A balancing act

Guest post by John M. Henshaw

The Segway is that crazy looking self-propelled scooter that rolls on two side-by-side wheels. Not everyone is in love with the way they look, but no matter what you might think of their aesthetics, you have to admit that they are pretty cool machines. And mysterious. Just how does a Segway keep from falling over, anyway? One might very well ask the same question about a human body, standing erect. Just how does that keep from falling over?

The means by which a Segway is able to stand up straight shares some things in common with the way humans balance themselves. Dean Kamen, whose company DEKA invented the Segway, has said that he was inspired by the human balance system when he was first developing the Segway.

photo by Tamorlan

From a purely physical perspective, a human body, standing upright, is frighteningly

photo by Lauren Bosak

unstable. Anytime an object’s center of mass is farther above the ground than the object is wide—as it most certainly is for a human standing up—that object is relatively easy to tip over. Think of a coffee cup (low center of mass compared to width) versus a champagne flute (high center of mass compared to width). The coffee cup is very difficult to tip, whereas champagne flutes are constantly getting knocked over. A human body, standing up, is a lot more like a champagne flute than a coffee cup.

And, like the champagne flute, we get knocked over plenty, particularly when we’re very young. Luckily, young kids, who haven’t fully mastered the art of balancing themselves, are far less likely to suffer serious injury when they take a tumble, since they are so much shorter than the average adult. As we grow, we get better and better at balancing ourselves. Other than when we’re walking on very slippery surfaces, such as ice, or playing contact sports, most adults rarely lose their balance and fall down. Things are frequently very different for the elderly. My dad is in his 80s, and he suffers from Parkinson’s. He still gets around remarkably well (he walks a mile or more every day) but his balance is not what it used to be, and in recent months he’s had a couple of scary falls.

Humans balance themselves through a complex combination of three senses: the vestibular system, proprioception or body awareness, and vision. The vestibular system, an immensely complex instrument located in the inner ear (one in each ear) tracks the motion and position of the head with extreme precision.

One quick exercise will give you some idea of how well the vestibular system does its job. Because of the vestibular system, your eyes have an amazing ability to maintain their focus on whatever you’re looking at, even when your head is moving. Your head, for example, bobs up and down a bit when you walk, but most of us have no problem maintaining our visual focus on a stationary object while we’re walking past it. You have your vestibular system to thank for this. Its ability in this regard is admirable, but it does have its limits. As you read these words, shake your head as if saying “no.” The words you are reading should remain remarkably stable. But if you shake your head vigorously from side to side, it’s relatively easy to make the words jiggle a little bit as you read them. You’ve reached the limit of the ability of your vestibular system and eyes, working in concert, to keep a visual image stationary while your head is moving.

John M. Henshaw is Harry H. Rogers Professor and Chair of the Department of Mechanical Engineering at The University of Tulsa. His books, A Tour of the Senses: How Your Brain Interprets the World, and Does Measurement Measure Up? How Numbers Reveal and Conceal the Truth, are published by JHU Press.

(The views expressed in this guest post belong to the author and in no way reflect the official opinion of the JHU Press.)

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