ROTTERDAM, Netherlands — The sun beams down on a warm Dutch spring morning, and the Iceman's students look wary as they watch him dump bag after bag of ice into the tub of water where they will soon be taking a dip.
The plan is to try to overcome the normal human reaction to immersion in freezing slush: gasping for air, shivering uncontrollably, and getting back out again as soon as possible.
Instead, under the direction of "Iceman" Wim Hof, the group of athletes is going to stay in the water for minutes practicing his meditation techniques, seeking possible performance or health benefits.
Hof, 52, earned his nickname from feats such as remaining in a tank of ice in Hong Kong for almost 2 hours; swimming half the length of a football field under a sheet of ice in the Arctic; and making the Guinness record books for running a half-marathon barefoot in Finnish snow in deep subzero conditions.
He tried to climb Mt. Everest in 2007 wearing only sandals and shorts, but suffered frostbite and turned back at 7,400 meters (24,300 feet) – he wants to test the limits of human potential, not die trying. He climbed Mt. Kilimanjaro instead the same way in 2009.
Hof tells his students meditation in the cold strengthens mind and body. Some scientists also say ice bath treatments may have circulatory benefits for athletes, or help them recover quicker after training, although this remains controversial.
For most people, hypothermia begins shortly after exposure to freezing temperatures without adequate clothing, and it can quickly lead to death once the body's core temperature falls below 90 degrees Fahrenheit (32 degrees Celsius).
Hof says he can endure cold so well because he has learned to activate parts of his mind beyond the reach of most people's conscious control, and crank up what he calls his "inner thermostat."
In one well-documented demonstration in 2008, Hof remained encased in a glass box filled with ice on a New York City street for 71 minutes, at that time a record. Doctors monitoring his vital signs said his body temperature descended gradually to 93.6 degrees as his heart rate rose slowly into the 120s. He didn't shiver.
It was as if he were running a race without moving.
Hof describes what he does as a kind of internal conversation, in which his mind and body send each other signals. During the Hong Kong stunt, he said he mentally directed warmth toward a specific part of his lower back when he sensed it was feeling too cold and starting to hurt.
"I never had a teacher, and I never had lessons, other than hard Nature itself," he says in an interview at his apartment in Amsterdam.
"If you do it wrong, it hurts and you take some knocks, and if you do it right, then you really learn."
Dr. Anders Cohen, chief of neurosurgery at Brooklyn Hospital Center, who had never heard of Hof, said he wasn't surprised at Hof's ability to influence his body temperature, given the growing body of evidence that Tibetan monks who practice "Tummo" meditation have similar abilities.
"In a way it makes perfect sense," he says. "They spend thousands of hours practicing this, while we spend that time doing other things," he says.
A new medical test released last month suggests Hof may be able to exercise some influence over other body functions considered involuntary.
"We have one result, from one person, that is extraordinary, but it doesn't prove that meditation is responsible," said Professor Peter Pickkers of Nijmegen's Radboud Medical University, who oversaw the test and has no commercial ties to Hof.
The Iceman was injected with endotoxin, a component of bacteria. Although harmless, the bacterial material essentially tricks the body into thinking it is under attack.
In most people, exposure provokes flulike symptoms: headaches, muscle pain, and fevers. These last several hours and then go away with no lasting ill-effect. Hof reported experiencing only a mild headache.
Pickkers said the unexpected part came in the laboratory: Hof produced only half as much inflammatory defense protein as average among more than 200 other healthy male test subjects. The apparent reason, Pickkers said, was a sharp rise in levels of cortisol, the "stress hormone" known to suppress the immune system.
Hof appeared to be intentionally influencing a system thought to be automatically regulated, Pickkers said.
"If you get into a fight on the street, and your heart races, that happens by itself, you can't just summon it up," Pickkers said. "What he has shown is that he can with his meditation, apparently, summon it up that his cortisol rises like that."
He said the next step would be to see whether others using similar techniques can do the same.
Cohen found the Nijmegen results intriguing. "It would be unwise to ignore this just because we don't understand the mechanism," he said.
Cohen, who is also a former professional tennis player, says science is divided about whether cold water or sauna treatments actually aid athletes, though many use them. One theory is that forcing blood vessels to contract and expand can strengthen them and improve circulation. Athletes often use cold baths after practice to reduce muscle inflammation and soreness.
However, Cohen said it would be difficult to conduct a rigorous test of whether meditation in cold conditions could benefit sick people, since it would be unethical to put them at risk.
Hof tells his students at the Rotterdam workshop that viewing mental and physical training as separate may hinder their performance during competition.
"Technically you're completely trained and ready and everything," he says. "But there is still a difference between how you feel – the flow isn't there – because there's no unity," he says, gesturing to his head and chest.
Hof describes the three main elements in his method as controlled breathing, paying close mental attention to signals coming from the body, and crucially, keeping an open mind.
Edith Bosch, who won silver and bronze medals in judo at the 2004 and 2008 Olympics, said her only remaining goal is gold. "If this helps me improve, to cope better with matches, then, yeah, it's definitely worth the effort to try," she says.
Hof says that as he grows older, he wants to avoid what he calls the "circus sideshow" of extreme physical tests, and become more of a teacher.
But daredevil habits die hard. To prove that he can also adjust his "inner thermostat" downward, he's planning to run a marathon in desert conditions – without drinking any water.
Pickkers, aware of this plan, shakes his head in dismay.
"I have warned him not to do this, it can be extremely dangerous or lethal," he said. "But if you had asked me ahead of time whether I thought he would have had a different reaction than anybody else to the endotoxin test, I would have said, 'no.'"
Sterling reported from Amsterdam.