Select few redefine human abilities
Tanya Streeter has a lung volume almost twice the normal size for her build, helping her set free diving records.
Most of the activities profiled in this Special Report are extremely dangerous and are performed or closely monitored by highly trained individuals. You should not attempt them yourself.
(CNN) -- To the average person, swimming a mile in the frigid waters off Antarctica, diving more than 500 feet on a single breath, or climbing the world's highest mountains without the help of extra oxygen would be deadly.
But a select few in the world decide to test their bodies and their wills, and challenge medicine to redefine what is humanly possible.
"It's hard to explain how they can do that because if you take the numbers that we know from medical school, it just shouldn't happen," said Dr. Kenneth Kamler, author of "Surviving the Extremes," a chronicle of his medical adventures in treacherous locales such as the Amazon and Mount Everest. "But it does happen. It happens in every kind of human activity. People exceed what you would calculate as their limits."
Taking a deep breath
Tanya Streeter's remarkable lungs and willpower have helped her break world records in free diving, a sport in which competitors dive deep beneath the water's surface on one breath. In 2002, she completed a dive of 525 feet -- a length equivalent to a 50-story building -- on a single breath of air, setting a new record.
Streeter, a native of the Cayman Islands, also has amazed the world with her breath-holding talents. Her time of six minutes and 16 seconds is just five seconds short of the women's record.
Studying how Streeter can function so well without oxygen, University of Texas professor Ed Coyle learned that she has a lung volume almost twice what women her size usually have.
Coyle also focused on the oxygen levels in Streeter's blood when she's holding her breath. Streeter is regularly able to push below 50 percent. By comparison, in an operating room, surgeons consider blood oxygen saturation of less than 70 percent the point at which the brain and heart can be damaged by lack of oxygen.
Streeter said she hopes her abilities can offer researchers insights into conditions such as asthma, sleep apnea or sudden infant death syndrome, and pass on a lesson to others about redefining their own limits.
"You have to accept that somewhere in you, you have a personal limit, but chances are, it's nowhere [near] where you think it is," Streeter said. "Chances are it's going to be much farther, deeper, longer than you thought."
Swimming to Antarctica
Lynne Cox has been pushing past limits for more than 30 years. At 14, she completed a 27-mile swim across California's Catalina Channel. A year later, she logged the then-fastest English Channel crossing time for a woman or man.
Over the years, Cox specialized in long-distance swims in icy water that would kill the average person in minutes. Her training culminated in 2002 with a swim of more than a mile through 32-degree waters, recounted in her memoir "Swimming to Antarctica."
But how can Cox -- with only a swimsuit for protection -- achieve feats that would leave most people permanently damaged or dead?
To find out, doctors from the University of California-Santa Barbara once had her swallow a tiny thermometer with a radio transmitter. Cox said that unlike most people, her body got warmer as she swam in cold water.
In 2002, Lynne Cox swam to Antarctica, withstanding 32-degree water in only a swimsuit.
"I swam for four hours in 50-degree temperatures, and my body temperature went from 97.6, a little bit lower than normal, to a 100.2," Cox said.
As an Olympic-level endurance athlete, Cox can work her muscles so hard and so long that she generates more heat than she loses, doctors said. And that heat is kept inside by what Cox calls her "internal wet suit," a little layer of extra fat, spread evenly around her body.
"The [Antarctic] swim itself was extremely beautiful and harsh," Cox said. "The harshness of knowing that if you stay in the water a moment too long you can go into cardiac arrest. There's a knowledge that you really are on edge here, and that you can push yourself too far."
Pulling their weight
Svend Karlsen and Jon Andersen throw the logical limits of human strength out the window. They are two among a handful of professional strongmen in the world, with bodies tipping the scales at more than 300 pounds.
An eight-year veteran of the sport, Karlsen has broken 30 Norwegian power-lifting records, and in 2001 captured the title of world's strongest man.
"I can do these things that I know hardly anybody ... can do this on this planet," Karlsen said. "I have a God-given talent for lifting big things. That's in my genes."
Svend Karlsen, left, and Jon Andersen credit genetics and training for their extreme human strength.
University of Pennsylvania researchers are studying just how much a strongman's success is due to genetics. When they injected a gene called IGF-I into muscle, it not only increased mass, but also sustained it even when weight training stopped.
Those findings may help stem the loss of muscle mass in older people or those with degenerative disorders like muscular dystrophy. But it raises questions about potential abuses by athletes.
The International Federation of Strength Athletes says strongmen are tested twice a year for 30 banned substances, but IGF-I is not among them.
Andersen, ranked fourth-strongest in the United States, said he has seen steroid abuse in his field but that he prefers to push his limits naturally, training 12 hours a week and putting away 25 pounds of beef and 14 supplement shakes every seven days.
"I tell people if you take five years of your life and truly commit yourself to any one thing, at the end of the time, you're going to have some major results," Andersen said.
Climbing every mountain
Ed Viesturs has been called the No. 1 adventure athlete in the world and the "Chevy truck of mountaineering."
He has climbed Mount Everest five times, and, without the use of supplemental oxygen, has reached 13 of the 14 world's highest peaks, all towering more than 26,000 feet high. This spring, he attempts to top Annapurna in the Himalayas, the last of his 14 peaks.
Ed Viesturs has climbed Mount Everest five times and reached 13 of the 14 world's highest peaks.
"I like things that are difficult, physically and mentally," Viesturs said. "Things that are really challenging, things that really maybe take a long time but really push me to my limits."
At least 1,200 people have climbed Mt. Everest, most with bottled oxygen to help them breathe. Because of the thin air, climbers take in about one-third the oxygen they would get at sea level.
Viesturs said his body is able to adapt to high altitudes more efficiently than most. Also, doctors said Viesturs has a 7-liter lung capacity, compared with the 5-liter capacity of an average person of his height.
There is a point in climbing where there's not enough oxygen to sustain life, the so-called "death zone." But Viesturs said he believes that even that zone can be surpassed.
"Because Everest is 29,000 feet and we've gone that far, if there's a peak that's 29,500 feet, I'm sure humans could climb it," he said. "I don't think we've reached our limits."
Kamler, the doctor-explorer, agreed, saying he is awed at the human body's capabilities.
"I'm willing to consider almost anything as possible now with the human body," Kamler said. "And the more I study the human body the more amazed I become by what it's capable of doing."