Black bears are extremely well-insulated animals, with a thick fur coat over a thick layer of fat. This keeps them quite warm in the winter:
But once spring arrives and temperatures rise, these same bears face a greater risk of overheating than of hypothermia. How do they dump heat without changing insulation layers?
Heller and Grahn discovered that bears and, in fact, nearly all mammals have built-in radiators: hairless areas of the body that feature extensive networks of veins very close to the surface of the skin.
Rabbits have them in their ears, rats have them in their tails, dogs have them in their tongues. Heat transfer with the environment overwhelmingly occurs on these relatively small patches of skin. When you look at a thermal scan of a bear, the animal is mostly indistinguishable from the background. But the pads of the bear’s feet and the tip of the nose look like they’re on fire.
These networks of veins, known as AVAs (arteriovenous anastomoses) seem exclusively devoted to rapid temperature management. They don’t supply nutrition to the skin, and they have highly variable blood flow, ranging from negligible in cold weather to as much as 60 percent of total cardiac output during hot weather or exercise.
In humans, AVAs show up in the face, feet, and hands — which means that a refrigerated glove can cool humans quickly:
The newest version of the device is a rigid plastic mitt, attached by a hose to what looks like a portable cooler. When Grahn sticks his hand in the airtight glove, the device creates a slight vacuum. The veins in the palm expand, drawing blood into the AVAs, where it is rapidly cooled by water circulating through the glove’s plastic lining.
The method is more convenient than, say, full-body submersion in ice water, and avoids the pitfalls of other rapid palm-cooling strategies. Because blood flow to the AVAs can be nearly shut off in cold weather, making the hand too cold will have almost no effect on core temperature. Cooling, Grahn says, is therefore a delicate balance.
“You have to stay above the local vasoconstriction threshold,” said Grahn. “And what do you get if you go under? You get a cold hand.”
Even in prototype form, the researchers’ device proved enormously efficient at altering body temperature. The glove’s early successes were actually in increasing the core temperature of surgery patients recovering from anesthesia.
“We built a silly device, took it over to the recovery room and, lo and behold, it worked beyond our wildest imaginations,” Heller explained. “Whereas it was taking them hours to re-warm patients coming into the recovery room, we were doing it in eight, nine minutes.”
Overheating is a problem for athletes — and not just marathon-runners:
But the glove’s effects on athletic performance didn’t become apparent until the researchers began using the glove to cool a member of the lab — the confessed “gym rat” and frequent coauthor Vinh Cao — between sets of pull-ups. The glove seemed to nearly erase his muscle fatigue; after multiple rounds, cooling allowed him to do just as many pull-ups as he did the first time around. So the researchers started cooling him after every other set of pull-ups.
“Then in the next six weeks he went from doing 180 pull-ups total to over 620,” said Heller. “That was a rate of physical performance improvement that was just unprecedented.”
The researchers applied the cooling method to other types of exercise — bench press, running, cycling. In every case, rates of gain in recovery were dramatic, without any evidence of the body being damaged by overwork — hence the “better than steroids” claim. Versions of the glove have since been adopted by the Stanford football and track and field teams, as well as other college athletics programs, the San Francisco 49ers, the Oakland Raiders and Manchester United soccer club.
When I studied exercise physiology years ago, fatigue was described through the three energy systems: ATP depletion, or lactic acid accumulation, or glycogen depletion, etc. This new research suggests that temperature is the primary limiting factor for performance:
In 2009, it was discovered that muscle pyruvate kinase, or MPK, an enzyme that muscles need in order to generate chemical energy, was highly temperature-sensitive. At normal body temperature, the enzyme is active — but as temperatures rise, some of the enzyme begins to deform into the inactive state. By the time muscle temperatures near 104 degrees Fahrenheit, MPK activity completely shuts down.
So, cooling should improve performance, but it shouldn’t necessarily improve training, if you want to adapt to overheating.