heat lost from the head will be the same in a kicker versus non-kicker. But since the non-kicker has a lesser total circulating blood volume - because the blood in the legs just remains in the legs, rather than circulating through the heart and the rest of the body, proportionately more heat is lost through the head since the head makes up a greater percentage of effective blood flow. But the absolute blood flow is the same. If the swim cap "preserves" one degree celsius of core body temperature in a non-kicker, it will also "preserve" the same one degree in the kicker."
Heady words from experienced people.
Other commonly held beliefs among open water swimmers include:
1. When there are dolphins in the oceans, the swimmers is safe from sharks (read here).
2. Commercial jellyfish ointments will prevent jellyfish barbs from firing into the skin of open water swimmers.
3. Black wetsuits lead to shark attacks (read here).
4. Shark risks increase at dawn and dusk (read here).
5. More people have been in space than have swum across the English Channel (read here).
Photo shows Jackie Cobell with a swim cap on.
Copyright © 2012 by World Open Water Swimming Association
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