Springing Down the Street
by Steve Miller
Think about the shape of your body. Don't think about whether you're tall and thin, or short and more round. Think about the real shape of the human body—most of your weight is on top, above two relatively thin legs. That's not an easy design to balance!
Try standing with your feet together and your hands by your side. Close your eyes and tilt your head back. Notice how much you sway back and forth. Pay attention to your body and you will feel muscles constantly tightening and releasing to keep you from toppling over. Now compare that to your dog balancing on its four legs or a beetle on its six. A lot easier for them, isn't it? That's why most walking robots look more like bugs than people (except for C3PO, but then most robots don't complain as much as him either!). And have you ever seen a spider trip and fall?
Now let's add one more complication to human locomotion—when you walk, you are on only one foot most of the time. So how do you keep upright as you walk? Well, one way to keep your balance is for your brain to send signals to your muscles telling them to contract and pull you back into position—that's why although you started to sway when you were standing still, you were able to recover. Constant tiny adjustments of your muscles pulled you back up whenever you started to lean in any direction.
Now, walk slowly and really concentrate on what your body is doing—which muscles you are using when. Notice when your ankles, knees, and hips bend. There's a lot happening. But researchers in biomechanics labs are finding that you may not be using your brain to control all of these walking motions. Michael Coleman was a graduate student at Cornell University, working with Andy Ruina, a professor of theoretical and applied mechanics, when he built a two-legged machine from Tinkertoys® in an attempt to figure out how human bodies work.
“It can't stand still, but it can walk without any motors to control it,” Ruina says of the toy walker. “We've found that machines can do interesting things—and human bodies may operate as machines. A lot of how we move might be by design rather than control. If a construction made of pieces of plastic can walk by mechanical forces, maybe a construction of bones and tendons does the same thing,” Ruina points out.
So if we move like a machine, can we design a simple model to explain the mechanics of human locomotion? Actually, humans usually use one of two completely different gaits, or ways of moving the feet and legs. You can probably guess what they are—walking and running. (There are a few others—skipping, galloping, hopping—but these are used for fun or for special instances such as moving around on the Moon.) When you walk, you always have one foot on the ground. Biomechanists use the model of an inverted pendulum to explain how this works. As you lift one leg, the mass of your upper body moves forward in an arc, swinging over your stationary foot like an upside-down pendulum. Some of the energy of this motion carries you into the next swing over the opposite foot. Your muscles lift your mass (or weight) to a maximum height in the middle of your stride. As your body comes back down, the force of gravity adds energy to your next step. As your body swings back and forth in a pendulumlike motion over your legs, as much as 65 percent of the total energy generated by your muscles is recovered.
Running is a completely different gait than walking, and requires a different model. When you run, you don't transfer mass gently from one foot to the other. Instead, your body acts like a mass bouncing up and down on two springs—your legs. The muscles of one leg propel you into the air; then you come down on the other foot. The leg bends at the ankle and knee. Energy is stored and released, not by gravity, but by your tendons, muscles, and ligaments. Tendons and ligaments are the connective tissues that hold your bones together and connect them to muscles. They are stretchy, like a rubber band. When you are running, the force of your body hitting the ground stretches them. A large part of the energy storage is in a tendon (the Achilles) that runs around the back of your ankle, connecting your foot to the back of your leg, and a tendon in the front of your knee, which connects the upper and lower parts of your leg. As you move upward into your next stride, the stretched tissues spring back to their relaxed length, returning the stored energy to the body.
Max Donelon is a graduate student in the Integrative Biology Department at the University of California at Berkeley. He researches the way people and animals move when they run. Donelon says, “Every animal that has been measured acts like a spring-mass system when it runs—humans, horses, and even cockroaches.”
Like walking, running can be explained, at least partly, as a mechanical system that doesn't require complete control from your brain. In fact, running might even be mechanically easier than walking. “To run seems very complicated,” says Donelon, “but if you look at the whole system as a mechanical function, it's easy to understand. People built robots that ran before they built ones that walked. Their legs behaved like a mass on a spring.” The study of human gait isn't just interesting, it's also useful. An understanding of the biomechanics of locomotion provides information for the design of walking robots, artificial legs, and improved running shoes, as well as a better understanding of nature.
So, if you want a two-legged walking, talking, complaining robot like C3PO, where should you look for a model system, with the details all worked out by nature? Try looking in the mirror.
- How is energy recaptured by the body when a person is walking? Write a few sentences to explain in your own words how the motion people use to walk helps to recapture some of the energy that was expended.
- When a person is running, how are the person's legs acting like springs? In your answer, include details about the transformation of potential to kinetic energy.
- A person begins walking slowly and then speeds up to walk more quickly. How would the acceleration of a person's walking gait change if this person were walking on the Moon versus on Earth? Why?