Daredevils and other thrill seekers need more than nerves of steel: They often require stretchy throat muscles, empty stomachs, and, occasionally, long poles. Thanks to scientific inquiry, we can reveal how those who perform death-defying stunts live to see another day.

Deep Breaths

Most of us probably can't hold our breath for much longer than 30 seconds or so. When you are deprived of oxygen, rising levels of carbon dioxide inside your body trigger a muscle spasm that forces you to gasp for air. But some hardy souls can push through or train away the pain.

The American illusionist David Blaine -- when he's not busy freezing himself in ice or sitting in a plastic cage suspended above London's River Thames -- occasionally submerges himself in water and tries to break world records for breath-holding.

Blaine's 2006 “Drowned Alive” stunt had him spend seven days and seven nights in a water-filled sphere, breathing and fed through tubes. The stunt was supposed to culminate in his attempt to break the world record for breath-holding without the use of breathing pure oxygen beforehand, but he failed and was pulled up by support divers nearly two minutes before he reached his goal.

In April 2008, Blaine set his first Guinness World Record on "The Oprah Winfrey Show," holding his breath for 17 minutes, 4.4 seconds underwater after inhaling pure oxygen for 23 minutes before the stunt.

Blaine's record has been broken several times since. The current Guinness World Record holder for breath-holding underwater is the Danish Stig Severinsen, who managed to keep going for 22 minutes flat at the London School of Diving in May. Severinsen huffed oxygen for 19 and one-half minutes before his attempt. But even his record seems to have fallen, though not officially yet, according to Time Magazine, which reported on on German diver Tom Sietas' 22-minute, 22-second feat in June.

Hyperventilating with pure oxygen before a dive is just one aspect of a technique called “lung packing,” which allows a diver to extend his or her breath-holding times and dive deeper. Before a diver takes the plunge, he or she first inhales to the maximum, then swallows additional air to force the lungs to expand even further. They usually fast for a while before dives, so their ballooning lungs won't hit the barrier of a full stomach.

Holding your breath underwater while floating, also called "static apnea," is just one element of the terrifying or thrilling -- depending on one's point of view -- sport of freediving. AIDA International, the global freediving association, maintains standards and world records in eight different categories, including static apnea. Other disciplines include “constant weight,” in which a diver uses fins and arms to descend and ascend without pulling on a nearby safety rope; “variable weight,” in which a diver descends with help of a ballast weight and ascends under his or her own power; and “no limit,” which allows a diver to descend with weight and ascend with assistance, be it a balloon, inflatable vest, or other device.

“No limit is the absolute depth discipline,” AIDA says.

Better Firewalking With Physics

Firewalking's journey from ancient religious practice to corporate-retreat activity rests on the physics of heat transference. While most firewalkers will suffer no injury, the practice is still dangerous -- as nearly two dozen people found out in July at a motivational seminar, where they suffered second- and third- degree burns on their feet. (The company that hosted the seminar, Robbins Research International Inc., noted soon after the incident that most of the 6,000 people who participated in the firewalk did so without injury.)

University of Pittsburgh physicist David Willey has dissected the experience both as a scientific observer and as a participant: He took part in a 1998 firewalk across 165 feet of hot coals, which at the time set a new world record.

The coals used should not be cherry-red. They have to be allowed to cool to 1,000 degrees Fahrenheit, and to build up a protective layer of ash that blocks radiant heat from the foot. The coals still transmit heat through a process called conduction, which happens when more energetic molecules -- in this case, inside hot coals -- collide with less excited molecules, the ones in the soles of the firewalker's feet, and transfer energy.

What makes firewalking possible is that the charcoal's thermal conductivity, or ability to transfer heat, is very low, Willey explained. Human flesh is also a relatively poor conductor of heat, just about four times stronger than charcoal. In contrast, most metals have a thermal conductivity several thousand times greater than charcoal.

Another factor in avoiding injury is timing -- you're much less likely to get burned when your foot spends little time in contact with the coals. But don't sprint across.

“It is neither necessary nor advisable to run, a brisk walk is reported to work best, with each step taking half a second or less,” Willey wrote on his website.

Let Me Clear My Throat

To become a card-carrying member of the Sword Swallowers Association International, or SSAI, you have to be able to gulp down a nonretractable, solid-steel blade that's at least two centimeters wide and 38 centimeters long (about three-quarters of an inch by 15 inches). Practitioners develop their skills through daily training for months or years, desensitizing their gag reflex by putting fingers, spoons, knitting needles, or other implements down their throats. Typically after mastering these shorter implements, a sword swallower in training then attempts to fully plumb his or her depths with a wire coat hanger.

Once budding performers progress to actual swords, they must train themselves to be able to relax muscles in the pharynx and esophagus that are normally not under our voluntary control. They must also learn to keep from vomiting as the sword passes through the lower esophagus and into the stomach.

At least one sword swallower has been documented “sucking” in a sword by filling his pharynx with air. The sword is typically lubricated with spit, although one performer reported using butter.

Obviously, professional sword swallowers are aware of the medical risks associated with shoving sharp metal down their throats. Through training, performers can minimize their risks, but accidents and side effects do occur. Some are chronicled in a 2006 paper in the journal BMJ by Gloucester radiologist Brian Witcombe and SSAI executive director Dan Meyer, who reported on medical histories volunteered by 46 sword swallowers.

Nineteen of the sword swallowers said they'd gotten sore throats when they were learning to swallow, or after performing too frequently, or when they downed more than one sword, or when they swallowed an oddly shaped blade. Sometimes performing was followed by pain in the lower chest, for which the sword swallowers rarely sought medical attention.

Injuries are more common when a performer is distracted, when he or she commits a technical fault, or when the audience interferes.

“One swallower lacerated his pharynx when trying to swallow a curved saber, a second lacerated his [esophagus] and developed pleurisy after being distracted by a misbehaving macaw on his shoulder, and a belly dancer suffered a major hemorrhage when a bystander pushed dollar bills into her belt causing three blades in her [esophagus] to scissor,” Witcombe and Meyer wrote in their BMJ paper.

Men On Wires

The long pole that tightrope walkers carry is a way for these daredevils to enlist a bit of physics to help them avoid a very long fall. By holding the pole perpendicular to his or her body, the tightrope walker, also called a funambulist, is doing two things: lowering the center of gravity and increasing the moment of inertia.

Lowering the center of gravity toward the tightrope helps stabilize the performer. The closer the center of gravity of a balanced object is to the point it is supported upon, the harder it is for small amounts of force to knock one over. A moment of inertia is an object’s resistance to rotation: In the tightrope walker’s case, the pole resists being torqued and adds further stabilization to the funambulist gripping it.

Another important consideration for wire walkers is the sag of the rope, which can greatly affect a performer’s balance as small movements become amplified.

In a paper published in the Journal of the Royal Society Interface this April, researchers from Harvard University determined that the ideal tension for both high wires and slacklines -- lower ropes people use to perform acrobatic tricks without the aid of a pole -- is one where the rope sags three to four feet in the middle.

"All your sensory-control information can be easily tuned to the dynamics of the rope," study author Paolo Paoletti told ScienceNOW. "The time that you need to react coincides with the time that the rope makes one swing."

If the rope is extremely tight, vibrations are quicker, while a slacker rope gives bigger motions as a person moves along it. The “sweet spot,” which allows for a wire walker to react to the rope or wire’s motion, lies somewhere in the middle between tight and loose, the scientists found.

Watch This Jump

Stunt motorcyclists are probably more acquainted with the less-pleasant effects of gravity than any other kind of daredevil.

Superficially, a long motorcycle jump is a simple physics problem: Know the velocity and angle at which a motorcyclist leaves a ramp, and with some accounting for friction and air resistance, you can figure out how many school buses a motorcyclist can clear.

But successfully sticking the landing isn’t simple math: The rider has to work to pull the nose of the bike up to a point where everything comes together more or less smoothly.

And, sometimes, mechanical failure can overcome careful planning. Evel Knievel’s 1974 jump of the Snake River Canyon in Idaho -- the U.S. Interior Department wouldn’t let him use the Grand Canyon in Arizona -- culminated in disaster when the parachute from his rocket-powered X-2 Skycycle deployed far too soon. Instead of landing on a pogo-sticklike contraption on the other side of the canyon, Knievel was blown back into the gorge and crashed after a 600-foot drop.

Knievel survived to defy gravity another day, but it’s a welcome reminder that -- even for professional daredevils -- balancing the equation is no guarantee of success.