Daredevils and thrill-seekers need more than nerves of steel: they often need stretchy throat muscles, empty stomachs, and occasionally, long poles. Thanks to scientific inquiry, we can unpack how those that 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, building 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 illusionist David Blaine, when he's not freezing himself in ice or sitting in a plastic cage suspended above the Thames River, occasionally submerges himself in water and tries to break world records for breath-holding.

Blaine's 2006 “Drowned Alive” stunt, in which he spent seven days and seven nights in a water-filled sphere, fed by and breathing 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, he set his first Guinness World Record on the Oprah Winfrey Show, holding his breath for 17 minutes and 4 seconds underwater after inhaling pure oxygen for 23 minutes beforehand.

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

Hyperventilating with pure oxygen before a dive is just one part of a technique called “lung packing,” which allows a diver to extend their breath-holding times and dive deeper. Right before a diver takes the plunge, he or she inhales to their maximum, then swallows additional air to force the lungs to expand even further. They usually fast for a while before a dive, 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 your view) sport of freediving. The international freediving association AIDA 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 the diver descends with help of a ballast weight and ascending under their own power, and “no limit,” which allows divers 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 this past 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 the majority of the 6,000 people that participated in the firewalk did so without injury.)
 

University of Pittsburgh physicist David Willey has dissected the experience both as a scientific observer and 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 1000 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 foot – and transfer energy.

What makes firewalking possible is that the charcoal's thermal conductivity, or ability to transfer heat, is very low, Willey explains. Human flesh is also a relatively poor conductor of heat, just about four times stronger than charcoal. In comparison, 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 writes on his website.

Let Me Clear My Throat

To become a card-carrying member of The Sword Swallowers' Association International, you have to be able to gulp down a non-retractable, solid steel blade that's at least two centimeters wide and 38 centimeters long (about ¾” by 15”). 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 their depths with a wire coat hanger.

Once a budding performer progresses 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, though at least one performer reported using butter.

Obviously, professional sword swallowers are aware of the medical risks associated with shoving sharp metal down one's throat. 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.

19 of the sword swallowers said they'd gotten sore throats when they were learning to swallow, or after performing too frequently, or when they swallowed more than one sword, or 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, commits a technical fault, or when the audience interferes.

“One swallower lacerated his pharynx when trying to swallow a curved sabre, 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 body, the tightrope walker, also called a funambulist, is doing two things: lowering his center of gravity and increasing his moment of inertia.

Lowering the center of gravity towards 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 him 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 past April, researchers from Harvard University determined that the ideal tension for both highwires and slacklines -- lower ropes where people often perform acrobatic tricks without the aid of a pole -- is one where the rope sags 3 to 4 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 wirewalker 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.

A long motorcycle jump is, superficially, 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

And sometimes, mechanical failure can overcome careful planning. Evel Knievel’s 1974 jump of the Snake River Canyon in Idaho (the U.S. Department of the Interior wouldn’t let him use the Grand Canyon) ended in disaster when the parachute from his rocket-powered Sky-Cycle deployed far too soon, and instead of landing on a pogo-stick like 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.

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