A Secret Weapon in the Palm of Your Hand?
We’ve done a lot of homework up to this point. By now, you are better educated on the matter of heat in endurance racing than 99% of your peers. Now that you understand the underlying science, we’re ready to cut to the chase.
What works and what doesn’t in the “extreme heat garment” market?
Let’s go down the line of a few products and assess their claims.
First is the idea of hand coolers, gadgets that propose to help keep you cool by allowing you to hold something very much like ice in your hand during your run. Among this group is DeSoto, who debuted a new and improved line of arm coolers this year just in time for the Ironman World Championships.
On their website, they claim that:
Backed by medical research proving that cooling the palms and wrists results slows the rising of core body temperature, reduces onset of fatigue, and accelerates recovery time…
The greatest advantage in palm cooling is running in heat. The purpose of this product is to keep your core body temperature from rising when “crossing that bridge” between aid stations that can provide ice, cold water, or cold sponges.
DeSoto then refers to three separate peer-reviewed studies to back up the validity of their technologies.
I dug up the three papers and read through them. There are some serious issues. Two of the three reports used a highly sophisticated unit to provide the hand cooling during testing. Priced at $800, it’s essentially a mini-fridge for your hands. Far from just inserting a few ice cubes into the palm, you actually immerse your whole hand up to the wrist in a sealed container that constantly circulates ice-cold water into it from a cooler. Therefore, the method in the research really isn’t the same thing that De Soto is offering. One report explicitly states that the device had an added advantage by sealing the hand in its own pressurized micro-environment, citing that “Hand cooling alone had little effect on exercise duration…”
Given this information, it’s worth examining just how much extra heat you can remove from your body by putting an ice cube in your hands.
Let’s make some assumptions. First, let’s assume the ice is able to cover the entire surface area of an athlete’s palm, which gives the hand coolers the maximum possible chance. Second, let’s assume that an athlete wearing the arm coolers fills up on ice at every aid station on the run, and therefore never runs out of ice.
Now for a few numbers, which we’ll need for an equation.
Ice freezes at 32⁰F (274.15⁰K). We’ll assume our athlete’s skin surface temperature is 99⁰F (310⁰K).
Studies have found that the surface area of the palm of an adult human’s hand ranges between 132 and 146cm2, and a brief overview of anatomical literature indicates the maximum skin thickness on the palm of the hand is about 1.5mm. Our primary method of heat transfer is conduction (contact between skin and the ice). The equation for energy transfer via conduction, also known as Fourier’s Law, is:
Rate of Heat Transfer (watts) = (conductivity of skin) x (surface area of palm) x (temperature of hand – temperature of ice) / (thickness of skin)
The thermal conductivity of human skin is estimated to be anywhere between 0.2 and 0.4 W/m⁰K.
Assuming the best possible skin conductivity, the largest hand, and that the ice is directly against the skin, the hand coolers would help you expel 280 watts of heat energy (140 watts from each hand). But if we go with less-than-ideal conditions, and assume a smaller hand and a more modest skin conductivity, our rate of heat dissipation drops to about 190 watts (95w from each hand).
Even 190 watts is actually pretty good!
It’s around 15% of your total heat production on the run. But one has to keep in mind that this requires ideal conditions. The ice is touching the entire palm of your hand and doesn’t melt. We are also not factoring the extra insulation of the hand fabric itself between your skin and the ice. In fact, these real world considerations will greatly reduce the product’s effectiveness. As the ice melts, it will approach your skin surface temperature and touch less skin. This means you will probably only experience the ideal heat dissipation rate for a few moments after each refill. You also have to consider that it will take some extra time and energy to grab ice and put it into the hand coolers at every aid station. In practice, it’s quite dubious as to whether any hand cooler product will yield a substantial benefit.
DeSoto’s product is a little more problematic, though, because it incorporates sleeves. According to their advertising, the company says that “the arm coolers do provide skin protection by blocking UVB rays to the arms and back of the hands and keeping you cool.” This is a claim made by manufacturers of several such products, but they all have one tiny problem in common—namely, choice of color. White is the go-to color for everyone’s “beat the heat” gear. Is that really the right choice? Science says not. That’s next.
1. Dennis A. Grahn, Vinh H. Cao, and H. Craig Heller, “Heat extraction through the palm of one hand improves aerobic exercise endurance in a hot environment.” Journal of Applied Physiology 99: 972–978, 2005.
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