You are on a rooftop, looking across empty air 1,350 feet above the ground. Your foot dangles over the ledge and touches a steel cable just centimeters wide. As you shift your body forward, hands gripped tight around a balancing pole, you find yourself suspended over a gut-wrenching void.
Acrophobics would surely hope to wake from this nightmare in a cold sweat. But for seasoned tightrope walkers, the dizzying feat can be accomplished if you understand the physics of the human body.
“Posture is the absolute most important thing,” says Sonja Harpstead, a tightrope instructor at Circus Warehouse in New York City.
The key to balancing on a tightrope is to lower the body’s center of gravity toward the wire. Just as it's harder to topple a stout vase than a tall slim one, a human is less likely to fall if the bulk of their mass is closer to the ground—or, in this case, the wire.
Amateurs often try to accomplish this shift by leaning forward. That may lower your overall mass, but it also interferes with your sense of location in space. Then, it's hello pavement.
Harpstead instead instructs her students to stand up straight and lower their hips by bending their knees. This brings a person's center of gravity closer to the wire while allowing them to keep their bearings.
At the same time, a tightrope walker has to remember that the wire itself tends to rotate. Each step along the cable invites it to spin underfoot, potentially throwing the walker off balance. To keep from falling, the walker must increase something called rotational inertia—effectively, positioning the body so that it fights against the wire's want to rotate.
As any child on the playground knows, the best way to improve your balance on a cylindrical object is to stick out your arms horizontally. This spreads out your mass and improves your ability to fight rotational forces, giving you enough time to correct your motions if you start to slip. Many tightrope walkers boost this effect by carrying a long balancing pole.
For his daredevil walk between the Twin Towers of the World Trade Center in 1974, currently depicted in the Joseph Gordon-Levitt vehicle The Walk, French high-wire artist Philippe Petit carried a 26-foot balancing pole he crafted for the act.
“After a few steps, I knew I was in my element and I knew the wire was not well rigged (we had some tremendous problem during the whole night of rigging) but it was safe enough for me to carry on,” says Petit in an episode of the PBS series “American Experience.” “And then, very slowly as I walked, I was overwhelmed by a sense of easiness, a sense of simplicity.”
Says Harpsted, “The pole increases your rotational inertia so that each tiny little movement you do does more, and in general the little bit of wiggle that happens in your personal body means less in relation to the whole system.” As an added bonus, the ends of the pole bend down, helping to lower the walker’s center of gravity even more.
The state of the wire itself can also play a role in the walker’s ability to successfully cross it. In the ideal scenario, the wire would be infinitely taut, says Paolo Paoletti of the University of Liverpool’s School of Engineering. The more slack in the wire, the more likely it is to undulate under your feet as you take each cautious step, making it harder to balance.
Tightrope walking is a science as well as an art. And further studying this entertaining act could be useful for improvements in healthcare, adds Paoletti.
Healthcare professionals could use tightrope walking itself as a tool to detect early muscle degradation in elderly patients. Moving on solid ground masks oncoming muscular issues, but with regular tests on tightropes, where movement is much more difficult, specialists could identify issues before they become problematic. Physical therapists may also seek to use tightrope exercises to build strength and balance with patients in need of muscular-skeletal strengthening.
But even for those who are able bodied, the stress and pressure of walking high off the ground, along a thin wire, with no safety net are enough to keep all but the truly determined grounded.