Your metabolism probably averages something around 100 watts over the course of a day.1 Our bodies aren’t terribly efficient at converting this energy into motion, so we have to get rid of quite a bit of waste heat. Sweating is one of the main mechanisms, It works particularly well when our sweat can evaporate from our skin - something that gets into a bit of curious physics about water.
These mechanisms work fairly well for most of us, but athletes have a real problem. I’m far from an athlete, but regular produce about 150 watts of work, in addition to my base load, when I’m rowing. In an hour I’ll go through over a liter of water and will still be very thirsty with my clothes drenched in sweat. It turns out, if I’m using good form, I’m about 20% efficient so my 150 watts of work requires about 750 watts from my metabolism. I’m left with about 600 watts of wasted energy to dump somewhere - most of it as waste heat. Think about holding a half dozen 100 watt incandescent bulbs...
Although I have about two square meters of skin, my sweating primarially takes place in preferential areas we’re all know. At that power level I am still regulating core temperature well and excessive waste heat is not a roadblock to my performance.
Recently I had a bit of surgery and remember being frigid when I started coming out of the anesthesia. They had wrapped in blankets to deal with the problem, but there was still quite a bit of shivering . It turns out the anesthesia disturbs our ability to regulate our core temperature and there in lies an interesting story.
About 20 years ago a couple of doctors at Stanford noticed you could warm surgery patients by increasing the blood flow to the skin of their palms. Very very curious stuff indeed. It turns out one of the ways we control our core temperature is by regulating the blood flow to the skin in non-hairy parts of our body. Notably the plams, soles of the feet, and cheeks - another neat mechanism.
A real athlete often needs to summon a very intense performance and can produce fairly amazing amounts of power. Those who specialize in events where burst performances are requried are paritcularly impressive. Colleen can produce over 700 watts for a half minute - a not far off a horsepower. She is probably about 20% efficient doing this (many of our athletic motions are in the 15% to low 20% range), so she needs to get rid of about three kilowatts of power. The burner on your electric range on high radiates about half that much.2 One of the limits of intense bursts of powe - beyond running out of fuel local to the muscle very early on and then developing too much lactic acid shortly thereafter - is the spike in core body temperature. A world class sprinter would probably die if they were to maintain their power level for - say - 20 seconds. Their 10 second intense burst of power can raise their core temperature to a dangerous 105°F.
If you could artificially increase blood flow in one of the critical areas, you could help dump some waste heat and perhaps increase the endurance of an athlete. It would be great during breaks in football and, in fact, the effect is being exploited with a device known as “The Glove”. A player inserts his hand into a chamber that seals at the wrist. A slight vacuum is applied under the palm to stimulate blood flow and quickly drop the core temperature in a few minutes.
1 An average adult might require about 2100 nutritional Calories a day. That is roughly equivalent to 2400 watt-hours. To find the average power in a day, simply divide by the number of hours in a day. You’re a light bulb:-)
2 Of course the hot plate only radiates over an area of about 0.1 meters whereas Colleen has 20 times that area, albeit not evenly distributed.
It was too tempting to use this photo of a very sweaty President. In fairness he probably was the most athletic